Wood Beam Design

[jheidt2543]

I have run across a problem regarding an existing building. The floor framing is composed of a plywood subfloor on 2x8 joist @ 16" o.c. spanning 10'-0". Joist bear on a builtup wood beams spaced at 10' bays and consisting of (3)-2x10's bolted together and spanning 17'-0". Even using Douglas Fir's high stress levels (I don't know the grade of the lumber used), the beams don't come close to carrying the load (100 psf live load and 15 psf dead load). Yet the floor has been in service for 32 years! We are looking at it now due to major water damage.

I propose to change out the beams to (3)-2x12's No. 2 Southern Pine and cut the span to 8'-6". The beam satifies the moment requirements but, still fails on horizontal shear. Here are a few questions to condider:

1. Why did these beams work for 32 years? The full story is that the floor was springy and had some deflection in storage areas, but by calculation it shouldn't have stood up.

2. Are code required live loads so high that that buildings don't actually attain those loads?

3. How is it that I can increase the size of the beam, reduce the span in half and still not satify design requirments, yet I have significantly improved the original design? If it worked for 32 years, why not put it back the same way?

 

[LPPE]

A 100 psf live load isnt too much to ask for for storage, but for an office space it is ridiculous. Picture your office packed with people standing shoulder to shoulder, front to back. Thats about 100 psf. You'll never see that loading.
Old growth lumber has higher strengths than todays fast growth lumber. These low values are represented in NDS. The values in NDS, if I remember correctly, are the absolute lowest you can have for that particular grade, with actual strength rarely ever approaching that low.
Also, you said the floor was springy and had deflection problems. I wouldn't say that it worked for 32 years, then.
Maybe try a flitch beam?

 

[Taro]

Three possible reasons why it hasn't collapsed...
1. Code prescribed live loads are conservative as you surmised.
2. There is a factor of safety built in to the allowable design stresses and the structure can carry significantly more load before it actually collapses.
3. There may be some alternate load path that is not accounted for in the analysis or strength due to "nonstructural" elements that are disregarded in design.

 

[ishvaaag]

The causes for it standing may be...
1. The loads in place are far less than those assumed. 100 psf in an office, average? Only when/where used for storage. More likely for most cases under 15% of that.

2. The strength safety factors for wood have been and stay huge. You accept the wood work at 70 to 110 kg/cm^2 and good woods can stand over 5 times that. Keeping the safety factors as high serves to reduce vibration and mid to long term deflection, since the young modulus of wood is moderate.

 

[ishvaaag]

Errata: I meant under 25% of that, not 15%.

 

[Trussdoc]

The NDS values for horizontal shear (90 psi for Visually Graded Southern Pine)are extremely conservative, which is why your calcs for the 2 x 12 system came up with failure.
Did you consider the 2.0 increase factor allowed by NDS for un-split wood? Also, if it's a major concern for you, look into using one of the engineered wood products, such as a parallam, which has a fv of 290 psi.

 

[KootK]

As Buddy Showalter has mentioned in previous posts, The next version of the NDS is eliminating the Ch factor. Instead, they're just going to multiply the base allowable shear stress by 1.95. I've been using this in my calculations since I found out.