residential attic floor joist beam possibilities

[wynn1]

"Can a structural engineer please help me with this problem".

I would like to make a storage area out of the attic above my garage. I do not really want to put that much stuff up there but things tend to add up over the years. The center area of the attic where I want to put the stuff is 9 ft wide between the trusses and 22 ft long and 10 foot high.

The Problem is that the trusses that span the garage are 28 ft long and are made out of 2 X 4s. The stringers are made out of two 14 foot 2 x 4 beams connected at the center with pound on metal plates.

Therefore, I was thinking of putting three 11 7/8 LVL beams to span the 22 feet perpendicular to the trusses and spaced 5 feet apart and then putting 2 X 6 X 10 ft beams parallel to the stringers to span the 9 foot floor that would be supported by the LVL beams. One beam in the center under where the stringers connect together, one beam under where the joists connect to the stringer.

I was thinking that three 2X4 beams on each side of the LVL beam that I screwed into the existing side wall and set on a 2 X 6 that set on the garage floor would support the load.

Will this work? Are there issues that I need to think of? What is the approx load per sq foot that I need to stay under?.

 

[boo1]

The light loading is typically 30 PSF.

Why not put LVL joists (like 4' oc) next to the trusses and attach 3/4" T&G plywood to them?

 

[wynn1]

Frankly as a EE I never thought of putting in eleven 28 foot beams rather than three 22 foot beams. I will need to get a quote from the lumber store. When you consider the cost of the three 11 7/8 in beams plus the 2X6s It might be a good bit less.

I was planning on using ¾ in plywood for the floor.

One detail that I forgot to mention is that the front of the two car garage is supported by an 18 foot beam that looks as if the builder built it. This is one of the 28 foot ends. I have no idea how much it will support. I would assume that it is designed to support the snow load and not a lot more.

Therefore, I would guess that I would need to put another beam next to it to make sure that I do not have an issue. I can not see how it is built. I can just see one side that has plywood nailed to something.

The other end wall has 2 X 4 beams 18 in apart with two 2 X 4 on top that the trusses sit on.

I must admit that I am way out of my area of expertise. It has been over 20 years since I took my static and dynamics courses. But, I am trying!!! I keep on reading the Weyerhaeuser LSL fact page and I am convinced that I do not understand it.

But, I would think that three 22 foot 11 7/8 LVL beams under the 9 foot area that I plan to put the stuff would support more load than the eleven 28 foot 4 inch LVL beams. Just a gut feeling.

Which solution would support more?? Am I missing something??

The loss of a foot of head room in the garage is not really an issue to me. The garage is 8 foot 10 in and the garage door is approx 7 foot.

 

[RDK]

Check the beam bearing on the floor for the potential to punch through the concrete in shear.

Typically you will have 3,000 (compressive Strength) psi concrete. It may be as thin as 4 “. Shear is typically 1/10 of the compressive strength. Thus you will have 3000x0.1x4=400pounds per inch of perimeter of your beam support available to resist the load. Your 3 2x4 will have a total perimeter of 20 inches or a resistance of 20x400=8,000 lbs. If the beam is loaded more than that then a shear failure is a real possibility. In this part of the world we have a 40-psf snow loading plus a 40-psf live load. Therefore your floor will support a load equal to 100 sq ft (8000/(40+40))

I would suggest a steal bearing plate under the columns as a minimum. The floor may not be fully supported underneath, this is especially true adjacent to the house. The backfill may have settled after the garage slab was poured and you may have a void under the column. I would suggest that you also consider removing a section of the floor and placing a 3’x3’ footing on compacted granular material and making it at least 6” with 3 #3 or #4 rebar each way.

Check with your local structural engineer to make sure that you are following local codes and practices.



Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[boo1]

one problem with your configuration is you are loosing head room. I have used wood I-beams to span you lenght, the cost is not that much.