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Truss Reactions
2

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

RE: Truss Reactions

I'm not familiar with the software you are using but it sounds like you have your reactions as "Pin, Pin" rather than "Pin, Roller". You need to release the lateral restraint at one of the supports to allow the bottom chord to take the tension otherwise it will just go straight into the reaction.

RE: Truss Reactions

(OP)
Ahhhhhhh awesome, thanks mate! I'm not sure how i would go about ensuring that the connection acts like a roller. Any suggestions? :)

RE: Truss Reactions

(OP)
And if i pin one side, shouldnt all of the lateral force be attracted to that pinned side?

RE: Truss Reactions

Is the truss made from concrete members? Or is it a steel truss being supported by a concrete member? If it's the latter, you can detail the connection on one end to allow lateral movement.

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.

RE: Truss Reactions

(OP)
Hi Trenno, thanks for the response. It's a steel truss supported on each end by a steel member. If I pin one end and roller the other, the pin should take all lateral loads shouldn't it?

RE: Truss Reactions

The only reason you are seeing horizontal loads when you adopt a pin-pin situation, is because each end of the truss cannot move in the horizontal direction. Now when the truss wants to deflect, under vertical loading, the bottom chord wants to 'kick out' but because the ends are all locked up that's why you're seeing those large lateral reactions.

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.

RE: Truss Reactions

(OP)
Once again thanks for the response trenno. If a lateral wind load were to be applied to the truss, wouldn't the pinned side of the truss essentially have to resist this force? Otherwise wouldn't the truss 'slide'?

RE: Truss Reactions

Yes, that is correct. If you apply horizontal loads, the horizontal reaction will be all at the pinned end. But that is only for analysis purposes. In the real structure, depending on how it is built, the force may be divided between the ends.

RE: Truss Reactions

(OP)
Cheers for that hokie. So my problem is I didn't want to have to design the beam which the truss is sitting on for biaxual bending. I'm guessing now my best bet would be to design these perimeter beams to wind loading in the minor axis and dead in the major?

RE: Truss Reactions

The usual process goes something like this:

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

(OP)
KootK, excellent post mate, thank you very much! Ok, so ive attached some sketches below. Just ignore the member sizes for the moment. All members in the truss are to be 100 x 50 RHS welded together. The trusses are going to sit on the perimeter beams with a cap connection with a bolt either side - acting like a pin connection. The columns are slightly offset from the perimeter beam and are bolted to an existing house on one side.

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.




RE: Truss Reactions

the left hand support (beside the house) looks pretty "flaky" ...

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

Like rb1957, I think that your lateral support reaction is probably at the left end of the truss at that whacky column that I'll leave in your capable hands.

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

I agree that the column looks 'flaky'. Also, Elevation A shows the fascia to be level whereas the plan seems to suggest it is sloping down toward the north.

BA

RE: Truss Reactions

(OP)
I was planning on bolting the posts into the existing brickwork. Yeah the the drawings are just s rough sketch st the moment. What do you guys mean by flaky? I was more so just trying to understand yh loading path from the trusses into the perimeter beam. My question is do I need to put the lateral load caused by the dead load from the truss into the perimeter beam? Shouldn't the bottom chord prevent this from happening even though it's pin pin at each end? I thought I'd only need put the wind lateral load into it.

RE: Truss Reactions

ok, looking at the truss in isolation, looking at Elev B, how much lateral load (acting to the right) will come out at the RH side (where you have only vertical posts) ? IMHO, not much, so your structure will "naturally" react this load on the LH support.

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

Quote (OP)

Shouldn't the bottom chord prevent this from happening even though it's pin pin at each end?

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

I almost spit my coffee out when I saw that column. I expect making it structurally adequate would be difficult.

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.

RE: Truss Reactions

(OP)
I didn't want to tie it into the existing roof because i don't know how its trusses are tied to the top plate. I dont think the bending in the column is that bad (see below). I should note that the structure isn't clad on the walls so the loads arent incredibly high.

Moon161, as long as i have modelled it correctly with the appropriate loads, how can it fail?

RE: Truss Reactions

I would rule out the doglegged columns just because they look silly. I know some folks don't want freestanding columns, but I don't know why not.

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.

RE: Truss Reactions

(OP)
Thanks for the response Hokie. I totally agree with you and that would be my preference also. Unfortunately, this is what the architect wants so ill have to do my best to make it work.

RE: Truss Reactions

Sparkes, I see, hands tied.

RE: Truss Reactions

i see you're cantilevering the RH posts (large moment at the ground) ...

another day in paradise, or is paradise one day closer ?

RE: Truss Reactions

(OP)
Yes that's correct RB1957. Will most likely be 450 dia bored piers for the footings.

RE: Truss Reactions

With the posts being cantilevered, I feel as though the compression force in the bottom flange goes away. You really should analyze the trusses with the support beams and columns modelled appropriately. I would bet you'll end up with proper truss action.

RE: Truss Reactions

(OP)
You make a good point jayrod. Thanks mate. Sorry, which compressive force were you referring to and what exactly to you mean by truss action?

RE: Truss Reactions

Apologies, the horizontal reactions you were seeing at the truss supports. I mistakenly took the lack of tension in the bottom chord to mean compression.

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.

RE: Truss Reactions

your plot is showing moment in the upper truss chords, yes?

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 ?

RE: Truss Reactions

Quote:

BAretired: Elevation A shows the fascia to be level whereas the plan seems to suggest it is sloping down toward the north.

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.

RE: Truss Reactions

Quote (rb1957)

if the truss elements are bends, all normal truss behave is "off the table". Traditional truss elements react axial load only.

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.

RE: Truss Reactions

it's fine to analyze truss elements as beams, just don't expect the results to be the same as "rod" trusses.

another day in paradise, or is paradise one day closer ?

RE: Truss Reactions

Quote (jayrod12)

Quote (rb1957)
if the truss elements are bends, all normal truss behave is "off the table". Traditional truss elements react axial load only.

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.

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

Quote (G&I)

we still performed bending checks on anything that may have a significant bending moment between panel points.
Hence designed accordingly. Meaning top chords are checked for combined bending and compression. Otherwise the still called a truss and treated as pinned at each joint (typically).

RE: Truss Reactions

Nowadays, the overwhelming majority of "trusses" have transverse loads between panel points and are, therefore, not true trusses by definition. Most steel joists. Most prefabricated wood trusses. Even for big industrial and institutional stuff, I see economics leading to dealing with chord bending in preference to panel point load alignment much of the time. Outside of the bridge world, lining up loads with panel points almost seems to be a forgotten/discarded art.

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

it's like many details in my line ... in the old days much work was taken to carefully align things to minimise these secondary bending moments (or to carefully analysis details of a structure or analysis), these days we just crush them with the FEA sledge.

another day in paradise, or is paradise one day closer ?

RE: Truss Reactions

(OP)
If I had a truss sitting on a beam and the truss purely only had dead load, there shouldn't be any horizontal load even though it's pinned st each end to the beam. However in theory, if you modelled this there would be horizontal loads unless one end is s roller. Am I right in saying this ?

RE: Truss Reactions

Quote (OP)

However in theory, if you modelled this there would be horizontal loads unless one end is s roller.

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.

Quote (OP)

If I had a truss sitting on a beam and the truss purely only had dead load, there shouldn't be any horizontal load even though it's pinned st each end to the beam.

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


Quote (Sparkes1992)

If I had a truss sitting on a beam and the truss purely only had dead load, there shouldn't be any horizontal load even though it's pinned st each end to the beam. However in theory, if you modelled this there would be horizontal loads unless one end is a roller. Am I right in saying this ?

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

Below is a 2D model that I believe represents your situation well conceptually. For practical design, I'd still probably just go 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

I'm really not a fan of those whacky cantilevered columns, are the deflections OK? Even if it works on paper it just doesn't feel right.

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.

RE: Truss Reactions

(OP)
Thanks for the responses all. Nice sketch by you kootk. So let's say the bottom cord is twice as stiff as the perimeter beam in the minor sxis, am I right to assume it will take twice the load?

Bowling danish, good idea mate, that's pretty much what I went with.

RE: Truss Reactions

Who's doing the waterproofing detail for the roof to roof valley? That looks like it'll be pretty tricky and could be the cause for a future call back when they keep getting leaks. The dogleg column will want to deflect down and the eave, when the existing truss is loaded will deflect up. If you can lap the trusses, you stand to reduce these deflections and simplify the structure. You just need accurate information on the location of the existing trusses and how they are tied in. If you design for a 0.3 m play in truss location, you can defer the final locations to the contractor and have the trusses fabricated while he installs the new footings.

RE: Truss Reactions

isn't the original gutter still there (in the valley) ?

another day in paradise, or is paradise one day closer ?

RE: Truss Reactions

Quote (sparkes)

So let's say the bottom cord is twice as stiff as the perimeter beam in the minor sxis, am I right to assume it will take twice the load?

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

If I was designing this I would probably recommend to the client to remove the fascia and tails at the existing structure so the new trusses can bear either on or adjacent to the existing wall. Less wasted coffee will likely result

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