Analysis of steel Warren type truss. Margin and compression concerns.

[r13]

Can you provide span length and truss depth, also the original design load?

Where you get the 22 kips from? Is it reaction at one support end, or the total reaction (11 kips/support)?

 

[RPMG]

Try posting a free body diagram. Someone will check your chord force, which is just typing a few numbers into a calculator.

 

[JoshPlumSE]

You can't set the safety factor to 1.0. Since you're doing ASD design (not LRFD) that safety factor takes into account variability of loading, materials, construction, et cetera.

I get something like 15.6 kips in axial capacity for tension. Or, 12.7 kips in axial compression..... just looking at the AISC table 4-11. But, that's assuming concentrically loaded angles. I believe your capacities came from Table 4-12 which is for eccentrically loaded angles.

Now, if you had already factored up your loading, then your capacity would be more like the LRFD values of 23.4 kips for tension and 19.1 kips for compression. (concentrically loaded angles again).

I just want to be sure that this is a true truss as opposed to a Open Web Steel joist like we'd get from SJI. If it's an SJI joist, I wouldn't be surprised if the capacity were higher.... because the manufacturers do a lot of testing and such to justify their values.

 

[STrctPono]

I would assume you ran the statics analysis correctly. I would, however, want to know what loads you applied. Where did they come from? What load combination did you use? And most importantly, did you factor the loads?

 

[phamENG]

To determine the eccentricity of the loading, we'd need a section cut through the truss. If all single angle members, it is likely that they are eccentrically loaded.

If this is a roof, you won't need to consider snow. Roof live load is a minimum 20psf, so it will control over the minimal snow loading you have.

I'm not familiar with the program you're using - the support on the right is a roller, correct? A scissor truss should generally be modeled pin-roller. Modelling it as a pin-pin is a sure way to "fail" several members in compression.

 

[phamENG]

Also, your units are in inches, correct? I doubt a scissor truss made up of L2x2's could span over 500 feet...

 

[phamENG]

Under the right circumstances, yes. But the minimum roof live load is still 12psf. What's the truss spacing? I didn't notice it in the previous posts.