Tag Archives: Vult

Feedback Needed From RDP’s and Building Officials

I am asking for feedback from RDP’s and Building Officials because:

There is a method to my madness. Seriously. I want to make sure we are doing things 100% correctly. In my humble opinion there are currently numerous post frame buildings being constructed where wall girts do not meet Code or acceptable engineering practice.

I have developed a professional respect for a builder based in Northern Idaho. Recently I visited his website and saw some photographs leading me to ask about how he solves “barn style” wall girt design issues. He was right on top of it – his photos were of older buildings and he switched to all bookshelf style wall girts years ago, I applaud him for doing so!

Lots of architects, engineers and building officials read my articles, thank you! Your wisdom is appreciated. Attached is an example set of wall girt calculations. If there is an error in any direction, or something missed, your feedback would be more than appreciated. Thank you in advance.

Code is 2015 IBC (International Building Code)

Building Summary

Building Footprint Width 40′
Building Footprint Length 60′
Building Footprint Height 17′
Square Footage (area contained by embedded poles) 2400 ft2
Total Roof Area 2745 ft2
Total Wall Area 3191 ft2
Building Eave Height 17′
Roof Style GABLE
Slope 4/12
Roof Height 20.33′
Building Conditioned Yes

Wind Summary

Vult 110 mph
Vasd 85 mph
Risk Category I
Wind Exposure B
Applicable Internal Pressure Coefficient 0.18
Components and Cladding Design Wind Pressure
Zone 1 -19.78
Zone 2 -32.985
Zone 3 -49.217
Zone 4 -23.522
Zone 5 -27.936
Zone 1 Positive 11.826
Zone 2 Positive 11.826
Zone 3 Positive 11.826
Zone 4 Positive 21.188
Zone 5 Positive 21.188
Duration of Load for Wind 1.6
Structure type Enclosed

wall girt size: 2″X6″
spacing between girts = 22.5″

girt span = 139.875″
supported by 2×4 blocking every 139.875″

Fb: allowable girt pressure
Fb‘ = Fb * CD * CM * Ct * CL * CF * Cfu * Ci * Cr NDS 4.3
CD: load duration factor
CD = 1.6 NDS 4.3
CM: wet service factor
CM = 1 because girts are protected from moisture by building envelope
Ct: temperature factor
Ct = 1 NDS 4.3
Cfu: flat use factor
Cfu = 1 NDS 4.3
Ci: incising factor
Ci = 1 NDS 4.3
Emin: reference adjusted modulus of elasticity
Emin = 470000 psi NDS Supplement
Cr: repetitive member factor
Cr = 1.15 NDS 4.3
lu: laterally unsupported span length
lu = 139.875″
le: effective length
le = 1.63 * lu NDS table 3.3.3
le = 244.496″
CF: size factor
CF = 1.3 NDS 4.3
CL: beam stability factor
CL = 1 NDS 3.3.3
Fb‘ = 850 psi * 1.6 * 1 * 1 * 1 * 1.3 * 1 * 1 * 1.15
Fb‘ = 2033.2 psi

fb: girt test pressure
fb = 6 * 0.6wall_wind_force / 144 * girtSpacing * span2 / 8 / (b * d2) NDS 3.3
fb = 6 * 17.389 psf / 144 in.2/ft.2 * 24″ * 139.875″2 / 8 / (1.5″ * 5.5″2)
fb = 937.255 psi
937.255 ≤ 2033.2 stressed to 46% 6″X2″ #2 OK in bending

Fv‘: allowable shear pressure
Fv = 135 NDS Supplement Table 4-A
Fv‘ = Fv * CD * CM * Ct * Ci NDS 4.3
Fv‘ = 135 psi * 1.6 * 1 * 1 * 1
Fv‘ = 216 psi NDS Supplement

fv: shear girt pressure
fv = 3 * (0.6wall_wind_force / 144 * girtSpacing * span / 2) / (2 * b * d) NDS 3.4
fv = 3 * (17.389 psf / 144 in.2/ft.2 * 24″ * 139.875″ / 2) / (2 * 1.5″ * 5.5″)
fv = 36.854 psi

36.854 ≤ 216 stressed to 17% 6″X2″ #2 OK in shear


Δallow: allowable deflection
l = 139.875″
Δallow = 139.875″ / 90
Δallow = 1.5542″
Δmax: maximum deflection
Δmax = 5 * 0.6W * spacing * span4 / 384 / E / I from http://www.awc.org/pdf/DA6-BeamFormulas.pdf p.4
E: Modulus of Elasticity
E = 1300000 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 * 12.173 psf / 144 psi/psf * 24″ * 139.875″4 / 384 / 1300000 psi / 20.796875 in.4 components and cladding reduced by .7 per footnote f of IBC table 1604.3
Δmax = 0.37401″ ≤ 1.5542″

Building Department Checklist 2019 Part 1


I Can Build, I Can Build!

(First published six years ago, it was more than past time to update to reflect current code requirements!)

Whoa there Nellie…..before getting all carried away, there are 14 essential questions to have on your Building Department Checklist, in order to ensure structural portions of your new building process goes off without a hitch.  I will cover first seven today, finishing up tomorrow, so you have a chance to take notes, start your own home file folder of “what to do before I build”.  Careful preparation will be key to having a successful post frame building outcome.

#1 What are required setbacks from streets, property lines, existing structures, septic systems, etc.?

Seemingly every jurisdiction has its own set of rules when it comes to setbacks. Want to build closer to a property line or existing structure than distance given? Ask about firewalls. If your building includes a firewall, you can often build closer to a property line. Creating an unusable space between your new building and a property line isn’t very practical. Being able to minimize this space could easily offset the small investment of a firewall. As far as my experience, you cannot dump weather (rain or snow) off a roof onto any neighbor’s lot, or into an alleyway – so keep those factors in mind.

#2 What Building Code will be applicable to this building?

Code is Code, right? Except when it has a “residential” and also has a “building” version and they do not entirely agree with each other. IBC (International Building Code) only applies to post frame buildings, not IRC (International Residential Code:


Also, every three years Building Codes get a rewrite. One might not think there should be many changes. Surprise! With new research even things seemingly as simple as how snow loads are applied to roofs…changes. Obviously important to know what Code version will be used.

#3 If building will be in snow country, what is GROUND snow load (abbreviated as Pg)?

Make sure you are clear in asking this question specific to “ground”. When you get to #4, you will see why.  Too many times we’ve had clients who asked their building official what their “snow load” will be, and B.O. (Building Official) replied using whichever value they are used to quoting.  Lost in communication was being specific about “ground” or “roof” snow load.

As well, what snow exposure factor (Ce) applies where building will be located? Put simply, will the roof be fully exposed to wind from all directions, partially exposed to wind, or sheltered by being located tight in among conifer trees qualifying as obstructions? Right now will be a good time to stand at your proposed building site and take pictures in all four directions, and then getting your B.O. to give their determination of snow exposure factor, based upon these photos.

#4 What is Flat Roof Snow Load (Pf)?

Since 2000, Building Codes are written with flat roof snow load being calculated from ground snow load. Now design snow load has become quite a science, taking into account a myriad of variables to arrive with a specific roof load for any given set of circumstances.

Unfortunately, some Building Departments have yet to come to grips with this, so they mandate use of a specified flat roof snow load, ignoring laws of physics.

Make certain to clearly understand information provided by your Building Department in regards to snow loads. Failure to do so could result in an expensive lesson.

#5 What is “Ultimate Design” or Vult wind speed in miles per hour?

Lowest possible Vult wind speed (100 miles per hour) only applies in three possible states – California, Oregon and Washington for Risk Category I structures. Everywhere else has a minimum of 105 mph.  Highest United States requirement of 200 mph for Risk Category III and IV buildings comes along portions of Florida’s coastline.  Don’t assume a friend of yours who lives in your same city has your same wind speed.  The city of Tacoma, WA has six different wind speeds within city limits!

Vult and nominal design wind speed Vasd are NOT the same thing. Make certain to always get Vult values.

#6 What is wind exposure (B, C or D)?

Take a few minutes to understand the differences:


A Building Department can add hundreds, or even thousands, of dollars to your project cost, by trying to mandate an excessive wind exposure.  Once again, a good place for photographs in all four directions from your building site being shared with your Building Department.  Some jurisdictions “assume” worst case scenarios.  Meaning, your property could very well have all four sides protected and easily “fit” category B wind exposure requirements.  However, your jurisdiction may have their own requirement for every site in their jurisdiction to be wind exposure C, no matter what.  It’s their call.

#7 Are “wind rated” overhead doors required?

Usually this requirements enforcement occurs in hurricane regions. My personal opinion – if buying an overhead door, invest a few extra dollars to get one rated for design wind speeds where the building will be constructed. Truly a “better safe, than sorry” type situation.

I’ve covered seven most important questions for your Building Department Checklist, and they really weren’t so difficult, were they?  Come back tomorrow to find out the last seven!


Fire Resistance, Condensation, and Wind Speed

Fire Resistance, Condensation, and Wind Speed

DEAR POLE BARN GURU: Do you know if WMP-10 metal building insulation facing is ok to have exposed in a commercial building in regards to its fire resistance rating? JON

DEAR JON: WMP-10 facings are flame resistant, however you should consult with your local building code enforcing agency to determine if they will allow it to remain exposed given your use of the structure. An alternative might be Johns Manville FSK-25 faced batts which are laminated with an FSK (foil-scrim-kraft) facing, which enables the insulation to carry a fire hazard classification rating of 25/50 or less per ASTM E 84. The FSK-25 facing also serves as an excellent vapor retarder and may be left exposed where codes permit. The FSK-25 batts are a lightweight fire-resistant thermal and acoustical fiberglass insulation made of long, resilient glass fibers bonded with a bio-based binder.

Personally, I’d look at using unfaced fiberglass or rock wool batt insulation then covering the interior surface with 5/8″ Type X gypsum wallboard. Probably less expensive and would afford greater R-values with less of an investment.


DEAR POLE BARN GURU: I recently had a small pole barn constructed in Northern NJ which I’m about to insulate. Needless to say, the information regarding this is very confusing. The end goal here is to not have a condensation problem. With no insulation on the walls currently the metal walls sweat. The roof consists of metal roofing on top of “double bubble” on top of purlins with ridge vent and soffit vents.  The walls will be filled with 6″ fiberglass and a poly vapor barrier applied. The ceiling will either be OSB or gypsum attached to the bottom of the trusses with blown insulation on top with no vapor barrier. With that said, my question is with this configuration, will the gable ends above ceiling height sweat or do they need to be insulated? If so what would be the recommended insulation?


DEAR CONFUSED: With proper ventilation in your attic I won’t say it will be impossible to have condensation on the inside of the attic gable endwalls, however the probability should be small. If you want to make certain, an inch of closed cell foam can be sprayed on the inside of the endwall steel and it will eliminate any chance.

DEAR POLE BARN GURU: Are your plans for stick built frames or CBS frames or both? If only for frame built what is the wind ratio? SUNSHINE in JUPITER

DEAR SUNSHINE: Our buildings are neither stick built or concrete block – they are post frame buildings.

Since January 1973 anemograph stations within the United Kingdom have tabulated for each clock hour the mean hourly speed and the maximum gust (of approximately three second duration). The ratio of maximum gust speed to the mean speed for individual hours as an effective height of 10 meters is referred to as the gust ratio. The mean wind ration is the ratio of the extreme gust speed to the extreme hourly mean speed, both having a return period of 50 years. This ratio turns out to be 1.60.

Here in the colonies, we design using Vult (Ultimate Wind Speed). Until the 2012 IBC (International Building Code) we designed for Vasd (Allowable Stress Design) which is 60% of Vult.

One of the beauties of post frame construction is the buildings can be designed to support any wind load situation needed.