Analysis of 40-year-old bar joist in HWVZ

[Streng72]

I have been asked to verify the structural stability for a 40-year-old bar joist in the Miami area. I am analyzing the roof joist using a computer model based on measurements taken in the field. The bottom chord is a double bar (3/4” dia each), the 28”-9” long top cord is a 2L 2x2x1/4, and the webs are solid 7/8” dia. bars. Per field observations there are three rows of cross bridging, which seems alrigth for this span length. I have assumed that all joints are pined and that the two end supports in the joist are fixed. I am using Fy=36 ksi (I am not very sure about that Fy yet).

After applying all the loading the bottom chord fails in compression due to wind uplift. I took the compression force and run manual calcs to determine the unsupported length which gives me about 47 inches. The max compression force is about 6.2 kips.
I am inclined to suggest they provide additional bridging at 4.5 feet o.c. (the bar joist ‘bays’ are 1.5 feet wide. I am assuming there is some fixity at the points where the bridging connects to the joist then I could apply K=0.8)

My question is: Does the bridging effectively restrain the bottom chord so that the KL=0.8 x 4.5’ is satisfactory?. Shouldn’t I use K=0.8?

Any input will be greatly appreciated.

 

[BAretired]

Sorry...ryy is not correct. Let's try again.

A = 2(pi*0.85^2/4) = 2(0.5675) = 1.1349
Ixx = 0.0512 in^4
Iyy = 0.0512 + 2(0.5675)(0.85/2)^2 = 0.256 in^4
ryy = ?(0.256/1.1349) = 0.475

Ly/ryy = 54/0.475 = 114
Lx/rxx = 18/(.85/4) = 84.7 (so Ly controls)

Cr/A = 100 MPa or 14.5 ksi
Cr = 14.5* 1.1349 = 16.4 kips (factored load)

Safety Factor = 16.4/6.2 = 2.65 (no problem).

I think this is correct, Streng721, but it is best if you check it for yourself.

BA

 

[Brad805]

BA, shh, some of us might be wasting a couple of minutes sometimes. It is nice to be the boss somedays. I find it very interesting to read how others practice in different areas and it is always handy to have a few resources that have proven to be knowledgable.

Brad

 

[Streng72]

BAretired,

Many thanks for taking the time to do all these calcs. You REALLY helped me out (my first time analyzing an old bar joist with a PC model).

With these hand calcs I was able to figure out what was wrong with the computer model. That's why I always prefer to run hand calculations over PC models. Many times you don't know if you have one parameter wrong in the computer that yields just the wrong output.

Thanks again !

 

[BAretired]

You're welcome, Streng72.

BA