Truss Reactions
Truss Reactions
(OP)
Hi Guys,
I've been modelling a simple truss in the program spacegass with a dead and live loading on the top chord. I seem to be getting large horizontal reaction forces. Shouldn't the bottom chord of the truss be acting in tension hence stopping the top chord from opening up? How do i ensure that the perimeter beam that the trusses are sitting on recieve no lateral force? Any help would be much appreciated.
Jack
I've been modelling a simple truss in the program spacegass with a dead and live loading on the top chord. I seem to be getting large horizontal reaction forces. Shouldn't the bottom chord of the truss be acting in tension hence stopping the top chord from opening up? How do i ensure that the perimeter beam that the trusses are sitting on recieve no lateral force? Any help would be much appreciated.
Jack






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In a pin-roller situation with only vertical loads, you won't get any horizontal reactions. The horizontal loads are internal forces within the truss members filtering out and finally ending up as an axial load in outermost intermediate member.
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Now when you have a pin-roller, the reason why you don't see any horizontal reactions is because the truss bottom chord is allowed to 'kick out' and isn't restricted to expand in the horizontal direction. This lateral deflection essentially alleviates the horizontal reaction.
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1) Decide where you want the lateral reactions to go. This can be the left end of the truss, the right end of the truss, both ends of the truss, the roof diaphragm, or any number of less common scenarios. Make your decision based on available strength, available stiffness, detailing convenience, and your own preferences as he designer.
2) Whatever provides the lateral support for your truss, usually we design that stuff separately from the truss itself, using the reactions from the truss.
3) Design the truss using one of these approaches:
3a) If lateral effects are small relative to gravity effects, ignore them and make one end of your truss pinned and the other fixed arbitrarily.
3b) If you're confident that you know where the lateral loads will go, or where you want them to go, model your truss supports to reflect that.
3c) If you're unsure of the load path and want a little more conservatism, place the lateral support (pin) as far from the source(s) of the lateral load as possible to maximize the number of truss members involved in resisting the lateral load. Note that this may not be universally conservative in all cases and some case specific engineering judgement is required.
I'd recommend posting some sketches of your truss and and the framing upon which it bears (plan and section). You've got some wildly competent structural engineers participating in your thread. With some additional information in hand, they'll tell you just what ought to be done and why. They might even agree.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Truss Reactions
What i wanted to do was put as little lateral load into the perimeter beams as possible. I'm just struggling to model this at the moment. Any help would be fantastic.
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it looks like part of your problem is having an unobstructed face (away for the house, with just some vertical posts). So the lateral loads have to be reacted at the house, unless you make these posts cantilevers (maybe the lateral loads are small ?). Maybe add a truss in the horizontal plane to take the loads towards the house side ?
are you trying to minimise the interaction with the house ? why not tie into the roof structure (as shown, rafters and such) ?
another day in paradise, or is paradise one day closer ?
RE: Truss Reactions
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
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BA
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I think we were also talking about the "other" lateral load (out of the page on ElevB), which again is naturally attracted to the house side.
the kinks in your LH support make it look "flaky". do you intentionally choose not to support on the existing roof ? why ?
another day in paradise, or is paradise one day closer ?
RE: Truss Reactions
This is only true if your truss is modelled pin-roller.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Truss Reactions
The support on the left looks flaky because it's not a direct load path, it develops a moment in the column, needs a lateral reaction at the ground, and would potentially have a lot more compliance than an appropriate tie into existing structure, or a straight column between the truss end and the foundation.
Problems that could arise from this are:
Applicable code/standard?
expensive design, detailing & fabrication of the dog legged column to make it structurally adequate.
Excessive motion of roof under dynamic loading, paint/drywall/plaster cracking problems.
Failure and other bad things.
Issuing extra prints if people actually do spit their coffee when they see it.
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Moon161, as long as i have modelled it correctly with the appropriate loads, how can it fail?
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My preference would be to build the verandah roof as a monoslope, high at the house and pitched to the outside. That has the advantage of not taking water from the verandah back to the house, creating an effectively internal gutter. You will still have the gutter, but with less runoff. You would need some vertical cladding to stop heavy rain from bouncing off the house roof into the verandah. For that, you might use translucent sheeting to allow light in.
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another day in paradise, or is paradise one day closer ?
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Truss action is just terminology. In this case I meant it to mean you'd have the proper tension forces you are expecting in the bottom chord.
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if the truss elements are bends, all normal truss behave is "off the table". Traditional truss elements react axial load only.
another day in paradise, or is paradise one day closer ?
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I second this sentiment. It's fine to design wacky shaped stuff, but you need to start with the right geometry. More than once I've had to point out to an architect that what they're showing in plan and elevation is a warped surface.
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Although true, this is commonly ignored provided the top chords are designed accordingly. Roof trusses, open-web steel joists and others all violate this idea but are still analyzed as trusses.
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another day in paradise, or is paradise one day closer ?
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I agree to an extent. However, when I worked for a truss manufacturer, we still performed bending checks on anything that may have a significant bending moment between panel points.
RE: Truss Reactions
RE: Truss Reactions
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Truss Reactions
another day in paradise, or is paradise one day closer ?
RE: Truss Reactions
RE: Truss Reactions
This is true. Of course, in your case, modelling one of the supports as a roller effectively means [i]not[i] modelling the beams as lateral load supports. Not that there's necesarily anything wrong with that.
This is not strictly true if the connection between the truss and both beams is capable of transmitting lateral thrust. The top chords of the truss produce a lateral thrust which has two (at least) potential places to be resisted:
1) tension in the truss bottom chord.
2) weak axis bending in the beams.
Some of the horizontal thrust is expected to travel through both mechanisms, always. How much goes to each is a function of how stiff each mechanism is. In typical arrangements, the relative axial stiffness of the bottom chord would greatly exceed the weak axis flexural stiffness of the beams and it would be appropriate to design the truss, in isolation, as though it were pin-roller supported.
Here, what percentage of your horizontal chord thrust winds up being resisted by the support beams? And where have you placed modelling supports (pins/rollers) in your 3D model? Are you using a 3D model or 2D?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Truss Reactions
Not entirely. If you have a truss with only dead load, pinned at both ends, there will be a horizontal reaction at each support but zero force in the bottom chord because the pins prevent the chord from stretching.
BA
RE: Truss Reactions
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Truss Reactions
What's driving the truss locations? Why don't you align the trusses with the 'whacky columns' and design it as a fully welded fixed frame. Then you can have pin-pin supports at the base. Much cleaner.
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Bowling danish, good idea mate, that's pretty much what I went with.
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another day in paradise, or is paradise one day closer ?
RE: Truss Reactions
Correct.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Truss Reactions