Basic Wood Design Question for Joists 1

[Dinosaur]

Colleagues,

I am a bridge designer so I deal with steel and concrete and big time loads so I just don't have a feel for typical residential wood design. I offered to help a buddy design or scope out what it would take to build a shed on his property. He has some grand ideas and I'm trying to help him keep his perspective.

If someone could help me understand why I am seeing a much larger member size for joists in this problem, I'd appreciate the insight.

The building will be 24 x 16 in plan with the width as you face the shed being the smaller dimension, 16 feet. He wants to build a second floor for storing material and some tools (ladders and such) and he would like the second floor to cover the back half of the shed, 12 feet deep and 16 feet wide. He hopes to avoid any columns.

I estimated the plywood floor to require 5/8" plywood if we used joists on 16" centers. When computing the joist size I believed that if we used a strong edge beam, we might be able to span the 12 foot direction with the 2x joists. I thought a 12 feet span at 16" centers would be achieved with a 2x8 easily but maybe we could get away with just 2x6 even at 100psf live load. Well I did the calcs and I was nowhere near a 2x6 so I thought what did I screw up. I went to look it up in a table online for reference and the table indicated a 2x10 would be needed for 40 psf live load and I had hoped to accommodate 100 psf.

If my description is clear, I would appreciate any insight as to how a 2x10 is required for 16" centers and a 12 feet span for a joist. Thanks in advance for your help.

 

[molibden]

I'm not from the US but that sounds about right for 4,8 kN/m[sup]2[/sup] live load (=100 psf).

 

[SlideRuleEra]

Dinosaur - There are several reasons why wood beams are larger:

Wood is much lighter (lb per cubic foot) than either steel or concrete. By weight, wood is stronger than either steel or concrete. Compare the weight of a 12' long 2x10 with the weight of either a steel or concrete beam to do the same job.

Wood is more flexible than either steel or concrete. It is common to have a wood beam that can carry a load (suitable section modulus) but will deflect too much (to small a moment of inertia). The best way to dramatically increase the moment of inertia is to go with a deeper beam (say 2x10 instead of a 2x8).

A more subtle difference is shown in my sketch.
Steel can be shaped with flanges to give high section modulus and high moment of inertia in a beam that is not too deep.

Reinforced concrete is designed so that the concrete is at it's best (compression) and steel is at it's best (tension).

With wood, we are stuck with what can efficiently be cut out of trees (rectangles).

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[jayrod12]

That sounds about right to be totally honest.

for 100psf, if deflection isn't a concern (usually isn't for garage storage), 12 ft span for a 2x10 at 16" o/c is right on the money. Almost too close for comfort actually. If it was pedestrian accessible, it'd be 2x12 @ 16 or 2x10 @ 12

 

[molibden]

SlideRuleEra, also elastic modulus of wood is much lower than of steel.

What I mean to say is that today we can manufacture timber beams of any shape: I, T, two meters high beams (glulam), cross-laminated or one-way slabs, ribbed slabs, hollow sections... It's elastic material so we compensate with height of the beams (greater moment of inertia).

 

[HouseBoy]

Dinosaur,
I do wood design all day long and I agree with jayrod that 2x10 @ 16" o.c. is right at the line.
I would look at it from a standpoint of how accurate is the live load estimation. Aside from any "code" requirement, a couple things that I would have in mind (for scale) are:
A residential garage floor is designed for 50 psf
True storage might get up to 100 psf and surely it is possible to have loading greater than that but "ladders and such" doesn't sound like 100 psf to me. File cabinets do sound like 100 psf or more.

Regarding deflection, one consideration is the behavior of drywall cracking due to excessive deflection. There is also the "feel" of the floor that you might want to consider.
Don't know if you would have drywall on the underside of the loft.
Still, all together, I'd say 2x10 at 16" is a good choice for the situation you describe.

 

[Yt.]

Perhaps you could consider to add straps at tension side.