Tag Archives: wind shear loads

Post Frame Purlin Blocking

Every time I begin to rest on my laurels and think I have covered all post frame (pole barn) building basics up jumps yet another one to bite me where I deserve to be bitten due to my overlooking it.

Our independent drafting team at Hansen Pole Buildings (thanks Kristie) came up with this question recently.

“As we are building our building, a question came up: what is the reason for purlin blocking? Why do we need it? What’s the important purpose for it? We will be doing this step tomorrow and actually considered skipping it (sorry, bad of us I know). Is this all explained in the CM, because I have looked and couldn’t find the why’s. I bet ALOT of people skip this step and just wanted to see why we have it.”

Well, our 500 page Construction Assembly Manual covers lots of “how tos” and very few “whys”. Biggest reason is we would hate to make it into a 700 or 800 page manual. We try to cover it all and continually add to it and improve it, so every time we get a question not covered by it, we add more information. Even though these subjects do not make a dime for Hansen Pole Buildings, we have recently expanded sections on Site Preparation and Concrete Floors. It is all part of us delivering “The Ultimate Post Frame Building Experience™” https://www.hansenpolebuildings.com/2019/05/the-ultimate-post-frame-building-experience-2/

Back on task – I will preface this by letting you readers know Kristie and her husband are currently erecting their own Hansen Pole Building.

There exist two types of purlin blocking:

At endwalls (this is Kristie’s case) – Building Codes require airflow from vented soffit on gables overhangs be blocked off. Ventilation for dead attic spaces must be accomplished by either a combination of eave and ridge vents or by gable vents. Venting through end overhangs will disrupt airflow for a properly ventilated attic space.

Structurally a solid load path must be provided in any building to transfer wind shear loads from roofing to ground. Purlins overhanging an end truss and attaching with a hanger such as a Simpson H-1 do not accomplish this. Brackets will not prevent purlin rotation under extreme loads. Properly placed, endwall overhang soffit panels can be attached to these same blocks, as they serve a plethora of duties.

Purlin blocking can also be “mid-span” – when a 2×10 or larger member (girt, purlin, floor joist, etc.) is 2×10 or greater mid-span blocking is required if a member is unsupported for more than eight feet.

There you have it and if you win on Jeopardy thanks to this, I will work for a percentage.

Why Post Frame Wall Girts Overhang Posts

Client and reader SCOTT in BOULDER posed this question recently:

“Is there a specific reason why the girts have an over hang to the outside an 1 ½ (inches). Can they be set inside flush with the posts?”

Mike the Pole Barn Guru responds:

There actually are a plethora of reasons why your (and all Hansen Pole Buildings) have wall girts set so girt outside face is 1-1/2 inches beyond the columns and building line.

From a standpoint of installing roofing and siding alone – your particular building happens to be 36 feet by 36 feet. 12 sheets of installed wall or roof steel  cover roughly 36’2″. If your finished dimensions were 36′ you would end up having to rip at least six panels of steel full length.

In order to adequately transfer wind shear loads from roof to ground, an effective connection is essential between the 2×8 pressure preservative treated splash plank and columns. Most efficient structural solution is with nails through splash plank into column faces – putting splash plank on outside of columns. Holding wall girts out 1-1/2″ places their face in the same plane with outside of splash plank. If splash plank were to be placed between columns, it would likely entail having to rotate it flat on top of your concrete slab on grade and utilize anchor bolts for an adequate load transfer. This would eliminate using splash plank as a concrete form for your building’s concrete floor. It would also create a “joint” or “seam” between the concrete floor and underside of what would now be a bottom plate, entailing having to design and install some sort of permanent sealant to prevent water intrusion.

Other structural members essential to wind shear transfer include eave struts, wainscot girts and your overhead door header – again all members where resistance to loads is best achieved by attachment directly to column faces, rather than creating some far more difficult connections.

A side benefit is wiring can be done in external walls by running around columns in this 1-1/2″ space – reducing or eliminating needs to drill holes for electrical.

Considering a post frame (pole barn) building where all exterior framing is flush to the outside of columns? Make sure it has been designed and plans sealed by a RDP (Registered Design Professional – architect or engineer) to insure all connections are adequate and a proper load path has been followed from roof to ground.