Truss Bearing Beam Sizing Help

[jcook5003]

Hey Guys-

I'm in the planning phase of my garage build. It's going to be 24' wide using common 4:12 pitch trusses on 24" centers.

I live in Southwest Virginia, specifically Blacksburg. I need to know how to calculate the load bearing beam sizing and post spacing for the support beam for the trusses.

I have two ideas, but from an engineering perspective I have no idea if they will work.

Idea 1: 6x6 Posts on 8' centers, double 2x12 headers. 32' long over all.

Idea 2. 6x6 posts on 12' centers and use one 2x12 or larger LVL that is 24' long. This eliminates two posts and saves money over the built up double 2x12 header above. It also saves two 2' x 2' footings. The garage is slightly smaller but should still be sufficient for my needs.

So.......all that in mind. I have no idea to actually calculate the load that will be on those beams. Therefore I'm not sure I trust running blind building them.

Can someone help me with a table or equation that can help me figure out these items?

 

[downsizingnow48]

Are you referring to size of header over garage door? If so they depend on width of garage door opening your county building dept will most likely have a handout saying what they are looking for. Also might consider 5:12 pitch also common in rural Virginia given possible snow loads.

 

[/pine]

Realistically you have to combine both the live (snow/ice) and dead loads weight of all materials...then you have to know what the capacities of the beam material is (yellow pine is quite strong)...also a plywood or steel "flitch" plate will increase the load bearing capacity of your beams...

Yous should be able to find load/span tables that meet you local local building codes either online or from the local building department...search for "framing manuals"...if you know and GC's they should have the manuals...

 

[KennyG]

I think you are trying to size headers tying the posts together and supporting the ends of the trusses. If so, you just need to use a standard beam table. Here is one I've often used:
http://www.awc.org/pdf/WSDD/C2B.pdf

To use this, determine how much load you will have on each beam segment and go into the tables to find the beam size to provide that capacity. As an example, if you have 8 foot post spacing and a 24 foot total truss span, each beam segment will support a roof area of about 8' by 12' (ignoring the angles) or about 100 square feet. You will have to figure out how much dead and live (snow) load you will have but if you assume 40 lb/sq ft, that would be about 4000 lbs on each beam span. Depending on the strength you assume for the lumber (I usually use 1000 psi but that's really conservative) 2x10's are probably OK and 2x12's are overkill.

Now for the fun part - opinions. I wouldn't go with more than 8 foot spacing for an enclosed structure. Too much chance of warping between the posts. Sounds like you are looking at a hybrid structure - post walls and truss/sheathing roof. If you aren't going more than 8 or 10 feet tall, why not just stick build 2x4 walls? If you are really going post beam why not a metal roof and trusses on 4 foot centers? Also, I don't quite understand the built up beam reference. With posts, you put one beam on the inside and one on the outside.

 

[KennyG]

Another quick thought. For a garage, use 4x6 or even 4x4 posts. They will be fine at 8 foot centers for that size building. My pole barn (40x48, 12 foot ceiling) has 4x6 posts.

 

[jcook5003]

I think you are trying to size headers tying the posts together and supporting the ends of the trusses. If so, you just need to use a standard beam table. Here is one I've often used:
http://www.awc.org/pdf/WSDD/C2B.pdf

To use this, determine how much load you will have on each beam segment and go into the tables to find the beam size to provide that capacity. As an example, if you have 8 foot post spacing and a 24 foot total truss span, each beam segment will support a roof area of about 8' by 12' (ignoring the angles) or about 100 square feet. You will have to figure out how much dead and live (snow) load you will have but if you assume 40 lb/sq ft, that would be about 4000 lbs on each beam span. Depending on the strength you assume for the lumber (I usually use 1000 psi but that's really conservative) 2x10's are probably OK and 2x12's are overkill.

Now for the fun part - opinions. I wouldn't go with more than 8 foot spacing for an enclosed structure. Too much chance of warping between the posts. Sounds like you are looking at a hybrid structure - post walls and truss/sheathing roof. If you aren't going more than 8 or 10 feet tall, why not just stick build 2x4 walls? If you are really going post beam why not a metal roof and trusses on 4 foot centers? Also, I don't quite understand the built up beam reference. With posts, you put one beam on the inside and one on the outside.

This is exactly what I was referring too. I was hoping to get 12' ceilings, so that's why not stick framed. Plus all the additional concrete for the trench footer. I'm on a budget.

I was going with a sheathed roof to avoid the typical "sweating metal" syndrome that happens with pole barns around here. I was spacing my trusses 2' OC because that is how they are engineered to run according to my local lumber yard. Is there another option? They are common 4:12 clear span garage trusses. I chose them because they would be very affordable at $42 each, and were a nice common width.

 

[Egon]

Do you have building codes to abide to?

Gota put in wind loading. You'll need bracing for lateral stability.

Trusses can be designed for different spacing.

 

[KennyG]

Conventional framing trusses are designed for 16/24 spacing. Pole barn trusses are for 4 feet or 8 feet spacing. If you are sheathing the roof you still would need stringers if you go over 24 inches so I don't know what the relative cost would be.