When it comes to designing wood trusses, the spacing of each truss according to each design is crucial. If you miscalculate any bit of the spacing and don’t consider the use of the building you’re constructing, then you’re likely to end up with a compromised structure that can present a hazard.
Today, we want to run through a design scenario to discuss how we approach design and our consideration of spacing. If you have any questions along the way, please don’t hesitate to reach out to our team. We’re happy to help – 580-852-3660
Here’s the design scenario:
Take trusses that are spaced 4’ apart. They are loaded for a post frame construction that has purlins on the top cord with metal screwed down to that.
The bottom of the truss is open and exposed from underneath although the building will be completely enclosed. So on this 30’ span truss, it is resting on 5 ½ inches at each end.
On that small surface, there is 1560 lbs. of weight on the bearing point. This is every 4’. So if the trusses were spaced out to 8’, there would be double that amount of weight.
How strong of a header and what material will the header be made out of to hold up a truss with the truss spaced every 4’ and the posts every 8’?
It will need to hold 1560 lbs. for long term if you want your building to last. The minimum bearing size for this is 2 inches.
What happens if the trusses are spanning 50 feet on 4’ centers?
Now each end of the truss will have a bearing weight of 2600 lbs. The bearing size will have to increase a bit wider and this is quite a bit more weight to hold.
When most people talk about a post frame building, they think about a shop or barn that has posts spread apart and purlins in-between with metal screwed to that. This does not require as much material to put up a barn or shop and is very cost effective for a large building to park and store equipment or materials.
Now let’s say someone would like to have half of a 30×70 to be finished out for living area or office space. How finished out will it be?
If you just build a wall with posts every 8’ with a few purlins running horizontal to screw metal too, what will you attach to that to make it look good? And what about the ceiling?
Most finishing material needs to have a stud at least every 2’. So you might be better off in that case to just frame a wall and sheet it on the outside and do what you like on the inside.
However, here is another thought: If a person just does purlins and metal and sprays foam against the metal on the inside to insulate, if anything ever damages the metal on the outside, it will be next to impossible to replace the metal. Hard foam glues it together which makes a very tight metal building. The gluing together is a positive. It makes the building much more solid.
If a wall gets built with normal studs, is sheeted and then has metal attached, they can easily replace a sheet whenever they want and also do something different on the outside if they want later.
You may also want to consider the roof: A post frame truss is only loaded enough to handle it’s own weight and the weight of the purlins and metal and enough to stand up to the outside elements.
It is not designed to have anything extra added to the bottom cord. If there is anything added to it, it must have extra loading applied and that quickly doubles and triples the amount of load it is required to hold. This means the bearing points have a huge amount of extra weight suddenly.
It’s smart to work with a truss company like Timberlake TrussWorks during the planning stages if you plan on using part of a post frame for a metal building and part of it for a home or office. As you can see, what the person plans to do with the inside makes a huge difference as to how the wall should be finished and also the spacing of the trusses.
Once any kind of decking is applied to the trusses either to the top or bottom, it’s always best to start putting trusses on 2’ centers. If you do that, you will probably want a regular framed wall to handle the tremendous amount of weight from the roof loads.
Typical post frame loading is:
20 lbs Top cord Live Load (covers most Oklahoma snow loads- may need more in Kansas)
5 lbs Dead load top cord
1 lb dead load bottom cord for the truss weight
Typical house loading:
20 lbs per square foot live load top cord
10 lbs top cord dead load
10 lbs bottom cord dead load.