# 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.

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 = 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

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

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

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

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

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%

# Building Your Own Pole Barn Trusses

Wants to Build His Own Pole Barn Trusses

Reader DANIEL in HAMPSHIRE writes:

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.

# Snow Load, Clear Span Scissor Trusses, and a Window Replacement

This Wednesday the Pole Barn Guru answers reader questions about whether or not a 30 year old building correct snow load, the possibility of clear spanning scissor trusses to eighty feet, and assistance with the replacement (or repair) of a window in a Hansen Building from 2014.

DEAR POLE BARN GURU: How do I figure out the snow load rating for my Morton barn since it is 30 years old and it has poles, trusses and purlins. The purlins are 2 x 8 and 20″ on center with 36 of them over 50 feet and 6 trusses nine feet tall and spanning 50 feet in length and 8 feet apart. Is the rating more than 55psf or not? MARK in PORTVILLE

DEAR MARK: Your roof purlins appear to be adequate to support this type of a snow load. As to trusses, I would reach out to Morton Buildings with your site address and they should be able to pull up truss drawings for your building. If not, you would need to retain services of a Registered Professional Engineer who could do an actual inspection of your trusses and run calculations to determine exactly their capacity.

DEAR POLE BARN GURU: Can you 80 foot clear span scissor truss on a 14-16 foot eave? Commercial shop use. Northern Indiana. ANDREW in AVILLA

DEAR ANDREW: Can and should are not often same.

Yes, an 80 foot clearspan scissor truss can be done, expect it to be either designed with a flat top and a peak “cap” or to be parallel chord with a deep heel and joined together onsite at center. It will prove to be far more economical to utilize flat bottom chord trusses with a taller eave height – this would also allow for full height interior clearance from wall-to-wall. One of our Building Designers will be reaching out to you to further discuss your building needs.

DEAR POLE BARN GURU: I built my Hansen monitor pole barn in 2014. It took this long but a nasty storm broke out my window in the upper floor of the building. What’s the best way to replace a window and frame in a metal sided building. Do I need to remove the surrounding ribbed sheet metal panels and are there any tricks to that? With an eight-year-old build should I use new screws as the gaskets might be dried out from baking on the southern facing wall? Any methods you can suggest to get me going are appreciated. Thanks DAVE in FERNLEY

DEAR DAVE: Unless your window’s vinyl frame was actually damaged, in most instances a glass company can do a repair of just broken glazed portions. I would suggest a call to Capital Glass in Reno (775)324-6688 as this appears to be in their wheelhouse and they service Fernley.

Rarely does glass repair require steel panels to be removed. In an unusual case where there is no alternative, and steel must be taken off, your siding screws have EPDM gaskets. These are UV resistant and have a manufacturer’s warranty they will outlast your steel siding.

Should you have some photos of your completed building, we would greatly appreciate your sharing them with us.

# Responsibility for Collapsed Pole Buildings

• Question Whether County is Responsible for Collapsed Pole BuildingsEllensburg’s (Washington) Daily Record published this letter from DAVE on March 5, 2022:

“To the Editor: Anybody passing through the Nelson Siding area in the Upper County some seven miles west of Cle Elum, will notice collapsed pole buildings due to the snow we had in January.

The weight of the snow (83 pounds) in that area was well below the county building codes weight limit, and yet these buildings collapsed.

The question is, what responsibility does the county building have regarding in passing this failed construction design.

I believe the insurance companies for these people that own these buildings that were built to code to satisfy the county, should expect the county to bear some if not all of the responsibility for the reconstruction cost of these buildings.

I, Dave xxxxx, private property owner over 37 years, live just one quarter of a mile east of the first two of the four pole buildings that collapsed.”

I will begin this with your local building permit issuing authority has absolutely no liability for failures due to inadequate structural design or construction. When a building permit is issued, most jurisdictions place a stamp on approved plans to advise accordingly.

Kittitas County (where these buildings are located), happens to be (in my humble opinion) on top of my list for providing to potential building owners and builders accurate site specific minimum climactic data for design. They do, however, only require a licensed design professional (Architect or Engineer) to stamp, prepare or oversee preparation of plans and calculations for pole (post frame) buildings meeting any one of these criteria:

An eave height over 16 feet
Having habitable living space
Two stories where a second floor is over 200 square feet
In areas where ground snow load (Pg) exceeds 70 psf (pounds per square foot)

To start with, Building Department loading requirements are minimums. IBC (International Building Code) 101.3 reads as follows, “The purpose of this code is to establish the minimum requirements to provide a reasonable level of safety, health and general welfare through structural strength….”

Nowhere does it say by following these minimum requirements, your building (of any sort) will not collapse in event of a catastrophic situation. Although this previous article was written in respect to wind events, snow events are obviously problematic as well: https://hansenpolebuildings.com/2018/11/500-year-storm/

*example of collapse

Back in late 1996, when I was building, we had a portion of one of our fully engineered post frame riding arenas collapse at Thorp, Washington (just West of Ellensburg and East of Cle Elum). Building was designed to minimum Code requirement with a roof snow load of 34 psf. Our client had attempted to remove some snow from roof, and had pre-collapse photos showing roughly four feet of snow on their building’s roof! Actual snow load was more than double design requirements.

Learn how to calculate snow weight here https://hansenpolebuildings.com/calculating-loads/

A smart insurance company would hire someone like me to do a forensic evaluation of these buildings as built, in comparison to engineered plans approved for construction. I am not a gambling man, however I would put forth a wager I can find one or more deviations from these approved plans on any building. This could absolve insurance providers of any liability to pay claims.

It is possible there were some engineering deficiencies contributing to these collapses, most often I would look to truss bracing, or inadequate design of purlins in drift zones (unbalanced loading).

I would encourage readers to peruse this document for further information on preventing post frame building collapses due to climactic conditions: https://hansenpolebuildings.com/2019/04/steps-to-minimize-snow-load-failures/.

# 4×4 or Double 2×4 Bay Roof Purlins?

4×4 or Double 2×4 for 12’ Bay Roof Purlins?

Reader JOHN in HUNTSVILLE writes:

“If you have trusses spaced at 12 feet, can a 4x4x12 or two 2x4x12’s span the distance given the minimal snow loads in Arkansas? I know this is question #2 but what kind of joist hangers do you use (Simpson Number or equivalent) for purlin attachment to trusses?”

We typically would use 2×6 #2 on edge for these recessed (between truss pairs) roof purlins. Here are the calculations:

Assumptions:

Roof slope = 4:12 (18.435° roof angle)
Trusses spaced 12-ft. o.c.
Purlin span = 11.75-ft.
Purlin spacing = 24 in.
Purlin size 2″ x 6″ #2
Roof steel dead load = 0.63 psf steel American Building Components catalogue
Roof lumber dead load = 62.4 pcf * 0.55 lbs/ft.3 / (1 + 0.55 lbs/ft.3 * 0.009 * 0.19) * (1 + 0.0019) * 1.5″ / 12 in./ft. * 5.5″ / 12 in./ft. * (12′ – 3″ / 12 in./ft.) / 12′ / (24″ / 12 in./ft.) psf in purlin weight based on 0.55 G NDS = 0.963 psf
Total purlin dead load = 1.593 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 = 1.25 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 (not applicable to SYP)
Cfu: flat use factor
Cfu = 1 NDS Supplement table 4A
Ci: incising factor
Ci = 1 NDS 4.3.8
Cr: repetitive member factor
Cr = 1.15 NDS 4.3.9
Fb =1000 psi NDS Supplement Table 4-A
Fb‘ = 1000 psi * 1.25 * 1 * 1 * 1 * 1 * 1 * 1 * 1.15
Fb‘ = 1437.5 psi

fb: bending stress from roof live/dead loads
fb = (purlin_dead_load + Lr) * spacing / 12 * cos(θ) / 12 * (sf * 12 – 3)2 / 8 * 6 / b / d2 * cos(θ)
Lr = 20 psf using the appropriate load calculated above
fb = 21.593 psf * 24″ / 12 in./ft. * cos(18.435) / 12 in./ft. * (12′ * 12 in./ft. – 3″)2 / 8 * 6 / 1.5″ / 5.5″2 * cos(18.435)
fb = 1060 psi ≤ 1437.5 psi; stressed to 73.7 %

Deflection

Δallow: allowable deflection
Δallow = l / 180 IBC table 1604.3
l = 141″
Δallow = 141″ / 180
Δallow = 0.783″
Δmax: maximum deflection
Δmax = 5 * Lr * spacing * cos(θ * π / 180) * (sf * 12 – 3)4 / 384 / E / I from http://www.awc.org/pdf/DA6-BeamFormulas.pdf p.4
E: Modulus of Elasticity
E = 1400000 psi NDS Supplement
I: moment of inertia
I = b * d3 / 12
I = 1.5″ * 5.5″3 / 12
I = 20.796875 in.4
Δmax = 5 * 20 psf / 144 psi/psf * 24″ * cos(18.435° * 3.14159 / 180) * (12′ * 12 in./ft. – 3″)4 / 384 / 1400000 psi / 20.796875 in.4
Δmax = 0.559″ ≤ 0.783″

2×4 #2 and 4×4 #2 Southern Pine have Fb values of 1100

Sm (Section Modulus) of a 2×6 is 7.5625; (2) 2×4 nailed together would be 1.5″ width x 3.5″ depth^2 x 2 members = 6.125 I would = 10.71875; 4×4 would be 7.146 with I = 12.5052

The (2) 2×4 would be stressed to 82.7% in bending however Δmax = 1.085″ so would fail due to being over deflection limits

How about a 4×4? 70.9% in bending Δmax = 0.9296″ so would also fail due to being over deflection limits

For our 2×6 purlins, we specify a Simpson LU26

# 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.

This 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)

Assumptions:

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 = (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%

Deflection

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.

# End Truss Overhang Dilemma

Reader ANDY in HAYDEN has an end overhang challenge. He writes:

“Hello Mr Guru. I’m building a 30x40x12 post frame with 18″ eaves. My trusses builder doesn’t build drop cord ag trusses for my gable over hangs. I was advised to lower the gable truss on the corner post to allow room for my on edge 2×8 purlinings  to extend over the top. I have a 16×10 garage door planned for below that over hang, will this method work. Can ladders be used?. I would appreciate your help sir. I know if I had the money I could have ordered one of your kits. Trust me I wish but I was born with a spork in my mouth and I’m just chipping away monthly on my project. Thank you for any help.”

Mike the Pole Barn Guru responds:

Most of our clients were not born with any sort of silver or plastic ware in their mouths – me either. While my brother and I did not realize it growing up, we were probably upper lower class in family income, but we were happy, our parents worked hard and we learned well from them. I have joked, “We were so poor our mother used to spray paint our feet black and lace up our toes”. It was not quite as bad – but Mother did go without socks for some time so we could have clothes.

Moving forward – there are advantages to investing in an engineered complete building kit package and not try to piecemeal. I have written about piecemealing before https://www.hansenpolebuildings.com/2014/03/diy-pole-building/. Ordering trusses can, as you have just found out, be far more difficult than it seems. https://www.hansenpolebuildings.com/2020/02/things-roof-truss-manufacturers-should-ask/

Financing is highly affordable, with some amazingly low interest rates and most suppliers have options available to delay some deliveries until you are more prepared for them.

Before you get carried away with an overhang, look at your engineered truss drawings. Guessing your building has a pair of trusses every 10 feet and a single truss on each end, it will need to be designed to account overhang weight plus any other dead loads and snow loads. To accomplish this, your end trusses should be designed with either one truss at five foot (plus a notation stating they can support an 18 inch end overhang), or have a spacing of 6’6″. If neither of these has occurred you need to contact your truss supplier for an engineered repair. It may be cheaper to use a double truss on one end (notching into corner and end columns, and purchase a correct new truss for the opposite end.

In any case, before there is any structural deviation from your engineered plans YOU MUST CALL YOUR ENGINEER. My suggestions are merely my opinion and are not to be construed as my supplying or practicing engineering. If you deviate from your engineered plans in any fashion, all liability for structural integrity falls directly upon you.

Measurements below are using this for a measure of eave height https://www.hansenpolebuildings.com/2015/02/eave-height-2/

Having taken care of loading issues in some fashion, you can lower your end trusses by 7-5/8″ to adjust for vertical component at a 4/12 slope (other slopes change this hold down dimension). This should put the bottom of your end trusses at 10′ 10-1/2″ for a 2×6 top chord truss (again at 4/12) or 10′ 8-3/4″ for a 2×8 top chord.

Another advantage of a complete package is it should come with a detailed step-by-step assembly manual. At Hansen Pole Buildings this means 500 pages.

Your engineer can verify if you can for 2×6 top chord truss place a 2×6 as a header between top jamb and truss, or move top jamb up 1/4″ and use a 2×4.

Ladder framing nailed or screwed to the face of end truss to create end overhangs is probably not structurally adequate and it could very well sag, if not fall off.

Best wishes.

# Hi, I Should be an Engineer

Hi, I Should Be an Engineer. Can You Tell Me What I Left Out?

Seemingly every Spring I receive an email similar to this one from JOHN in UNION DALE, who it sadly appears has not done much (if any) homework in reading my articles.

JOHN writes:

“ Hi, I have been doing a couple of months homework on making my pole barn, my plan is a 30×50. Right now my plan is using (16) 6x6x16 pole about 52 inches in the ground, the spacing between posts will be 10 ft, now I have not decided on a concrete cookie before the setting the post or gravel first has a drainage layer the set the pole and then use about 5 bags of concrete for uplift protection and the normal back fill, for the posts I got post protectors, so the wood is separated from the soil, my plan is to use double  2×12 for the top strapping with the posts notched at the top for added snow load, has far has the roof it will either be a 4/12 or 5/12 pitch my plan is using 2×6 rafters that I’m making on the ground and hoisting up by myself and they will be on 48 inch on center, my purlins are going to be 2x4s about 2ft apart and standard metal to finish it off, if you can can you please let me know if I left anything out, thanks ps I forgot to say the door opening on a non-load bearing wall will be a 12ft wide and 10ft tall, I’m thinking about putting a door  on a load bearing wall a 10ft, all doors are going to be sliding barn doors.”

Mike the Pole Barn Guru Responds:

Well John, you have left out a crucial part. One no proper pole barn should be without. Plans designed and sealed by a Registered Professional Engineer specific to your building at your site. To build without them is, in my humble opinion, fool hardy and I cannot endorse your plan of attack or methods of construction without them. Outside of this – attempting to field construct your own roof trusses is not a good choice. Prefabricated trusses are truly a bargain, especially when considering risks involved should your home made trusses collapse injuring or worse killing you or a loved one.

For last year’s related article, please read: https://www.hansenpolebuildings.com/2019/05/self-designed-pole-buildings/

For extended reading on the misadventures of site built roof trusses: https://www.hansenpolebuildings.com/2018/12/site-built-roof-trusses/

# 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:

“Hello,

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.

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.

# Roof Collapses Due to Heavy Snow are Largely Avoidable

Roof Collapses Due To Heavy Snow Are Largely Avoidable.

Portions of this article are thanks to a February 25, 2019 article by Bill Steffan at www.woodtv.com

“Above pic. is the Negaunee Schools bus garage in Marquette Co., Michigan.  The roof collapsed under the weight of heavy snow over the weekend.  There were 16 buses inside the garage when the collapse occurred.  The collapsed triggered the sprinkler system and that led to a substantial accumulation of ice.  This was one of several buildings that had a roof collapse due to heavy snow in Marquette Co.

Another collapse occurred at Shunk Furniture.  The force of the collapse blew out windows in the building.  “The first buildings to be concerned about are the pole buildings, the large-span pole buildings with truss spacings of eight foot or greater,” said Gary Niemela, Owner of Skandia Truss.”Those are usually the ones to be concerned about. Probably want to take the heavy snowload off. If the snowload is three to four feet deep on those, you’re going to want to do something,” said Niemela.”

I am going to address several issues, all of them ones leading to a better investment of a new post frame building owner’s dollars.

What, a Building permit?

In a surprising number of jurisdictions across our country, post frame (pole barn) buildings are exempted from a building permit process for one of several reasons. In some areas, there are just no actual building departments. Next step up is a “Building Permit” is issued for a minimal fee (usually in a clerk’s office) usually to get it added to property tax reevaluations.

In my humble opinion every building should have RDP (Registered Design Professional – architect or engineer) sealed plans submitted to an authority who can do minimally invasive site inspections via one of a myriad of online live chat options. Permits and payments could be obtained electronically. This type of system could even be contracted out to third-party providers on a percentage type contract with carefully worded expectations so there is not someone having hurt feelings at a later date.

But I Have to Pay for a RDP!

Yes you do and a good one will save you more money than they cost (or give you a greater value) in efficient use of materials and ease of construction. Favorite articles is on this very subject: https://www.hansenpolebuildings.com/2018/08/minimum-design-loads-and-risk/.

Do Away With Risk Category I

I can hear people screaming now about how much more they are going to have to pay to get a building designed for a once in 50 year occurrence (Risk Category II) rather than once every 25 years. For practical purposes, you cut in half risks to life and property from a catastrophic failure. In many buildings added investment will be minimal, as compared to gain in reliability.

Insurance Company Discounts

Property insurers should offer some discounts for building from RDP sealed plans, as well as a further discount for buildings designed for above Code minimum climactic loads.

# 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.

Thanks”

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.

Face 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!

# 500 Year Storm and Wind Exposure

500 Year Storm and wind exposure.

Allstate® Insurance has a TV commercial featuring actor Dennis Haysbert. Haysbert sits in an open field and questions why there have been 26 “once in 500 years storms” in last decade, when term alone implies they should only happen every 500 years.

IBC (International Building Code) in Chapter 16 (https://codes.iccsafe.org/public/document/IBC2018/chapter-16-structural-design) Table 1604.5 lists Risk Category of Buildings and Other Structures.

Risk Category I includes buildings representing a low hazard to human life in event of failure – agricultural buildings and most detached residential accessory buildings fit into this category.

Risk Category II would be most homes and many low risk commercial, industrial and manufacturing buildings.

Risk Categories III and IV cover buildings with high occupancies or are essential to fire, life and safety (like fire stations).

IBC offers Minimum Design Loads modified by a given factor depending upon Risk Category. For a previous article about this subject please see: https://www.hansenpolebuildings.com/2018/08/minimum-design-loads-and-risk/.

Reflect, if you will, back upon paragraph one above and a 500 year storm.

Risk Category I buildings are based upon a once in 25 year probability of minimum design loads being exceeded. Risk Category II once in 50 years, while Categories III and IV are once in 100 years.

So, what does one do to protect against a once in 500 years storm?

When planning your new post frame building, this becomes relatively easy – have it designed for greater loads than bare Code required minimums. While this sounds simple, very few clients consider asking for it and even fewer post frame building sales people offer it!

Why would it not be offered?

Price

People are selling buildings using price rather than value.  Most are afraid to suggest increasing building price by a few percentage points, because they think it will cost them a sale!

I know there are numerous members in our post frame industry who are reading this article. To you I offer this challenge – as an option start offering to every potential client an ability to have their building designed for an extra even five or 10 psf (pounds per square foot) of snow load (in snow country of course). And, give them an option of withstanding greater wind speeds than Code minimums. Even upgrading wind Exposure B sites to Exposure C will increase ability to resist wind loads by about 20%.

A short wind exposure story can be found here: https://www.hansenpolebuildings.com/2011/11/wind_exposure/.

Now, sell your potential client benefits of having last building standing when Mr. Haysbert’s storm rolls through.

Sold itself, didn’t it?

# Builders Who Make No Upgrades in Twenty-Five Years

Builders Who Make No Upgrades in Twenty-Five Years.

Why?

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.
Why?
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!!

COREY in BILLINGS writes:
“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.

# Panic Mode! We’ve All Been There

When Clients get into Panic Mode

Most of us have been there with a major purchase – we were all excited about it and then somewhere before it gets delivered we start to second guess ourselves.

Here is an example:

Dear Mr. Xxxxxx ~

Thank you for your investment into a new Hansen Pole Building.

You wrote:

“When I originally talked with Mike I wanted a heavy duty building, It seems no one there was listening, I have my plans and the roof trusses show 2×4 construction and side posts 3.5 x 5.5,  I am very concerned that this is a very weak design. I know I already approved the plans but I will have to spend about 4000.00 more to locally to purchase 6×6 posts and 2×6 trusses. I guess I can sell the ones I receive.  You people must think I live near Seattle, not, I live on the eastern slope of the Cascade Mountains, your 2×4 double truss design appears obviously not strong enough to take the snow loads in my area. I am very concerned.”

(FYI – the “Mike” referenced is Hansen Pole Buildings’ Designer Mike Houska)

My response:

We take our client’s concerns to heart, it is part of why every Hansen Pole Building is structurally designed by a registered professional engineer.

In review of your building plans, I see it has been designed to meet or exceed a roof snow load of 45 psf (pounds per square foot) as well as an ultimate wind speed of 100 mph (miles per hour). The calculations for each and every member and connection on your building have been thoroughly reviewed by a Registered Design Professional (the engineer who seals your plans). I would venture a guess your Building Department has approved the plans as meeting the structural requirements.

The prefabricated roof truss designs for your building utilize 2×6 1650msr (or stronger) lumber for the top and bottom chords. When you go to your local lumberyard to purchase a 2×6 graded as #2 (the standard for framing throughout the industry), it has a bending strength of from 1105 to 1170 psi (pounds per square inch), depending upon the species of lumber. The 1650 msr being used for your truss top and bottom chords is at least 41% greater in bending strength. The interior members of the trusses (the webs) are indeed 2×4, as they would be in virtually any truss design. In truss configurations, the webs carry minimal loads for both compression and tension and are loaded to only 75% of their capacity on the interior double trusses. If your trusses have not yet been fabricated, it is possible we could upgrade to 2×6 webs, however the load carrying capacity of the trusses would not be increased by this change – you would basically just be spending money to spend money.

On to the column sizes. I’ve written extensively in the past on why a 4×6 (3-1/2″ x 5-1/2″ actual) sized column will outperform a 6×6 column in most cases. If you would kindly take just a few moments to read about it here: https://www.hansenpolebuildings.com/2014/08/lumber-bending/.

Things I do know – your building, built according to the engineered plans, will support the loads given, not only is it an engineered building, but we also provide a limited lifetime warranty to back it up!

I did a few trial calculations, increasing your roof snow load to up to 180 psf (four times what you invested in). Even at this load, 4×6 columns still work! The column is not “the weak link”.

Provided your lumber package is not ready to be shipped and your trusses have yet to be fabricated, it might yet be possible to increase the design roof snow loads, and/or the design wind speed beyond what the Code requirements are. In doing so, this again checks every member to insure you have no weak links.

If you desired to increase both by 25% (Ground snow load to 75, roof snow load to 56.25 and Vult to 112 mph) you would be looking at an up charge of \$1580 and would receive new sealed plans and truss drawings to confirm these loads. An increase of 50% (Pg = 90, Pf = 67.5, Vult = 122.5) would be \$2765. Either of these would, of course, be depending upon the status of your building in the production processes.

We will await hearing back from you as to your wishes.

Some notes – the increase in wind velocity does not increase in a linear fashion due to there being a square of the velocity in the calculation between wind speed and load being carried. If you have a concern about the adequacy of the loads being called about by your local Building Officials, we can quite easily give ideas as to what your added investment would be to increase either snow or wind loads. In many cases the difference is small in relationship to the total price of your building and peace of mind is always a bargain!

Mike the Pole Barn Guru

# When is it Time to Remove Roof Snow?

Regardless of what side of the climate change argument one is on – it has been snowing in Massachusetts this winter.

A lot.

Late January’s Winter Storm Juno alone brought up to 36 inches of snow in some parts of Massachusetts. https://www.weather.com/storms/winter/news/winter-storm-juno-snow-totals-wind-gusts

As if Juno wasn’t enough, another storm followed – leaving so much snow on the ground it forced the postponement of the celebratory parade through Boston for the Super Bowl Champion New England Patriots. https://www.cbsnews.com/news/flash-freezing-now-the-big-concern-in-northeast/

So, how much snow is too much for one’s roof?

As a basic rule of thumb, consider saturated snow weighs in at approximately 20 pounds per cubic foot. This weight is based upon a 25% moisture density, which may be conservative or liberal, as the actual moisture content of snow can range from approximately 1% to 33%.

Using the 20 pounds per cubic foot, this means every inch of snow will add 1-2/3 pounds per square foot of weight!

Any ice build-up on roofs would need to be added in as well. Use 5.2 pounds for each inch of ice depth.

For those who want to get scientific, the actual roof snow load can be checked by cutting a one foot square the full depth of the snow and ice build-up on the roof, dumping into a plastic bag and weighing the contents.

Modern buildings are designed for a snow load which assumes the roof snow load will be exceeded anywhere from once in 25 to once in 100 years, depending upon the Risk Category of the structure. The actual International Building Code language on risk categories can be read at: https://publicecodes.cyberregs.com/icod/ibc/2012/icod_ibc_2012_16_par023.htm

Buildings which were not constructed under Code requirements are often at far greater risk to collapse under snowfall. When rain falls upon snow, the weight of the roof snow can increase rapidly. Heating a building, in an attempt to melt the snow off a roof, can result in ice dams at the eave sides of the building – compounding the load problems.

Please be aware of the potential dangers of shoveling or raking snow from a roof. Besides the potential damage to the roofing materials and structure, there are such factors as a person sliding off the roof, falling off a ladder, overexerting themselves, or injury from snow sliding on top of them.

I can’t make recommendations on when to remove snow from any particular roof. It is up to the individual building owner to consider the benefits and dangers of snow removal and determine their own course of action. If your structure is in question, it is always best to consult a registered professional engineer.

# Under-Designed Ag Buildings

Does Anyone Else See How This Could Be a Problem?

Eric, one of the owners of Hansen Pole Buildings, had me check out a website today for a pole building supplier who is extolling the virtues of a particular “nailed up” laminated column, which has been the subject of some discussion in my articles. https://www.hansenpolebuildings.com/blog/2014/04/titan-timbers/

This particular supplier took verbatim the information provided by the nailed up column suppliers, without questioning the validity of the data supplied.

Me, being the curious sort, took a cruise around the pole building supplier’s website.

WHAT I SAW MADE BLOOD SHOOT OUT OF MY EYEBALLS!!

Xxxxx Buildings commitment to quality is second to none. This is amplified by the fact that all buildings meet or exceed the MN State Building Code. Xxxxx Buildings provides all customers ‘peace of mind’ by making sure the roof system loading for your building will keep you protected from natures elements. The roof system loading includes the trusses and the roof purlins.”

Now I am all over this! I appreciate people with a commitment to high quality and excellence in pole buildings. “…all buildings meet or exceed the MN State Building Code” is way cool….

Until I read their next paragraph:

“Ag Buildings
There is no code regulation of Ag buildings, (these buildings are exempt from the code) but suggested minimum loading would be 25 psf or 30 psf live load on the roof system. The definition of an Ag building would be a structure on agricultural land designed, constructed, and used to house farm implements, hay, grain, poultry, livestock, or other horticultural products. This structure shall not be a place of human habitation or a place of employment where agricultural products are processed, treated or packaged, nor shall it be a place used by the public.”

From one side of their mouths is “all buildings” meet Code, and out of the other – they are providing “ag buildings” with loads below Code!!

Here is the Minnesota State Snow Load map: https://www.dli.mn.gov/CCLD/PDF/bc_map_snowload.pdf

To get from a Pg (ground snow load), to a roof snow load, involves the multiplication by several factors. Learn more than you ever wanted to know here: https://www.hansenpolebuildings.com/blog/2012/02/snow-loads/

For discussion’s sake, we will assume these Ag buildings are unheated (most unoccupied buildings are) with the most common 4/12 roof slopes and steel roofing. The roof truss top chord live load under this combination should be 34.7 psf with 50 psf for Pg.

This provider’s, “suggested minimum loading would be 25 psf or 30 psf of live load on the roof system” is under designing these roofs to support snow by at least 13% and as much as 40%!!

You don’t own a farm, so what do you care?

When those under designed roofs collapse and the insurance companies pay to rebuild – it is YOUR insurance rates which are going to increase!

And if you do own a farm, I’d hate to be the one cleaning up the mess when your roof caves in…and hoping you are not in it when it does!

# 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.

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.

This 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.