Tag Archives: wind load

How Far Can a 2×6 Purlin Span?

How Far Can a 2×6 Purlin Span?

Reader WILL in COMFORT writes:

How far can a 2×6 purlin on a 6:12 sloped roof span?”

The following describes 2×6 SYP #2 purlins spanning a 14′ bay, with an on-center spacing of 24″ (sf).

Purlins are recessed between rafters with their top edges flush with rafter top edges. Purlins are mounted to rafters with Simpson Strong-Tie LU-26 joist hangers at both ends.

Effective simple beam span length (le) will be taken as 165.

Applied loads

Dead load, D[

Dpurlin: dead load from weight of purlin itself
Dpurlin = purlin density × ((b × d × le) / (sf × l))
Purlin density found via NDS Supplement 2015 Section 3.1.3:
density = 62.4 × (G / (1 + (G × 0.009 × moisture content))) × (1 + (moisture content / 100))
moisture content = 19%
density = 62.4 × (0.55 / (1 + (0.55 × 0.009 × 0.19))) × (1 + (0.19 / 100))
density = 34.56 pcf
Dpurlin = 34.56 pcf × ( ( 1.5″ × 5.5″ × 165″ ) / ( 24″ × 168″ ) ) × 1/12 in/ft
Dpurlin = 0.966 psf

Roof designed for 29g corrugated steel
Dead load from weight of steel (Dsteel) based on values from the American Building Components catalogue:
Dsteel = 0.63 psf

D: dead load
D = Dpurlin + Dsteel
D = 0.966 psf psf + 0.63 psf psf
D = 1.596 psf

Project load to a vector acting perpendicular to the roof plane:
D = D × cos(Θ)
D = 1.596 psf × cos(0.464)
D = 1.428 psf

A conversion from psf to psi will be made for ease of calculation:
D = 1.428 psf × 1/144 psi/psf
D = 0.01 psi

Roof live load, Lr

L: roof live load
Lr = 18 psf

Project load to a vector acting perpendicular to the roof plane:
Lr = Lr × cos(Θ) × cos(Θ)
Lr = 18 psf × cos(0.464) × cos(0.464)
Lr = 14.4 psf

A conversion from psf to psi will be made for ease of calculation:
Lr = 14.4 psf × 1/144 psi/psf
Lr = 0.1 psi

Snow load, S

S: snow load
S = 13.267 psf

Project load to a vector acting perpendicular to the roof plane:
S = S × cos(Θ) × cos(Θ)
S = 13.267 psf × cos(0.464) × cos(0.464)
S = 10.614 psf

A conversion from psf to psi will be made for ease of calculation:
S = 10.614 psf × 1/144 psi/psf
S = 0.074 psi

Wind load, W

W: wind load
W = 9.6 psf

A conversion from psf to psi will be made for ease of calculation:
W = 9.6 psf × 1/144 psi/psf
W = 0.067 psi

Wind uplift load, Wu

Wu: wind uplift load
Wu = -11.763 psf

A conversion from psf to psi will be made for ease of calculation:
Wu = -11.763 psf × 1/144 psi/psf
Wu = -0.082 psi

Lr ≥ S, so roof live loads will dictate in load combinations.

Bending test (fb / Fb′ ≤ 1.0)

Fb: allowable bending pressure
Fb′ = Fb × CD × CM × Ct × CL × CF × Cfu × Ci × Cr
CL = 1
CM = 1 because purlins are protected from moisture by roof
Ct = 1 NDS 2.3.3
CF = 1 NDS Supplement
Ci = 1 NDS 4.3.8
Cr = 1 NDS 4.3.9

S: section modulus
S = (b × d2) / 6
S = (1.5″ × (5.5″)2) / 6
S = 7.563 in3

w: pounds force exerted per linear inch of beam length
M: maximum moment
fb: maximum bending stress

Load combinations:

  1. D

CD = 0.9
Cfu = 1
Fb′ = 1000 psi × 0.9 × 1 × 1 × 1 × 1 × 1 × 1 × 1
Fb′ = 900 psi

w = (D) × sf
w = 0.008 psi × 24″
w = 0.186 pli

M = (w × l2) / 8
M = ( 0.18559992381479 pli × (165″)2 ) / 8
M = 654.797 in-lbs

fb = M / S
fb = 654.797 in-lbs / 7.563 in3
fb = 86.585 psi

fb / Fb′ ≤ 1.0
86.585 psi / 900 psi ≤ 1.0
0.096 ≤ 1.0

  1. D + Lr

CD = 1.25
Cfu = 1
Fb′ = 1000 psi × 1.25 × 1 × 1 × 1 × 1 × 1 × 1 × 1
Fb′ = 1250 psi

w = (D + Lr) × sf
w = 0.108 psi × 24″
w = 2.586 pli

M = (w × l2) / 8
M = ( 2.5855999238148 pli × (165″)2 ) / 8
M = 9121.997 in-lbs

fb = M / S
fb = 9121.997 in-lbs / 7.563 in3
fb = 1206.214 psi

fb / Fb′ ≤ 1.0
1206.214 psi / 1250 psi ≤ 1.0
0.965 ≤ 1.0

  1. D + W

CD = 1.6
Cfu = 1
Fb′ = 1000 psi × 1.6 × 1 × 1 × 1 × 1 × 1 × 1 × 1
Fb′ = 1600 psi

w = (D + W) × sf
w = 0.074 psi × 24″
w = 1.786 pli

M = (w × l2) / 8
M = ( 1.7855999238148 pli × (165″)2 ) / 8
M = 6299.597 in-lbs

fb = M / S
fb = 6299.597 in-lbs / 7.563 in3
fb = 833.004 psi

fb / Fb′ ≤ 1.0
833.004 psi / 1600 psi ≤ 1.0
0.521 ≤ 1.0

  1. D + Wu

CD = 1.6
Cfu = 1
Fb′ = 1000 psi × 1.6 × 1 × 1 × 1 × 1 × 1 × 1 × 1
Fb′ = 1600 psi

w = (D + Wu) × sf
w = -0.074 psi × 24″
w = -1.775 pli

M = (w × l2) / 8
M = ( -1.7748379435325 pli × (165″)2 ) / 8
M = -6261.628 in-lbs

fb = M / S
fb = -6261.628 in-lbs / 7.563 in3
fb = -827.984 psi

fb / Fb′ ≤ 1.0
-827.984 psi / 1600 psi ≤ 1.0
-0.517 ≤ 1.0

  1. D + 0.75Lr + 0.75W

CD = 1.6
Cfu = 1
Fb′ = 1000 psi × 1.6 × 1 × 1 × 1 × 1 × 1 × 1 × 1
Fb′ = 1600 psi

w = (D + 0.75Lr + 0.75W) × sf
w = 0.133 psi × 24″
w = 3.186 pli

M = (w × l2) / 8
M = ( 3.1855999238148 pli × (165″)2 ) / 8
M = 11238.797 in-lbs

fb = M / S
fb = 11238.797 in-lbs / 7.563 in3
fb = 1486.122 psi

fb / Fb′ ≤ 1.0
1486.122 psi / 1600 psi ≤ 1.0
0.929 ≤ 1.0

  1. D + 0.75Lr + 0.75Wu

CD = 1.6
Cfu = 1
Fb′ = 1000 psi × 1.6 × 1 × 1 × 1 × 1 × 1 × 1 × 1
Fb′ = 1600 psi

w = (D + 0.75Lr + 0.75Wu) × sf
w = 0.021 psi × 24″
w = 0.515 pli

M = (w × l2) / 8
M = ( 0.51527152330431 pli × (165″)2 ) / 8
M = 1817.878 in-lbs

fb = M / S
fb = 1817.878 in-lbs / 7.563 in3
fb = 240.381 psi

fb / Fb′ ≤ 1.0
240.381 psi / 1600 psi ≤ 1.0
0.15 ≤ 1.0

Purlin stressed in bending to a maximum of 96.5%

Shear test (fv / Fv′ ≤ 1.0)

Fv: allowable shear pressure
Fv′ = Fv × CD × CM × Ct × Ci
CM = 1 because purlins are protected from moisture by roof
Ct = 1 NDS 2.3.3
Ci = 1 NDS 4.3.8
V: max shear force
fv: max shear stress

Load combinations:

  1. D

CD = 0.9
Fv‘ = 175 psi × 0.9 × 1 × 1 × 1
Fv‘ = 157.5 psi

V = w × (le – (2 × d)) / 2
V = 0.186 pli × ( 165″ – (2 × 5.5″) ) / 2
V = 14.57 lbs

fv = (3 × V) / (2 × b × d)
fv = (3 × 14.57 lbs) / ( 2 × 1.5″ × 5.5″ )
fv = 2.649 psi

fv / Fv′ ≤ 1.0
2.649 psi / 157.5 psi ≤ 1.0
0.017 ≤ 1.0

  1. D + Lr

CD = 1.25
Fv‘ = 175 psi × 1.25 × 1 × 1 × 1
Fv‘ = 218.75 psi

V = w × (le – (2 × d)) / 2
V = 2.586 pli × ( 165″ – (2 × 5.5″) ) / 2
V = 202.97 lbs

fv = (3 × V) / (2 × b × d)
fv = (3 × 202.97 lbs) / ( 2 × 1.5″ × 5.5″ )
fv = 36.904 psi

fv / Fv′ ≤ 1.0
36.904 psi / 218.75 psi ≤ 1.0
0.169 ≤ 1.0

  1. D + W

CD = 1.6
Fv‘ = 175 psi × 1.6 × 1 × 1 × 1
Fv‘ = 280 psi

V = w × (le – (2 × d)) / 2
V = 1.786 pli × ( 165″ – (2 × 5.5″) ) / 2
V = 140.17 lbs

fv = (3 × V) / (2 × b × d)
fv = (3 × 140.17 lbs) / ( 2 × 1.5″ × 5.5″ )
fv = 25.485 psi

fv / Fv′ ≤ 1.0
25.485 psi / 280 psi ≤ 1.0
0.091 ≤ 1.0

  1. D + Wu

CD = 1.6
Fv‘ = 175 psi × 1.6 × 1 × 1 × 1
Fv‘ = 280 psi

V = w × (le – (2 × d)) / 2
V = -1.775 pli × ( 165″ – (2 × 5.5″) ) / 2
V = -139.325 lbs

fv = (3 × V) / (2 × b × d)
fv = (3 × -139.325 lbs) / ( 2 × 1.5″ × 5.5″ )
fv = -25.332 psi

fv / Fv′ ≤ 1.0
-25.332 psi / 280 psi ≤ 1.0
-0.09 ≤ 1.0

  1. D + 0.75Lr + 0.75W

CD = 1.6
Fv‘ = 175 psi × 1.6 × 1 × 1 × 1
Fv‘ = 280 psi

V = w × (le – (2 × d)) / 2
V = 3.186 pli × ( 165″ – (2 × 5.5″) ) / 2
V = 250.07 lbs

fv = (3 × V) / (2 × b × d)
fv = (3 × 250.07 lbs) / ( 2 × 1.5″ × 5.5″ )
fv = 45.467 psi

fv / Fv′ ≤ 1.0
45.467 psi / 280 psi ≤ 1.0
0.162 ≤ 1.0

  1. D + 0.75Lr + 0.75Wu

CD = 1.6
Fv‘ = 175 psi × 1.6 × 1 × 1 × 1
Fv‘ = 280 psi

V = w × (le – (2 × d)) / 2
V = 0.515 pli × ( 165″ – (2 × 5.5″) ) / 2
V = 40.449 lbs

fv = (3 × V) / (2 × b × d)
fv = (3 × 40.449 lbs) / ( 2 × 1.5″ × 5.5″ )
fv = 7.354 psi

fv / Fv′ ≤ 1.0
7.354 psi / 280 psi ≤ 1.0
0.026 ≤ 1.0

Purlin stressed in shear to a maximum of 16.9%

Deflection test (Δmax / Δallow ≤ 1.0)

I: moment of inertia
I = b × d3 / 12 NDS 3.3.2
I = ( 1.5″ × (5.5″)3 ) / 12
I = 20.797 in4

E: modulus of elasticity
E′ = E × CD × CM × Ct × Ci
CM = 1 because purlins are protected from moisture by roof
Ct = 1 NDS 2.3.3
Ci = 1 NDS 4.3.8

Δallow: allowable deflection
Δmax: maximum deflection

Load combinations:

  1. D + Lr

CD = 1.25
E′ = 1400000 × 1.25 × 1 × 1 × 1
E′ = 1750000 psi

Per IBC 1604.3 footnote d, dead load may be taken as 0.5D.
w = ((0.5 × D) + Lr) × sf
w = ( (0.5 × 0.01 psi) + 0.1 psi ) × 24″
w = 0.104 pli

Δallow = l / 150 IBC 1604.3
Δallow = 165″ / 150
Δallow = 1.1″

Δmax = (5 × w × l4) / (384 × E′ × I)
Δmax = ( 5 × 2.493 pli × (165″)4 ) / ( 384 × 1750000 psi × 20.797 in4 )
Δmax = 0.661″

Δmax / Δallow ≤ 1.0
0.661″ / 1.1″ ≤ 1.0
0.601 ≤ 1.0

  1. D + W

CD = 1.6
E′ = 1400000 × 1.6 × 1 × 1 × 1
E′ = 2240000 psi

Δallow = l / 150 IBC 1604.3
Δallow = 165″ / 150
Δallow = 1.1″

Δmax = (5 × w × l4) / (384 × E′ × I)
Δmax = ( 5 × 1.786 pli × (165″)4 ) / ( 384 × 2240000 psi × 20.797 in4 )
Δmax = 0.37″

Δmax / Δallow ≤ 1.0
0.37″ / 1.1″ ≤ 1.0
0.336 ≤ 1.0

  1. D + Wu

CD = 1.6
E′ = 1400000 × 1.6 × 1 × 1 × 1
E′ = 2240000 psi

Δallow = l / 150 IBC 1604.3
Δallow = 165″ / 150
Δallow = 1.1″

Δmax = (5 × w × l4) / (384 × E′ × I)
Δmax = ( 5 × -1.775 pli × (165″)4 ) / ( 384 × 2240000 psi × 20.797 in4 )
Δmax = -0.368″

Δmax / Δallow ≤ 1.0
-0.368″ / 1.1″ ≤ 1.0
-0.334 ≤ 1.0

  1. D + 0.75Lr + 0.75W

CD = 1.6
E′ = 1400000 × 1.6 × 1 × 1 × 1
E′ = 2240000 psi

Δallow = l / 150 IBC 1604.3
Δallow = 165″ / 150
Δallow = 1.1″

Δmax = (5 × w × l4) / (384 × E′ × I)
Δmax = ( 5 × 3.186 pli × (165″)4 ) / ( 384 × 2240000 psi × 20.797 in4 )
Δmax = 0.66″

Δmax / Δallow ≤ 1.0
0.66″ / 1.1″ ≤ 1.0
0.6 ≤ 1.0

  1. D + 0.75Lr + 0.75Wu

CD = 1.6
E′ = 1400000 × 1.6 × 1 × 1 × 1
E′ = 2240000 psi

Δallow = l / 150 IBC 1604.3
Δallow = 165″ / 150
Δallow = 1.1″

Δmax = (5 × w × l4) / (384 × E′ × I)
Δmax = ( 5 × 0.515 pli × (165″)4 ) / ( 384 × 2240000 psi × 20.797 in4 )
Δmax = 0.107″

Δmax / Δallow ≤ 1.0
0.107″ / 1.1″ ≤ 1.0
0.097 ≤ 1.0

Purlin stressed in deflection to a maximum of 60.1%

Load Duration Factor for Wood

Load Duration Factor for Wood

Load Duration Factor, or LDF, is based on wood’s ability to recover after a reasonable load has been applied for a given time. Wood is a stiff material but it is not completely rigid. Wood will flex under load, and once load has been removed, wood member will rebound or spring back to its original shape (if load was not excessive or applied for too long). LDF is relevant in regular service load cases expected to act on a structure and member in consideration must resist without failure. LDF does not address loads overstressing a member to breakage point.

Some loads are expected to act on a structure for short time periods, such as wind and seismic loads whose duration would normally be measurable in seconds. Other loads, such as snow, might last at least three months, depending on geography. Dead loads are permanent and they are expected to act on a structure for its life. Load Duration Factors allow us to increase wood’s load carrying capacity based on how long a load is expected to act on a structure—shorter time period, higher allowed increase.

Load Duration Factors are applied to member capacity or resistance, not to loads on member. Only those capacities or resistances related to wood’s ability to recover from a load are subject to LDF adjustments, i.e. moment and shear. Bearing capacity is not adjusted for LDF for IBC (International Building Code). Instantaneous deflection is also not affected by LDF because we are measuring how far a member flexes, not for how long. There is a deflection analysis type called Creep Analysis, it checks long term deflection beyond instantaneous deflections due to heavy loads acting on a member for long time periods. Our software does not analyze this deflection type because North American building codes do not require creep analysis. 

Load Duration Factors are tightly integrated into load combinations used to load a member, i.e. if a load combination has both dead and wind loads then load duration factor for this load combination will be higher value defined by wind. Even though a load combination may have more permanent loads, by default load combination will apply shortest acting load’s LDF. Our software will run through all load combinations and their LDFs as required by building codes and will pick one producing highest stresses as critical load combination for a particular design check.

IBC allows engineers to choose what load duration factor (CD) can be applied with a given load. Table below shows LDF default values and ranges our software allows.

Default LDF Range Loading
CD = 0.90 Dead load only
CD = 1.00 0.90 – 1.00 Floor and Heavy Snow Load
CD = 1.15 1.00 – 1.15 Snow Load
CD = 1.25 1.00 – 1.25 Roof Load, Construction Load
CD = 1.60 1.00 – 1.60 Wind Load, Seismic Load

Notice our software defaults to the highest possible factor (liberal, but correct most times). For example, a structure for an area with heavy snow loads lasting in place for half a year, it would be reasonable to reduce your snow LDF to 1.00. Always consult your local building officials when in doubt.

Building Your Own Pole Barn Trusses

Wants to Build His Own Pole Barn Trusses

Reader DANIEL in HAMPSHIRE writes:

“Good evening, I was wondering if I could ask for your help? I have a question regarding truss designs and truss spacing. I’m building a pole barn (50ft wide x 112ft long x 12ft tall). Prices of pole barn kits have skyrocketed just as much as steel buildings. Building this size 3 years ago would have cost a third of the price today. I’m building an indoor fish farm. If you like to know more of my back story you can visit www.steelheadsprings.com I don’t want to waste your time reading it here. I spent years collecting investors and putting up my whole life and it turned out its not enough. However, I found a solution, I must build it myself, I must build everything myself. I have good support here however I don’t have a specialist. Every time I speak to an engineer, they tell me it can’t be done. Right now my problem is trusses. Locally, each 3-ply 6x6x14 post columns retails anywhere between 400 and 500 dollars. I laminated mine for just under a $100. Steel brackets to mount said post columns into concrete with hardware retails around $125 each, I sourced a local shop to build mine for $40 each. Steel sheathing for walls and roof was sourced from social media from an out of business contractor for .30$ on the dollar. Currently trusses are outrageously priced! The few local places are pricing them anywhere between $600 and $900 for the 40-footer and between $800 and $1300 for the 50-footer. One building needs 15 trusses and another two need 8 trusses each. Prices just keep going up, so I’m forced to build the trusses myself. So, I turned to the web. I’ve been educating myself on designs and ideal styles that would suit my buildings.  Already have the concrete columns pored. Pillars are 18-inch diameter and 50-inch deep. Brackets are already installed at 8ft on center. I would like to use the saddle style truss and wedge it at the top. I have 20 inches of middle board notched out to accommodate a saddle truss. I want a 4/12 pitch with 8ft o.c. truss spacing and 2ft o.c. purlin spacing. Because I’m going 8ft o.c. truss spacing I must install the purlins upright on its edge. This works perfectly because it gives me plenty of room for insulation to be installed flush with the steel. I have no overhangs and my heel is 10″. I found a company on the web (medeek designs). They design the geometry of the trusses. I basically plug in the lumber and the software does the rest. It designs the truss and with a simple click of the mouse I can get exact dimensions of my tc, bc and the webbing. However, it does not explain what size of lumber I should use to achieve the desired clear span goal. I must go to an online retailer and look up a truss and copy their design to plug in the information. I need your help; my land is in an unincorporated county which basically allows me to do anything that I want. I just must follow simple rules with foundation and snow/wind loads. Top Chord live load is 30psf, Top Chord dead load is 7psf, Bottom Chord live load is zero and Bottom Chord dead load is 10psf. I chose 12ft height because it is just tall enough for my needs and it’s sturdy enough for the wind and snow loads. I almost built 4-ply columns, but I decided to go with three because I would obtain the same rigidity with girts spacing of 24-inches instead of 36-inches. I built a 20-ton gusset plate press, and I used the software to build a sample truss. I tested it to the best of my abilities, and it stood its ground. I watched a few videos where some people installed wooden “gusset” plates as additional support over the steel plates. Some even used glue. I know that I want to over engineer this truss to make sure it stands the time. It leaves a good story for the upcoming generations about how we built this from the ground up. I still recall hearing stories from my grandfather and father how they both built their homes. I will attach a few pictures of the drawings that I have. Both 50-foot and 40-foot trusses should be double fink as this truss is rated for 40-60ft clear span. I was going to use 2×8 for both top chords and bottom chords with 2×4 for the webbing. The 40-footer truss isn’t the problem because the truss only has one cut in the bottom chord at the 20ft mid-point. The 50-footer truss is the big issue. If we assume that 2×8 lumber is strong enough for the construction, where should the bottom chord be spliced/connected as my common sense calls for a one 20ft middle section and two 15ft outer sections. If that is ok, what about the top chord, where should the 20ft board be extended? I’m so sorry for taking so much of your time, I hope this is enough information and I hope it makes sense. Can you please help? Thank you.” 

Mike the Pole Barn Guru:

Let’s start with the disclaimer at www.medeek.com:

The truss designs produced herein are for initial design and estimating purposes only. The calculations and drawings presented do not constitute a fully engineered truss design. The truss manufacturer will calculate final loads, metal plate sizing, member sizing, webs and chord deflections based on local climatic and/or seismic conditions. Wood truss construction drawings shall be prepared by a registered and licensed engineer as per IRC 2012 Sec. R802.10.2 and designed according to the minimum requirements of ANSI/TPI 1-2007. The truss designs and calculations provided by this online tool are for educational and illustrative purposes only. Medeek Design assumes no liability or loss for any designs presented and does not guarantee fitness for use.

Moving forward, Building Codes and ANSI/TPI have had several changes since Medeek put this information out. Most jurisdictions are using 2018 or 2021 versions of Codes and ANSI/TPI 1-2016.

I have previously opined in regards to site built trusses: https://www.hansenpolebuildings.com/2018/12/site-built-roof-trusses/

I spent two decades in management or owning prefabricated metal connector plated wood truss plants. In my humble opinion – attempting to fabricate your own trusses of this magnitude is a foolhardy endeavor, for a plethora of reasons:

1) You want to build trusses only from a fully engineered design, specifying dimensions, grades and species of all wood members, as well as detailing dimensions of all connections. Besides dead and snow loads, design wind speed and exposure need to also be considered. Do NOT try to copy someone’s online design, as it is likely to prove inadequate.

2) It is unlikely you will be able to obtain lumber graded higher than #2, without a special order. A 40 or 50 foot clear span truss with your specified loads is going to need some high grade lumber for chords – expect to see MSR or MEL lumber (read more here: https://www.hansenpolebuildings.com/2012/12/machine-graded-lumber/).

3) You will be unable to purchase steel connector plates of sufficient size and thickness to connect members. This leaves you with having to invest in Struct 1 rated plywood to cut into gussets.

4) Should you have a failure from building your own trusses without an engineered design, your insurance company can easily get themselves out of having to pay your claim.

Per your statement, “I know that I want to over engineer this truss to make sure it stands the time.”

Do yourself a favor and find a way to invest in prefabricated trusses. It will give you peace-of-mind you will not get otherwise.

Wind Exposure and Confusion

If you are a registered design professional, or a building official, then you are trying to make sense out of this subject on a daily basis. Most people who are selling buildings (either constructed or kit packages), tend to ignore wind exposure, or pretend it somehow doesn’t exist.
What adds into confusion, for all involved, is (even though written by same group of experts) IBC and IRC definitions do not exactly align!

Choosing a proper wind exposure is crucial to your building’s proper structural performance. Exposure C buildings must withstand a roughly 20% greater wind force than Exposure B and Exposure D, yet another 20%! This can result in one or more of deeper and/or wider column embedments, more concrete required to prevent uplift, larger columns and/or more closely
spaced, larger dimension and/or higher grades for wall girts and roof purlins, changes in truss design (larger and/or higher graded chord lumber, more webs, larger steel connector plates), ‘beefier’ connections, etc. In a nut-shell, it can change nearly every structural member and connection. To ignore proper wind exposure can result in catastrophic failures.
For utter confusion’s sake, I’ll list 2021 code sections (just in case you need some “put me to sleep” late night reading material.)
(HINT: At end, I include a broad generalization providing a close idea for most building sites.)
Picture entering a code book resembling a surrealistic painting by Salvadore Dali.
IBC Section 1609.4‐‐Exposure Category: “For each wind direction considered, an exposure category that adequately reflects the characteristics of ground surface irregularities shall be determined for the site at which the building or structure is to be constructed. Account shall be taken of variations in ground surface roughness that arise from natural topography and vegetation as well as from constructed features.”
IRC Section R301.2.1.4 Exposure Category: “For each wind direction considered, an exposure category that adequately reflects the characteristics of ground surface irregularities shall be determined for the site at which the building or structure is to be constructed. For a site located in the transition zone between categories, the category resulting in the largest wind forces shall apply. Account shall be taken of variations in ground surface roughness that arise from natural topography and vegetation as well as from constructed features. For a site where multiple detached one- and two-family dwellings, townhouses or other structures are to be constructed as part of a subdivision or master-planned community, or are otherwise designated
as a developed area by the authority having jurisdiction, the exposure category for an individual structure shall be based on the site conditions that will exist at the time when all adjacent structures on the site have been constructed, provided that their construction is expected to begin within year of the start of construction for the structure for which the exposure category is determined. For an given wind direction, the exposure in which a specific building or other structure is sited shall be assessed as being one of the following categories:”

Exposure determination is not relegated to a nice, comfortable chart or table. This section’s main part explains ground roughness variations from natural topography and vegetation need to be take into account when determining Exposure Category.
IBC Section 1609.4.1‐‐Wind Directions and Sectors “For each selected wind direction at which the wind loads are to be evaluated, the exposure of the building or structure shall be determined for the two upwind sectors extending 45 degrees either side of the selected wind direction. The exposures in these two sectors shall be determined in accordance with Sections 1609.4.2 and 1609.4.3 and the exposure resulting in the highest wind loads shall be used to represent wind loads from that direction.”
Here we get started in determining Exposure Category, but this process is three‐step from here.
Breaking this babble down to something making sense isn’t easy, but a list helps:
1) Select wind direction for wind loads to be evaluated
2) Two upwind sectors extending 45° from either chosen wind direction side are markers.
3) Use IBC Section 1609.4.2 and Section 1609.4.3 to determine exposure in those sectors
4) Exposure with highest wind loads is chosen for this wind direction
Got all this? If not, you aren’t the only one. But wait, there’s more! Tune in next time for yet
another fascinating installment!

Spray Foam, Second Floor, and Bending Posts

This week the Pole Barn Guru discusses a reader’s concern about “condensation leaks” when using spray foam, advice on costs of attic trusses vs traditional second floor, and how to stop posts from bending when the wind blows.

DEAR POLE BARN GURU: I am building a 40×50 post frame building as my garage. It will have concrete floors and HVAC. I intend on insulating the entire building (walls & ceiling) with closed cell spray foam. I’ve read a lot about people having condensation leaks, so my question is: Should I wrap the walls (and/or ceiling) with Tyvek or spray foam directly to the metal? Second question is should I use plywood on the roof for better structure and to have something to spray foam to? Any advice is appreciated. Thanks. CHRIS in BLOOMFIELD

DEAR CHRIS: Closed cell spray foam is a great product, however it would not be my first choice for your climate zone. For best results, closed cell spray foam should be two inches or thicker to prevent condensation, and applied directly to the roof and wall steel. Hopefully you have a well-sealed vapor barrier beneath your building’s slab. With closed cell spray foam, you may experience condensation on your building’s interior, so do not be surprised should you have to mechanically dehumidify. Unless specified as necessary on your engineer sealed building plans, you should not add plywood or OSB to your roof system, as it will add unexpected dead loads to your building system.


DEAR POLE BARN GURU: Hello. We are wanting to build a 2 story pole barn winery. First floor winery and second would be air-bnb type rental. We aren’t sure if we should use attic truss or complete the build with a traditional second floor. Cost is probably biggest concern. Space second as we know attic truss would be less room. Would you do an attic truss or traditional 2nd floor type build and roughly cost difference between the 2? Building size will be roughly 40×60. Thank you for your time. CRAIG in ROCK CREEK

DEAR CRAIG: If cost is your biggest concern, then having rental on ground floor will be least expensive, easiest to climate control, more accessible for your guests and easiest to fire separate from your winery.

If your only option is to have rental above winery, going with a second floor type build is going to give you far less costly investment per square footage of rentable space.


DEAR POLE BARN GURU: I built a pole barn for my r/v a couple of years ago. I used 4×4 for my posts with a metal roof and purlins with no siding. The posts are set 3′ into the soil with no concrete. Posts are 10′ out of the ground. When we get a strong wind the posts bend slightly at ground level allowing the structure to flex. Is there a way to add strength to the posts or do I need to replace with a larger size post and should I embed the post in concrete or will it rot? MARK in BRADENTON

DEAR MARK: I am frankly amazed your building is standing! This response is not to be taken as a replacement for an actual engineered structural design and should be verified by an engineer prior to moving forward. You should replace 4×4’s with at least 6×6 (it may require 8×8’s depending upon design wind speed and exposure at your site) #2 Southern Pine columns pressure preservative treated to UC-4B (there will be a treating tag on one end of each). Columns should be at least 40″ in ground and backfilled with pre-mix concrete.

Boral Steel Stone Coated Roofing

Boral Steel® Stone Coated Roofing

Boral Steel® Stone Coated Roofing is manufactured from Galvalume® steel, then coated with stone granules applied with acrylic polymer adhesives. Result is a lightweight (1.5 psf – pounds per square foot) durable and cost-effective roofing system offering superior strength of steel and is ideal for new post frame barndominium or shouse (shop/house) construction. Boral Steel® is also 100% end of usable life recyclable..

Boral Steel® product’s natural aesthetic is ideal for pairing with barndominiums of all architectural styles, from Mediterranean to Contemporary and Transitional. This material is offered in numerous profiles and colors, providing a wide array and variety to satisfy even the most discerning of barndominium owners. Popular options mimic traditional shake, slate, tile, and shingle roofing.

When selecting any roof, it’s of paramount importance to consider regional climate and identify weather conditions most likely to occur where you are installing your roof. Storms, fire, hail, snow, and wind are all significant challenges for any roofing material. Good news is Boral Steel® roofing product installed with proper underlayment and attachment process helps provide safety, comfort, and protection from most severe climate conditions.

Stone coated steel roofing is one of the best possible materials for withstanding devastating hailstorms impacting many of our country’s regions, providing a highest possible UL-listed, Class 4 UL impact rating.

Stone coated steel roofing panels are proven to resist wind speeds in excess of 120 miles per hour, making this roofing solution ideal in regions where high winds occur, such as Florida, Hawaii and Caribbean coastal markets.

With its steel composition, Boral Steel® product can notably carry more weight attributed to snow loads than other roofing options. Additionally, due to stone coating, snow will not slide off a Boral Steel® roof as is common with alternative standing-seam metal roofs. Because snow melts slowly on a stone coated steel roof into your roof’s gutter system, it creates peace of mind with a safer roof perimeter.

Boral Steel® roofing product helps offer protection in severely cold climates where ice damming is problematic. These roof systems provide an “above sheathing ventilated” (ASV) space across the entire roof deck. This horizontal and vertical air space provides above-deck air flow and insulation helping mitigate ice damming and icicle formation issues common in heavy snow areas.

As shown by testing results to ASTM-E108, stone coated steel roofing also helps protect structures from fire spread. Boral Steel® product offers a Class-A fire rating when used with specific underlayment materials, providing extra protection from wind-driven embers, common during urban firestorms.

Total cost to install a product is the first factor many barndominium owners consider, so let’s see if stone coated steel roofing is in your budget. Note one square = 100 square feet.

  • Material Costs: $400-$550 per square for shingles, shakes and tiles and underlayment, fasteners, ridge cap, trim and other accessories required
  • Installation Costs: $450-$900 per square depending on factors affecting cost, as listed below

Total Installed Cost: $850-$1,450 per square.

While this seems like a broad price range, the range for asphalt shingles can be even broader. Best asphalt shingles can cost 3-4 times the cheapest option.

Did you know? Many online cost estimators fail to consider trim cost, underlayment, fasteners and other accessories, permitting, disposal and removal fees, etc., so their cost estimates can be inaccurately low.

Itemized Materials Cost
There’s a lot more to a roofing system than stone coated steel panels. Here’s a list of materials with their average costs.

Materials priced per square:

  • Roof decking: $100-$135
  • Sound-proofing underlayment designed for metal roofs: $75-$125
  • Stone-coated steel roofing panels (shingles, shakes, tiles): $175-$375
  • Battens (used on some roofs to create a grid to nail roofing material to): $45-$75
  • Fasteners: $3.00-$4.50

Materials priced per linear foot:

  • Moisture barrier (Ice-and-Water Shield) for valleys/eaves/rakes: $1.50-$2.75
  • Drip edge: Up to $1.25
  • Ridge vent, typically installed on both sides of ridge with center open: $3.35-$5.50
  • Stone coated steel ridge, hip, and rake cap to match shingles/shakes: $6.00-$9.00
  • Flashing and fascia: $2.00-$4.50

Factors Affecting Cost

No two stone-coated steel roofing projects are the same. Each has materials and installation variables to consider.

Material cost factors:

  • Style: Specific type of panels include shingles (lowest cost) to shakes (moderate cost) to barrel-style Spanish/Roman/Florida tiles (highest cost)
  • Panel thickness: Steel gauges range from 28-gauge (thinner) to 24-gauge (thicker), and thicker materials cost more
  • Type of coatings: Stone coated steel roofing is coated on both sides with various materials to improve resistance to corrosion and loss of stones.
  • Batten vs. Batten-less (direct-to-deck): Battens are installed in most applications. Installation without battens, or direct-to-deck installation, is possible, too.

Did you know? In high-wind areas such as High Velocity Hurricane (HVHZ), batten-less installation with stone coated steel roofing nailed directly to the roof deck is recommended because it holds material more securely. This factor affects installation costs too.

Why Is Engineering Design So Important?

Why is Engineering Design so Important?

Reprinted from the National Frame Building Association (www.NFBA.org) of November 2021

As we see in Chapter 1 of the Post Frame Building Design Manual (PFBDM), post frame construction has been around for hundreds of years. The performance, life expectancy, and reduction of material and labor costs are all reasons that this type of construction is becoming more popular today. We see not only construction in agricultural settings, but residential construction is rapidly growing in today’s price and time conscious market. However, structural design is critical to ensure long life and adequate performance of the building.

There is little question that quite a number of post frame buildings have been around for many years without the benefit of structural design prior to construction. There is also little question that building failures are due to inadequate construction and overloading (both snow and wind) are becoming more common. We often hear about “post frame” construction that has failed and upon inspection we find that the original construction was inadequate to meet the expected loads.

1. Roof diaphragms not adequately connected to roof trusses and purlins.

2. Roof trusses and headers modified for particular end uses, such as tall equipment, without the benefit of engineering design.

3. Posts “embedded” into the soil only 12 to 18 inches are common pictures provided from building failure investigations.

We are not saying that the way contractors have been building post frame construction for many years is wrong. However, due to increased loading (from changes in weather patterns) and material changes such as a decrease in strength of wood products due to accelerated growing or the use of screws and nail guns; the design of buildings today is far more complex than the original over designed buildings that were constructed years ago.

Many times builders and owners are after the fastest and least expensive construction they can find. Post frame construction, with wider spacing of posts and trusses, is often the solution they find. These goals can be realized through post frame construction, but construction of an adequate structure does come at a cost. Engineering design is the key to making sure that each element of post frame construction works to transfer the loads safely to the foundation of the structure. Everything from the thickness and strength of the roof deck through the connections to the trusses and in turn through the connections of the trusses to the posts or headers are keys to making the building work. Engineers, familiar with the design requirements of post frame construction through the PFBDM and other sources, are able to ensure that the expected loads will not overburden the structure.

One other area of construction that does require significant attention is the foundation of the post frame building. In many cases, posts must be buried into the soil to a depth below the frost line. This ensures that the building will not heave during the changes from fall to winter to spring each year. Too often we find that posts are inadequately buried in the soil and/or that no uplift restraint is included to prevent the building from failure at the foundation level. There are ways to make the foundation work properly. These methods are well documented in the post frame practices used by the design engineers. Understanding the foundation requirements and how to implement them in post frame construction is a key task for the post frame design engineer.

Finally, as post frame construction moves into the residential market, the requirements placed upon construction by building code become more apparent. Proof, at the plan check stage, is becoming more of a requirement for residential construction. On several occasions, the question has come up whether we should develop “prescriptive” construction requirements for residential buildings. Unfortunately, there are far too many variables from building height, to loading patterns (both snow and wind), and to the owner’s requirement that he gets “more than just a rectangular box”. Again, these unique requirements call for a design professional to mathematically prove that the structure and the materials used will be adequate for long-term performance.

The phrase “pay me now or pay me later” too often comes into play when the structure is not designed to meet the potential loads. To avoid this, the building code requires structural design calculations to be included with the submission for a building permit. The way “post frame construction has always been done” may be adequate to meet the loading requirements, but in today’s cost-cutting world one must be sure that we are not asking too much from materials or construction that are included in a design.

A Wood Purlin Design Question

Chances are good if you have to ask a structural design question, then you are in over your head.

Reader LARRY in DITTMER writes:

“Can you 2 by 4 flat on an 8 foot span Truss”

A few years ago, one of my neighbors bought a pole building kit from someone other than Hansen Pole Buildings. It was for a garage and sidewall columns and single roof trusses were placed every eight feet. Now I am relatively certain this building’s roof purlins were supposed to be 2×8 on edge between trusses – however for some obscure reason, they got installed flat wise! I am unsure as to how they were even able to get roofing installed without falling through.

building-plansThis is just one of many reasons why post frame buildings should be designed by a Registered Professional Engineer.

When it comes to designing whether a roof purlin can achieve a given span, it takes a lot of calculations – both for live or snow loads, as well as wind loads. In high wind areas, wind will fail purlins (or their connections) rather than snow! I have condensed calculations down to just bending and deflection and will use minimum snow loads in this example:

ROOF PURLIN DESIGN – Main Building (Balanced snow load)


Roof slope = 4:12 (18.435° roof angle)
Trusses spaced 8-ft. o.c.
Purlin span = 8-ft.
Purlin spacing = 24 in.
Purlin size 2″ x 4″ #2 Southern Pine
Roof steel dead load = 0.63 psf steel American Building Components catalogue
Roof lumber dead load = 0.587 psf
Total purlin dead load = 1.217 psf


Check for gravity loads

Bending Stresses

Fb: allowable bending pressure
Fb‘ = Fb * CD * CM * Ct * CL * CF * Cfu * Ci * Cr
CD: load duration factor
CD = 1.15 NDS 2.3.2
CM: wet service factor
CM = 1 because purlins are protected from moisture by roof
Ct: temperature factor
Ct = 1 NDS 2.3.3
CL: beam stability factor
CL = 1 NDS 4.4.1
CF: size factor
CF = 1 NDS Supplement table 4B
Cfu: flat use factor
Cfu = 1.1 NDS Supplement table 4B
Ci: incising factor
Ci = 1 NDS 4.3.8
Cr: repetitive member factor
Cr = 1.15 NDS 4.3.9
Fb = 1100 psi NDS Supplement Table 4B
Fb‘ = 1100 psi * 1.15 * 1 * 1 * 1 * 1 * 1.1 * 1 * 1.15
Fb‘ = 1600 psi

fb: bending stress from snow/dead loads
fb = (purlin_dead_load + S) * spacing / 12 * cos(θ) / 12 * (sf * 12 – 3)2 / 8 * 6 / b / d2 * cos(θ)
S = 21.217 psf using the appropriate load calculated above
fb = 21.217 psf * 24″ / 12 in./ft. * cos(18.435) / 12 in./ft. * (8′ * 12 in./ft.)2 / 8 * 6 / 3.5″ / 1.5″2 * cos(18.435)
fb = 2961.59 psi > 1600 psi; stressed to 185.1%



Δallow: allowable deflection
Δallow = l / 180 IBC table 1604.3
l = 96″
Δallow = 96″ / 180
Δallow = 0.533″
Δmax: maximum deflection
Δmax = S * spacing * cos(θ * π / 180) * (sf * 12)4 / 185 / E / I from http://www.awc.org/pdf/DA6-BeamFormulas.pdf p.18
E: Modulus of Elasticity
E = 1400000 psi NDS Supplement
I: moment of inertia
I = b * d3 / 12
I = 3.5″ * 1.5″3 / 12
I = 0.984375 in.4
Δmax = 21.217 psf / 144 psi/psf * 24″ * cos(18.435° * 3.14159 / 180) * (8′ * 12 in./ft.)4 / 185 / 1400000 psi / 0.984375 in.4
Δmax = 1.118″ > 0.533″; 209.68% overstressed in deflection

These calculations are based upon purlins every 24 inches on center. If you were to reduce spacing to say 11 inches on center then flatwise 2×4 #2 Southern Pine with a 20 psf roof snow load would be adequate.

If you were able to somehow acquire 2850f Machine Stress Rated 2×4 with a E value of 2300000 psi (very high grade material used by some truss manufacturers) spacing could be 18 inches on center.

Again – remember these equations are just for checking for bending due to a minimal snow load, wind conditions may dictate. Please consult with a Registered Professional Engineer for actual designs.

North to Alaska

While Alaska is America’s last great frontier, it doesn’t mean when we go North, we throw proper structural design out of a window.

Reader CRAIG in WILLOW has more challenges going on than he has dreamed. He writes:


I’m building a 42Wx50D pole barn. I have 6×6 columns spaced 10’ apart on more than adequate footings. Slab on grade 5-6inches thick (poor final grading ) with 6” mesh and pens tubing. Willow has a snow load of 90:10:10. With a 4:12 pitch, truss companies up here are recommending a set of two two-ply trusses for a total of 24 trusses. 2’ overhang.
My problem is figuring out how to support the load between the trusses. They won’t give me a recommendation. I was planning on using 2×6 between top chords spaced every 2’. These would be oriented vertically and installed with joist hangers. I don’t think they’d be strong enough. The top chords on the trusses are called out at 2×6 so it’d be difficult to hang a larger member on them.

If I can’t make this plan work should I frame in between the columns and build a stick frame wall to set normal trusses on every 2 feet? What about laying some size beam across the tops of the columns and then setting trusses at 2’ centers? I’m dead in the water and want if anything to have overbuilt. Can you help? Thanks.”

Here is my response:

You have a plethora of challenges going on. This is why I always, always, always (did I mention always?) tell clients to ONLY build post frame (pole barn) buildings from engineer sealed plans produced specifically for their building at their site. It is not too late to get one involved and it will be money well spent.

Challenge #1 It is highly doubtful 6×6 columns you have placed along your building sidewall are going to be adequate to carry combined wind and snow loads. An engineer can design a repair – probably involving adding 2x lumber to one or both columns sides.

Challenge #2 Your wall girts placed on column faces “barn style” will not meet Code requirements – they will probably fail in bending and absolutely will not be adequate for deflection. https://www.hansenpolebuildings.com/2012/03/girts/
Again – an engineer can design a repair and there are several choices. You could remove them and turn them flat like book shelves between columns – you would need to add material for blocking at girt ends. https://www.hansenpolebuildings.com/2018/09/making-framing-work-with-bookshelf-girts/ Or, more girts could be added to your wall. Or, a strongback (2×4 or 2×6) could be added to your barnstyle girts to form an “L” or a “T”. My personal preference would be a bookshelf design, as it creates an insulation cavity.

Now – on to your trusses and roof purlins.

Your snow load is actually 90 psf (pounds per square foot). 10 and 10 are dead loads – you may not need ones these large. If you are using light gauge steel roofing over purlins top chord dead load can be as low as 3.3. Steel over sheathing 5. Shingled roof 7. If using steel roofing, make sure it is capable of supporting this snow load over a two foot span. If using sheathing, 7/16″ OSB or 15/32″ CDX plywood will not span two feet with a 90 psf snow load. Second 10 is bottom chord dead load. It is adequate to support the weight of ceiling joists, two layers of 5/8″ Type X drywall and blown in insulation. For a single layer of sheetrock and minimal lighting five psf is probably adequate. No ceiling – 1 psf. Important – make sure truss people are using 1.00 for DOL (Duration of Load) for snow. With your snow load, chances are snow is going to sit upon your building’s roof for a significant time period. Again, an engineer can determine what loading is adequate for your situation.

Trusses – how about placing three of them every ten feet? They can be notched into your columns from one side so you have full bearing – when two trusses are placed each side of a column, they are not acting together to load share.

Your roof purlin dimension can be larger than truss top chords – just utilize larger purlin hangers and balance of purlin can hang below top chord of truss. An engineer can confirm adequacy of hanger nails to support imposed snow and wind loads. Given your load conditions, your engineer should be looking to use something like 2×8 #2 purlins every 12 inches or 2×10 #2 purlins every 19.2 inches. You would not want to go to 2×10 unless truss top chords are at least 2×8.

You could stick frame between columns to support trusses every two feet. Any stud walls over 10′ tall do need to be designed by a Registered Design Professional (architect or engineer) as they would be outside of Building Code parameters. Your slab edges would also need to be thickened in order to support added weight. A beam could be placed from column to column to support trusses, you are probably looking at something around a 3-1/2″ x 14″ 2800f LVL.

If you are considering insulating an attic space, be sure to order raised heel trusses. They are usually no more expensive and they afford full insulation depth from wall-to-wall. https://www.hansenpolebuildings.com/2012/07/raised-heel-trusses/

With all of this said – go hire yourself a competent Registered Professional Engineer today to resolve your challenges. Otherwise you are placing yourself and your building contents at peril.

I’d Rather Order My New Pole Building Myself

We humans want to do things ourselves. We love GPS because it keeps us from having to ask strangers for help or admitting we are lost.

I admit to, at one time in my life, being an extremist at “doing it myself”.

Then I learned….. by listening to experts I could learn so much faster.

Consider me – I’ve either personally made more mistakes or been a party to helping people fix theirs, than most can even begin to imagine.

Why should you repeat these sins?

Answer: You do not have to. Here is a case in point real life story thanks to reader ARNOLD:

“It would be really neat if when filling out information your page a potential customer could get the information without having to give name, email, and what all else.  Kind of a pain in the rear if you know what I mean.


Mike the Pole Barn Guru writes:

Thank you very much for your input. Certainly we could have our system set up so you could go online and actually even order a building, without ever having to talk to anyone. Think of it similar to be able to custom produce a massive set of somewhat Lego® like pieces online and have them delivered. We could do it……

And chances are you would end up regretting your decision forever.

Our system would allow you to make changes in climactic design. This could result in you not having a building meeting Building Code loadings. Worst case scenarios being you would either not be allowed to build, or (in jurisdictions with no plan reviews and field inspections) your building could fail and injure or kill someone. Decrease snow and/or wind loads or chose B for wind exposure instead of C could result in both savings as well as collapses. Your building department would also reject your plans…or even worse, your building, once you had constructed it. Planning on “doing it yourself” and not ever contacting your building department? In one word: Don’t!!! I’ve seen far too many customers snagged on their buildings after they were built. Worse case, the building department made them tear it down.

About Hansen BuildingsFace it, we humans are dimensionally challenged. Even though we have an idea a basketball hoop will be at 10 feet, we think our car needs a door this height. We want to make certain you design a building with adequate spaces for your activities. This includes properly sized doors, properly spaced, to actually allow prized possessions in or out without damage to your building or something treasured.

Our having you interact with a real live person has a goal of keeping you (as much as possible) from making crucial design errors causing you to hate your pole building forever. One of those mistakes would be us allowing you, as a serious future building owner, to order a post frame building from someone else. We firmly believe we have the absolute best value in a complete, engineered post frame building kit package – enough so we offer to go comparative shop for any client prepared to invest in a building. Call 866-200-9657 and ask us about this service. It’s free!

Wall Girts Are Not Sexy

Wall Girts Are Not Sexy

Thought I had forgotten about Features and Benefits? Guess again!

My 1990’s salesman Jerry was proud of his ability to rattle off a litany of features, without explaining to clients benefits of any of them. This one feature I can imagine meant little or nothing to clients, as wall girts are not sexy!

FEATURE: Bookshelf style 2×6 #2 and better, kiln dried wall girts

Interior StairwellBENEFIT: Set flat like shelves, girts oriented this direction are strong enough to withstand wind loads, stiff enough to meet Code deflection requirements and keep finishes such as gypsum wallboard (sheetrock) from cracking. Spaced 24 inches on center, they create a deep wall cavity for insulation.


On deflection limitations: https://www.hansenpolebuildings.com/2012/03/girts/

For insulation: https://www.hansenpolebuildings.com/2018/09/making-framing-work-with-bookshelf-girts/

WHAT OTHERS DO: Most often, “barn” style girts placed flat on outside of wall columns.

pole spacingConcept of girts being nailed to column exteriors is they (in theory) do not have to be trimmed therefore saving labor (as well as any need to measure). Being ignored in this is lumber typically comes approximately 5/8 inch over specified lengths, making trimming needed anyhow. Most common column spacings are multiples of exactly two feet – eight, 10, 12, etc., and those 5/8 inches add up across a very long or wide post frame building.

Usually girts as specified upon plans (or plans themselves) have never been checked by an engineer and they sail right through most plan checks, in part because they are done “how we’ve always been doing them”.

In some cases every other girt will have another member attached to form a “T” or an “L” – however intermediate member (one between T or L girts) still fails to meet Code deflection limitations and often proves insufficient to resist wind loads.

Generally, little or no consideration has been given to additional forces upon girts when buildings are partial enclosed or three sided: https://www.hansenpolebuildings.com/2014/03/three-sided-building/

WHAT WE DID IN 1980: Remember, Lucas Plywood & Lumber was in a region where low grade green lumber was king! We used green 2×6 #3, barn style, spaced upon two foot centers. Ignorance was bliss and we were happy, as this solution was cheap, however not structurally adequate.

If you are not well versed regarding issues surrounding green lumber, you will want to read this information: https://www.hansenpolebuildings.com/2011/09/499green-lumber-vs-dry-lumber/


Both Ends Open, Pole Barn Wind Load Challenge

The Both Ends Open, Pole Barn Wind Load Challenge
There are plenty of people who just do not understand the basic concepts of how wind loads are transferred through a pole barn (post frame building) to the ground. Included amongst these would be those who desire buildings which are enclosed on both long sidewalls and open on both ends. This is one of the worst possible design concepts one can come up with in a new post frame building.

Of course somewhere along the discussion between the Building Designer and the client this statement always seems to come up:
“Well Joe Blow has one down the road and his is still standing”.
My response to this is – “Joe has just been phenomenally lucky”.

In my years living in Eastern Washington, we made numerous trips from Spokane to Seattle. Driving across Interstate 90, one passes through the towns of Moses Lake and Ellensburg. This is prime grass growing country, where numerous hay storage buildings have been constructed over the years, with both ends open. The majority of these now have complex systems of braces and/or extra diagonal columns added to their sidewalls in attempts to maintain them standing vertical. More than a few of them only remain standing up because they are full of hay – the contents alone are what is keeping the buildings standing.

I’ve hashed through this challenge in the past, however it is apparent too few people have read and grasped the situation (read more here: https://www.hansenpolebuildings.com/2017/04/open-endwalls-hay-barn/).

For those of you who enjoy audience participation, please go find an empty shoe box and a pair of scissors.
Remove the lid (and the shoes) from the shoe box. Place it open side down on a table top. Push down on the box – pretty stable, isn’t it?
Next, cut both of the narrow ends completely out of the box. Again place it open side down on the table and push on it…..
Flat as a pancake, isn’t it?

The very same concepts work to keep buildings standing. Remove too much or all of the ends and the building does a fall down, goes boom.

Just because Joe happens to have a building standing which sound engineering practice says it should not be, does not make it right. Most folks are going to make a significant financial investment into a new post frame building and my personal preference is for them to not have their insurance company paying to replace the building.

Builders Who Make No Upgrades in Twenty-Five Years

Builders Who Make No Upgrades in Twenty-Five Years.


We’ve Been Building This Way for 25 Years
In the event you happen to hear this from a pole builder – run away from them as quickly as possible.
Because every three years there is a new version of the Building Codes and often those new Codes come with changes in the way wind, snow and or seismic loads are applied to the building. New methods and materials seem to appear on the market so fast they make one’s head spin. Technology moves at a breakneck speed and to be doing things exactly the same for 25 years means your proposed erector is pre-internet in thinking!!

“Good Morning,
I was speaking to Rachel and she gave me your email to see if you might be able to answer a question for me. I hired complete a 50’x 80’ x 12’ pole barn here in Huntley, MT. The company showed up on the job yesterday and drilled the holes and started setting posts. Posts are 8’ center. They set the corner posts and maybe 6 sidewall posts and 4 endwall posts. The other posts were placed in the drilled holes and left for completion today/tomorrow. When I inspected the posts that were placed but not set (no backfill) I noticed that there was no footing or no cleats attached to post base to prevent uplift. When I questioned the owner of the company what he was using for footings he stated nothing added just solid tamped. I immediately called him and questioned his reasoning and got the I have been building these like this for 25 years. My question is on average what is the post load in psi on the 50’ x 80’ x 12’ pole barn with a 40# snow load? My soil has a bearing capacity of 2100 psi.”
In my humble opinion, you need to stop them immediately. Just because they have been doing them this way for 25 years does not make it correct.

Mike the Pole Barn Guru Writes:

Assuming a 40# design roof snow load and minimal design dead loads (usually 3.3 psf top chord and 1 psf bottom chord) gives a total of 44.3 psf X 8′ on center X 50’/2 = 8860# downward If they are using 6×6 posts (5-1/2″ nominal) they are placing over 42,000 psf on the base of the column!!

Roughly 21 times the soil bearing capacity.

Each post should probably have a concrete pad 30 inches or so in diameter underneath and at least 6 (if not 8) inches thick.

If I were you, I’d be requiring the building contractor to submit engineer sealed plans for your building to you (even if you have to pay for the cost). Otherwise you are pretty well hung out to dry.

Dear Pole Barn Guru: How to Replace a Sliding Door with an Overhead

New!  The Pole Barn Guru’s mailbox is overflowing with questions.  Due to high demand, he is answering questions on Saturdays as well as Mondays.

Welcome to Ask the Pole Barn Guru – where you can ask questions about building topics, with answers posted on Mondays.  With many questions to answer, please be patient to watch for yours to come up on a future Monday or Saturday segment.  If you want a quick answer, please be sure to answer with a “reply-able” email address.

Email all questions to: PoleBarnGuru@HansenPoleBuildings.com


DEAR POLE BARN GURU: Have pole barn with sliding doors which are being wedged with weather changes. Looking for overhead door option for door that is 16′ wide and 12′ tall. Do you provide these and conversion labor to install? LOOKING IN LEBANON

DEAR LOOKING: Switching from sliding doors to an overhead door is going to pose a massive challenge to do correctly. This, in itself, is reason enough to spend the generally few dollars up front to use a sectional steel overhead door.

To begin with, the openings are not framed to the same size. It is easier to frame smaller than have to try to hack out and replace one or more columns. This will probably entail framing down to a finished hole 13’10” in width and 10’11” in height (measured from the top of the concrete floor) and installing a 14’ x 11’ residential overhead door. In order to get things looking right from the outside. All of the steel on this wall should be replaced, to give uniform color and no splices.

We can certainly provide a wall’s worth of steel siding, color matched powder coated screws, the appropriate steel trims, the overhead door and hardware to hang it. We are not contractors in any state, so we do not and cannot provide any labor to install.

You may want to look at what the real problem is – sounds like you have frost heaving, which is pushing the ground, or concrete, up at the location of the door. Just switching doors is not going to take away the problem.

If heave is the root cause of the problem, then remedial action can be taken by installing a French drain along the side of the building in front of the door. The sliding doors can also be taken off, and their overall height shortened enough to keep them from binding when the heave occurs.

DEAR POLE BARN GURU: How do I calculate what size of purlin I need based on my snow load, and the bay spacing of my pole barn? Thanks. CURIOUS IN CULDESAC


DEAR CURIOUS: From the ground, a roof purlin looks pretty simple – it is usually a piece of 2x material, fastened on top of or attached to the side of rafters or roof trusses. Roof sheathing (typically OSB – oriented strand board, plywood, or steel roofing) is then attached to the top of the purlins.

Purlins are not simple at all. They must carry all applied dead loads, live loads from snow as well as wind loads. They need to be checked for the ability to withstand bending forces (both compressive and from uplift), to not have too much deflection and be adequately attached at each end.

In snow country, purlins near the roof peak need to be checked for the added drift loads which are applied.

I could spend several thousand words and numerous pages to teach you how to be able to properly calculate the purlins for your individual case, however it is far more information than the average person wants to, or is able to, absorb.

The best recommendation – hire a registered design professional (RDP – architect or engineer) who has the ability to run the calculations to adequate design your purlins based upon the climactic (wind and snow) loads being imposed upon them at your building site. Or better yet, order a complete pole building kit package which has been designed by an RDP.

Building Code: Or Not?!

Things Which Scare the Pooh Out of Me

And we are not talking about things which go bump in the night or hide in closets waiting to jump out.

Hansen Buildings’ Designer Rick recently ran up against an interesting situation.

One of the responsibilities of clients is to verify the code information with their Building Department prior to ordering. As there are, at times, only questions which can be answered by the client, we have found it to be the best solution for all involved if the client checks out building code requirements with his local jurisdiction.

Building Code Snow LoadsThis particular client lives in the far northern United States, where it tends to snow…a lot.

Client does his part and gets this response:

“We don’t enforce the building code in Xxxxxx County, so you will have to have whoever designs your building refer to the xx State Building Code. We have a link for that on our County Website it is as follows: xxxx”

So Rick gives it a try and comes back to me with:

“You are sure right on this.  I just got off the phone with the county.  He actually used the words “we don’t care if it is built of straw” as long as your setbacks conform. 

This is my first encounter with a county that doesn’t even have a snow load, and I didn’t think there were any.

The link in the e-mail below gives me an error code and the link on the county web site for the XX State Building Code gives me Chinese.”

Now this particular building is going to be constructed where there is snow in the winter….lots and lots and lots of snow.

Personally, I am not a fan of government intervention, however I am a fan of people not being hurt or killed when under designed buildings collapse.

For people who are going to build in areas where the Building Code is not enforced – do due diligence, make every effort to find or calculate loads which will be adequate for your structure.

If you need help understanding your local building code, ask me.  I’m all about safety first.







Fabric Covered Building and Wind

One of the Hansen Buildings designers recently asked me what I knew about fabric covered buildings. He was speaking with a client who was comparing one of our post frame buildings versus a fabric covered structure.

My only up close and personal experience with a fabric structure was with the United States pavilion at the 1974 World’s Fair in my home town of Spokane, Washington. The original covering of the pavilion was a thick vinyl sheeting. It was allowed to remain until it began to deteriorate, become unsightly and was thought a safety hazard.

As I started to do more research, I found article after article about the May 2, 2009 failure of a fabric covered building with steel frame practice facility owned by the National Football League’s Dallas Cowboys. This structure collapsed under wind loads significantly less than those required under applicable design standards, according to a report released October 6, 2009 by the Commerce Department’s National Institute of Standards and Technology (NIST).

Located in Irving, Texas, the facility collapsed, during a severe thunderstorm. Twelve people were injured, one seriously. Based on the national standards for determining loads and for designing structural steel buildings, NIST researchers studying the Cowboys facility found the May 2 wind load demands on the building’s framework—a series of identical, rib-like steel frames supporting a tensioned fabric covering—were greater than the capacity of the frame to resist those loads.

Assumptions and approaches used in the design of the Cowboys facility led to the differences between the values originally calculated for the wind load demand and structural frame capacity compared to those derived by the NIST researchers. For instance, the NIST researchers included internal wind pressure due to the presence of vents and multiple doors in their wind load calculations because they classified the fabric covered building as “partially enclosed” rather than “fully enclosed” as stated in the design documents.

Even more damning, the NIST researchers determined the building’s fabric could not be relied upon to provide lateral bracing (additional perpendicular support) to the frames in contrast to what was stated in the design documents and the expected wind resistance of the structure did not account for bending effects in some members of the frame.

“Our investigation found that the facility collapsed under a wind load that a building of this type would be expected to withstand,” said study leader John Gross. “As a result of our findings, NIST is recommending that fabric-covered steel frame structures be evaluated to ensure the adequate performance of the structural framing system under design wind loads.”

The NIST report recommends such evaluations determine whether or not: (1) the fabric covering provides lateral bracing for structural frames considering its potential for tearing; (2) the building should be considered partially enclosed or fully enclosed based on the openings which may be present around the building’s perimeter; and (3) the failure of one or a few frame members may propagate, leading to a partial or total collapse of the structure.

Shortly after the Cowboys facility’s collapse, NIST sent a reconnaissance team of three structural engineers to assess the failed structure and wind damage in the surrounding area, and collect relevant data such as plans, specifications and design calculations. Using the data acquired during the reconnaissance, the NIST study team developed a computer model of a typical structural frame used in the practice facility and then studied the frame’s ability to resist forces under two wind conditions: the wind loads based on the design standard wind speed of 90 miles per hour (mph) and the actual wind loads based on conditions at the time of the collapse.

NIST worked with the National Oceanic and Atmospheric Administration’s (NOAA) National Severe Storms Laboratory to estimate the wind conditions at the time of collapse. The researchers determined, at the time of collapse, the wind was blowing predominantly from west to east, perpendicular to the long side of the building. Maximum wind speed gusts at the time of collapse were estimated to be in the range of only 55 to 65 mph!

In the conversion of actual wind speeds to pounds of force applied to a building the wind speed is squared. A 65 mph wind speed creates a force of 10.816 pounds per square foot (psf), whereas the required load carrying capacity of 90 mph would be 20.736. The structure failed to carry much more than half of the wind load force it should have carried!

This evidence could lead one to be highly skeptical about the ability of a fabric covered structure to adequately support wind loads.  If one is considering such a fabric covered building, my advice would be to carefully gather evidence (backup data) to clearly substantiate the building supporting wind loads…in all circumstances.