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?

 

[sdz]

I'm dealing with a similar problem at the moment with a quite shallow truss starting at 10 deg at the ends. A good structural analysis program should be able to calculate effective length for you. The main problem is buckling in the horizontal plane. Gusset plates probably won't do much to fix this. You are likely to get some restraint from the top chord which will reduce the effective length of the bottom chord. How much depends on how well the top chord is restrained and member sizes. If you can, put in a bracing system which will restrain the bottom chord laterally; that would be the most effective.

 

[KootK]

1) If there's no highly compelling reason to avoid bracing in the plane of the bottom chord, that would definitely be my recommendation. Such bracing will be highly efficient, very reliable, and may well benefit your building in other valuable ways such as bracing the tops of end walls etc.

2) If there IS a compelling reason to avoid bracing in the plane of the bottom chord, the mechanism shown below is often the way to get it done and, indirectly, does make use of the gussets in the sense that the gussets are part of the structure that effectively converts pure lateral buckling into something more akin to constrained axis bucking about the top chord. This is just semantics though; in conventional design, the gussets perform no meaningful bracing function as the other folks have made clear.

 

[KootK]

I'm dealing with a similar problem at the moment with a quite shallow truss starting at 10 deg at the ends.

In similar situations, I've used the scheme below to brace the bottom chord without introducing ceiling plan bracing.

 

[r13]

The reason I've shied away from recommending bracing in my original response is the size effect (L=25', changed to 20' later), also the notion that the truss bottom chord shouldn't be in significant compression, thus, strengthen the chord may be the optimum option. I am not quite certain on the strengthen now, as it turns out to be a bowstring truss, for which uplift due to wind could be hard to handle. But I am still concerned with the effectiveness of a long bracing. I am thinking it may be viable that to brace the end bas only, and use lighter tension ties in the middle bays to engage all trusses in the action. Just a random thinking though.

We need inputs and responses/comments from the OP.

 

[LJ_]

I was thinking that HSS filed tubes would be enough to handle de compression the total span of the truss is 100ft. So I think If I fill 10ft at each end It will reduce the buckling potential of the bottom chord. Also I think it will reduce the bottom chord out of plane unbraced length down to 80ft.

https://files.engineering.com/getfi...file=E620D17F-25F9-4958-BB26-CAB071DDB4BE.png

 

[LJ_]

Hello @kootK. Thank you for your response.

How do you calculate the new k in the bottom chord for the kL given that I provide the moment frame connection at the top of the truss.

Also, the purlins are welded to the top chord. They are channels. So they might be able to provide the moment frame rigidity. I wonder if the need to be the same section types as the truss members which is HSS.

 

[LJ_]

Hello @retired13,

This is a new construction but a lot of issues are happening in the construction phase. The design needs to be revisited because the specified material is not available. Also, Seems like no wind check was made previously and now it is failing. I am trying to avoid bracing in the perpendicular direction of the bottom chord because it will need trusses as the distance is 20ft.

 

[human909]

This is a new construction but a lot of issues are happening in the construction phase. The design needs to be revisited because the specified material is not available. Also, Seems like no wind check was made previously and now it is failing. I am trying to avoid bracing in the perpendicular direction of the bottom chord because it will need trusses as the distance is 20ft.

Use fly bracing. Like everyone has said the bottom flange needs to be braced. Fly bracing is the common solution for the bottom chord of truss. Perpendicular bracing is less ideal.

 

[r13]

With sturdy purlin, human's idea get a vote here.

 

[human909]

Yep. A purlin check is needed though it is generally not onerous and should generally pass unless you have really got the purlins light. Using fly braces both side will half your load, spacing them more frequently along the truss than is required by the truss will also reduce the brace load (I presume your code allows this.)

 

[r13]

Pairing is even a better idea, as one will get into tension, if the compression brace deforms slightly.

 

[KootK]

How do you calculate the new k in the bottom chord for the kL given that I provide the moment frame connection at the top of the truss.

There are a number of ways but mine would probably be this:

1) Decide what you want KL to be in your truss and introduce bracing frames to match that spacing.

2) Imagine that you have a discrete, lateral spring at each brace point on your bottom chord and use something like AISC Appendix 6 to work out what strength and stiffness that spring needs to have in order to serve as competent bracing.

3) Design the moment frames, including the beams, connections, and truss webs to have the strength and stiffness determined in [2] at the level of the truss bottom chord.

This is really the exact same procedure that I'd use for fly bracing. And that makes sense given that the fly bracing is really another version of a moment frame wherein the diagonal creates a superior version of the purlin to truss web moment connection.

I wonder if the need to be the same section types as the truss members which is HSS.

I see no need for them to be the same section and, frankly, would be surprised if they were in practice.

I was thinking that HSS filed tubes would be enough to handle de compression the total span of the truss is 100ft. So I think If I fill 10ft at each end It will reduce the buckling potential of the bottom chord. Also I think it will reduce the bottom chord out of plane unbraced length down to 80ft.

I would be very careful with that. The concrete fill will increase member stiffness where the fill exists but, given that you only plan to introduce it at the ends of the truss, I'd be surprised if it really had a significant impact on overall buckling capacity at all. And it definitely will not simply reduce KL by the length that is concrete filled.

 

[structSU10]

what about the bottom chord providing sufficient bracing for the compression truss verticals? if the bottom chord is too limber it could be a problem for that condition.

 

[KootK]

The Importance of Tension Chord Bracing.

Yeah, that's always a thing. One nice thing about a "real" uplift demand is that it give you an excused to cover tension chord buckling concerns at, usually, no extra cost.

 

[LJ_]

Hello again,
I revised the AISC Design Guide 24 and the hollow structural section book by Packer and the latter explains the procedure to calculate the K for the bottom chord given top chord and web elements and it also states that this K can be as low as 0.3 ir even less. Unfortunately this doesn’t work for my design either so I need to do fly bracing.
Now fly bracing round HSS is very expensive so I will ask the owner if it can be changed to round hss because the connection details will be difficult.

 

[JoshPlumSE]

I am skeptical about the analysis that showed this type of truss to have compression in the bottom chord for gravity load. Did you just build an FEM model and run it, or did you do a hand calculation?

One common mistake I see a lot of people make with truss analysis is that they build a model where both ends of the truss are pinned. When they really should have made one end pinned and the other end a roller.

If you compare the two analysis results, (pinned-pinned vs pinned-roller) you will usually see that a large compression reaction appears at each support for the pinned-pinned model. Then, the pinned-roller model barely (which obviously can't develop the compression reaction) is allowed to move, but it only moves a tiny fraction of an inch. So, if there is any give in the bolt connections, or the support, then the reactions causing compression will never develop.

Of course, there are ways in which I could see wind causing compression in the bottom chord. But, I'd start off making sure the analysis results are correct by further investigating the gravity load case.

 

[r13]

Can you use lighter truss at a closer spacing to minimize the difficulty in bracing?

 

[LJ_]

No unfortunately the spacing is set as 20ft.

 

[LJ_]

@joshplumSE The supports are welded to a base plate and then the base plate is anchored to a column. This is why I think it should be pinned pinned and not pinned roller

 

[jayrod12]

It's quite likely that your column will be flexible enough to move and create the roller intent.

As Josh indicated, run an analysis with a pin-roller and see how much lateral movement needs to occur for that to be true, and then see whether you feel that amount of movement could be achieved based on connections and member flexibility. If yes, then use the pin-roller results, if no, then pin-pin it is.