# Loads on Barn Girts

Oops! A Girt Failure

When last we peeked in on Leroy and the boys on the pole building across the street, our hero and his crew were making slow and steady progress. I’ve figuratively hammered on why “barn style” wall girts generally do not meet the requirements of the Building Codes. The problem is one of deflection and I have probably mentioned more than once the barn girts may very well meet the requirements for bending – meaning they may very well be designed to carry the design wind load and won’t break, they just bend too far!

Well…..

Leroy’s cohorts had a wall girt fail while under a load.

Always someone to prove me wrong!

The load, however was not from the wind (as might have been expected), but instead, it was from one of the crew standing in the middle of it!

Wood is unique in the sense that it has the ability to support higher stresses if the loads are being applied for short periods of time. This effect is quantified by the Load Duration factor, or CD for short. This helps greatly for temporary increases in loading (such as an earthquake, wheel load, wind load, etc.) will have additional strength, compared to permanent loading.

Cd for an impact load (lasting less than two seconds) is 2.0, for a 10 minute load (like wind or seismic forces) 1.6.

So how much concentrated load can a 2×4 1650msr wall girt placed placed as barn girts on the side of a building take?  If you like math – here we go…

The formula for maximum moment (refresh yourself on bending moments here: https://www.hansenpolebuildings.com/2012/09/bending-moment/) is Mmax = P x a x b / L, where P is the applied load in pounds, a and b are the distance from each end to where the load is applied and L is the length squared (all distances in inches). A 200 pound worker standing in the center of the girt would impart 4800 in-lbs (inch pounds) of force upon the eight foot long member!

4800 in-lbs / Sm (Section modulus of a 2×4 = 3.0625) = 1567.34 <= 1650 (fiberstress of the wall girt) X 1.6 for duration of load.

Moral of the story is a 200 pound worker standing on the top of the now broken 2×4 barn girt should have been able to safely be there for up to 10 minutes, and only have utilized just under 60% of the strength of the board. The caveat being if the nails in each end of the girt were adequate to support the worker’s weight!

# Them Girts, They Be a Bending

In order to follow the deflection criteria of the International Building Codes, other than for fairly small wall column spacings with low wind speeds, pole building wall girts need to be installed in a “bookshelf” fashion.

What even is “deflection criteria”?

Dictionary.com defines deflection as, “the deviation of the indicator of an instrument from the position taken as zero.”

In layperson’s terms, when you push on something with a known force, how far does it bend (or deflect)?

Prior to the adoption of the International Building Codes, wall members which did not support a brittle finish material (e.g. drywall), were not limited in how far they could deflect.

Not so with the new code.

For those of you who care to look it up and read along, Table 1604.3 lists deflection limits. For walls with brittle finishes, the limitation is L/240 (where “L” is the span or length of the supporting member). For flexible finishes, the limit is L/120. Footnote A includes, “For secondary wall members supporting formed metal siding, the design wind load deflection shall not exceed L/90.”

The smaller the value L is divided by, the larger the allowable deflection is.

Ignoring the width of nominal six inch building columns the allowable deflection of wall girts for an 8’ bay (with steel siding) would be 1”; a 10’ bay 1.27”; 12’ bay 1.54”.

Using the least possible loads allowed by code – which would be 85 mile per hour winds, B wind exposure and a building Occupancy class of I (I = a building which represents a low hazard to human life in the event of failure) combined with the stiffest possible commonly used lumber (Douglas Fir or Southern Yellow Pine #2) and spaced 24 inches on center, some problems occur.

Using 2×6 lumber on 10’ bays the actual deflection is 1.507” and on 12’ bays 3.226”. On 8’ bays IF 2×4 #2 is actually used (it is rarely available in lumber yards), then it would deflect 0.924” and be OK. Another issue with the 2×4 girts on 8’ bays, is this style of construction is most prevalent in areas where the minimum wind speed is 90 mph.

Keeping all variables the same, except increasing the wind speed to 90 mph, increases the deflection of a 2×4 #2 to over an inch on an eight foot bay!

Shopping for your new ideal dream pole building? If so, ask specifically how the wall girts are placed. Anything other than “bookshelf” style pretty much guarantees the building being proposed, is not code conforming!