Composite Truss

[slickdeals]

Folks,
I want to run through steps for the design of composite trusses and wanted to check if I was missing something.

I don't know if there is a simpler way to do this, but here is what I was thinking of.

1. Create two separate models of the structure, one with no composite action and another with the desired composite action. This will probably be adjusted by turning off horizontal shear transfer between steel and concrete.

2. In the first model, capture the axial forces/moments in the top and bottom chords based on non-composite behavior. This will include the effect of compression force + bending on the top chord and tension only on the bottom chord.

3. In the second model, capture the axial forces/moments in the top and bottom chords with post-composite loads (superimposed dead and live loads).

4. A superposition of these forces along with the appropriate load combinations will then be created. This will be used in a spreadsheet etc to check the interaction equations.

Can you guys chime in?

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[ash060]

How big of a truss are you talking about?

Is it about the size of a composite joist, or is it a really big truss?

Why composite truss? Are you trying to reduce the size of your compression chord. If the stress block is all in the concrete you will have tension in the top chord. In that case I would probably ignore the top chord in compression (like with composite joists) and that way get a more conservative bottom chord number.

So maybe design the top chord for construction only and than the concrete for the service condition. Maybe if you have large spacings between panel points you may get some tension in the concrete at the top over the panel point, but it probably won't overcome your axial compression from the chord force.

 

[slickdeals]

It is more of a generic question. Yes, it is intended for use in deep trusses with long spans. Composite to help with deflections and to keep the size of the compression chord reasonable.

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[msquared48]

Item #2 will need two load conditions, one with the construction live load to check the member stresses at that time, and one without to be used for the summation of the end model results. Othwewise you will be penalizing yourself here..

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[paddingtongreen]

This scares me, I 'm not sure why, perhaps because I'm a dinosaur.
I don't like placing thin slabs on long spans, there is difficulty getting the thickness right, the more the placement continues, the more the deflection, the more the already placed concrete is too low.
Could you not get most of the same effect from cambering the truss? Not the cross section, it's true, but control of the deflection.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ishvaaag]

Paddington, a usual solution to the thickness problem is to use prefab panels as bottom forms (or even "voiders" or "plate" insets), then you get the composite action through the concrete "channel" along the beam. Usually the panels in place produce some interstitial space, what can be a problem for composite action but canbe forfeited by using some separation (and forms) or be reduced by making a lower flange to the panels. In all these cases the excessive concrete can be more easily avoided for you have almost anywhere the form panel reference closer to the surface to make easy checks.

Really, you always have the form to do that but if less careful the panel forms may help to get some total constant thickness.

 

[paddingtongreen]

ishvaaag, I'm not sure I understand. I'm not just thinking of the thickness so much as finishing level, as in a surface on which to place a moving sidewalk.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ishvaaag]

Well, certainly if by care one can't produce a target level for the service situation then overload is always a risk.

Or contrarily, if such loss of flatness is unaceptable like it might be for a less refined use and with a structure otherwise sound, the problem stands for the same reason, you need a very controlled level.

You are right, Paddingtongreen. And in fact I saw the case as a student in no less than the Ministry of Commerce building in Madrid, fillings of 4" were being used (I was told, but might they use technical floors?) because temprane unshoring had caused such much visible deflections on the 12 m or so slabs on capitels.

 

[slickdeals]

A related question I was thinking about is this:

Assume you have a long span joist girder or a long span truss (say in the order of 100+ feet). It was probably assumed that there was no interaction between the deck and the joist/truss in the design of the chords (meaning that the truss was designed assuming no distribution of forces into the deck). This is well and good for the design of the chord members but what about the deck/truss attachments?

In reality, before any superimposed dead, live or wind loads are applied, the deck is attached to the truss and will start picking up load acting "composite" of sorts. There will be a shear transfer between the truss chord and the deck for gravity loading as the truss deflects down (compatibility).

Is this something that you account for in the design of the attachment between deck/truss in addition to the shear transfer (for diaphragm purposes)? I have had a few people raise their eyebrows and question "what force transfer for gravity"?

Thoughts?

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