Truss Reactions 2

[Sparkes1992]

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

 

[hokie66]

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.

 

[Sparkes1992]

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.

 

[moon161]

Sparkes, I see, hands tied.

 

[rb1957]

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

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

 

[Sparkes1992]

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

 

[jayrod12]

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.

 

[Sparkes1992]

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?

 

[jayrod12]

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.

 

[rb1957]

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 ?

 

[kipfoot]

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.

 

[jayrod12]

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.

 

[rb1957]

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 ?

 

[gravityandinertia]

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.

 

[jayrod12]

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).

 

[KootK]

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.

 

[rb1957]

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 ?

 

[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 s roller. Am I right in saying this ?

 

[KootK]

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 not modelling the beams as lateral load supports. Not that there's necesarily anything wrong with that.

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.

 

[BAretired]

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

 

[KootK]

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.