Another excellent question from a soon to be new post frame (pole building) owner.
DEAR POLE BARN GURU: The materials just arrived for my pole barn and construction is to start on Monday. The building will be 30×40 with attic trusses with an 8/12 pitch, 24 inches on center with a 1ft overhang (there is a total of 21 trusses). In the materials there was 4- 28ft 6x6s, and 14-18ft 4x6s. My question is will the 4×6’s be able to handle the load bearing of the attic trusses, or should they be 6×6’s as well? Thank you for your help. JIMMY IN PLYMOUTH
DEAR JIMMY: I tried to look you up in our data base, but found you are not one of our clients, however, I am always happy to help anyone who takes the time to ask a question. At Hansen Pole Buildings, every member (columns and boards) in the building is checked via a sophisticated and proprietary software system to insure all components and connections are properly designed to support the loads which will be applied to them – prior to going to the engineer who reviews all of the work and verifies it is correct. Only then are your components ordered and scheduled for delivery.
With 18 foot long columns, I would surmise you have a 14 foot eave height. Not knowing all of your load conditions (roof and ground snow load, design wind speed and wind exposure) I can only rely upon judicious experience when I say it would be highly unlikely a 4×6 column would be adequate to carry the loads.
In many cases a 4×6 is stronger than a 6×6 (here is why: https://www.hansenpolebuildings.com/2014/08/lumber-bending/). The one place where the 4×6 falls down (hopefully not literally) is as the columns get longer from grade to truss, the 4×6 tends to want to buckle in the narrow direction.
My recommendation – ask your contractor (or the supplier of the pole barn kit package) to show you the engineer sealed plans for your building. This is your only assurance the structure will be adequately designed to withstand wind and snow loads. If they cannot provide the engineering, for whatever reason, I recommend you delay construction until an engineered design is provided. Keep in mind also, an engineer sealed roof truss drawings is NOT the engineering for your entire structure (far too many folks believe if the trusses are engineered, the building is as well).
Best of luck to you, and if you do not mind, please share the results of your questioning.
To my loyal readers – my educated guess is Jimmy is going to find out the building he has ordered was not designed by an engineer and the columns as delivered are under designed. I would encourage anyone who is going to invest their hard earned dollars into a new post frame building to demand it be designed specifically for your needs (not something which is generic and maybe close) by a Registered Design Profession (RDP – architect or engineer).
Here is a secret I have shared before: