Steel truss bottom chord in compression 5

[LJ_]

Hello,

I have this truss that has the bottom chord in compression. The length between the truss supports is 90ft and the section is HSS. The joints in the truss are every 8 ft. The trusses are spaced 20ft.

When I check for compression in the bottom chord, the length is 90ft so my HSS fails. Can I use gusset plates in the joints so I can provide for out of plane rigidity and thus reduce the length of analysis?. If the length was 8ft my member wouldn’t fail. Or is it necessary to provide bracing by connecting the trusses in the joints.

What do you think?

 

[r13]

IMO, you should first reduce the truss spacing by half, if possible. At present arrangement, both options could be difficult to deliver good result.

 

[LJ_]

Sorry it was 20ft. So do you think the gusset plates would work instead of lateral bracing?

 

[r13]

Even at 20' spacing, bracing member can be quite massive/heavy and possess it's own problems. Try strengthen the chord if you can.

 

[Agent666]

Can you please post a sketch of how the gusset plate you note (combined with the rest of your system) is intended to provide bracing to the bottom chord to decrease its effective length for compression about both axes? I'm struggling to envisage how a gusset plate might be directly equivalent to discrete bracing of the chord here?

 

[BAretired]

What is causing compression in the bottom chord?

BA

 

[r13]

I believe it is fixed end truss with top chord braced.

 

[LJ_]

The truss is a custom truss And it is getting compression simply from gravity but also in the wind load case. It looks something like this without the gusset plates as the whole system is welded.

 

[r13]

Difficult to see the bottom chord in compression for gravity load. Is this a truss for vaulted roof, or bridge?

 

[LJ_]

This is not the truss. It is similar to this one. I can’t upload the real configuration. What I am asking is: given that I provide gusset plates in the joints, would you think it would provide enough fixity for out of plane compression buckling? I think it needs to be braced for out of plane and that the gusset plate is not enough. The top chord is beaded with the purlins.

This is for a vaulted roof.

 

[r13]

I am not familiar with it, but a buckling analysis may be is a good approach for this situation. I think Agent666 might have something to offer.

 

[Guest090822]

A simply supported truss should not have compression in the bottom chord. Double check your statics.

 

[r13]

While it helps weak axis bending, the gusset plates would not improve buckling strength much, and you still have the same problem to deal with. As suggest by Rick, you shall double check your model. If nothing wrong with it, the best is to perform a buckling analysis on the model, if you know how, and your program can handle the task. You can try to size brace for 2%/2.5% of the gravity load though.

 

[LJ_]

Thank you @TheRick109 for your suggestion. I checked my model and it indeed had a problem. Now it only has compression due to wind.

 

[BAretired]

It needs to be braced.

BA

 

[hokie66]

Wind uplift, presumably for a roof, is a real thing. Don't even think about leaving out the bracing.

 

[Agent666]

Adding gussets does virtually nothing to enhance the buckling capacity of the chord as others have eluded to.

It may create some additional restraint against rotation of the chord relative to the web members. But you already had a fully welded connection, how is making it more 'fully welded' going to help, and adding rotational restraint alone does not provide restraint against buckling in compression. You need lateral restraint of the member to drive a higher mode of buckling in compression.

The restraint in your situation may come from the bending stiffness of the web members as bottom chord tends to move laterally engaging the web members about their minor axis out of the plane of the truss, and the connection of the top chord to something that can resist the twist inherent in this method of bracing. i.e. moment connection to sufficiently stiff purlins if a roof.

Surely even if you cannot post the exact arrangement you can sketch something up that shows the general intent of the truss and the elements restraining it. You still have not mentioned anything about what is restraining the truss to know if the above method might be viable or if you need to provide either a larger chord, bracing or combination of all of the above.

 

[phamENG]

Gusset plates do not brace chords, they just connect members. Even brace forces have load paths. Make sure you can run yours all the way to ground.

 

[r13]

Is this is a new construction, or existing? Can you add weight to the roof?

 

[MegaStructures]

Or is it necessary to provide bracing by connecting the trusses in the joints.

Certainly sounds like a good option if your bottom chord isn't passing buckling checks. Adding gusset plates will do nothing. To limit buckling you would need to add significant MOI in the direction of buckling. Bracing the trusses out of plane sounds like the most efficient solution.