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Some Help with Basic Beam Design

01/21/2022 10:53 AM

Would someone please compare these two truss configurations and give me pros & cons for each? Just in general terms. I've a feeling my employer is not using their truss designs properly.

Let's assume equal load all the way across (as in heavy snowfall), and the ends of the beam are fixed to the posts.

As you can see, the difference is in where the bottom ends of the diagonal struts are attached. In "A" they attach to the horizontal beam, in "B" they attach to the vertical central strut.

Thanx!

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#1

Re: Some Help with Basic Beam Design

01/21/2022 11:40 AM

From the diagram, the diagonal struts meet the supporting struts at an angle, so at these joints there is shear stress. In B, the angle is closer to perpendicular than in A, so B would have less shear stress than A.

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#7
In reply to #1

Re: Some Help with Basic Beam Design

01/23/2022 3:20 PM

Shear is not the limiting force, I suspect. I always see framing to the C/L of the members, as in A, as PapaDoc states. A is correct. B is a mistake.

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#2

Re: Some Help with Basic Beam Design

01/21/2022 1:34 PM

As I see it the function of the infrastructure is to transfer weight to supporting wall/post/ground...The greatest point of deflection or weakest point would be the center of the horizontal cross beam...the arch roof is transferring weight to the supporting beams to ground and also supporting the center of the vertical beam at the weakest point of the horizontal beam...the diagonal support beams should not be trying to support the roof by using the horizontal beam but the center vertical beam....so the second one I think is the better design...

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#3
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Re: Some Help with Basic Beam Design

01/21/2022 1:51 PM

Even better...

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#4
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Re: Some Help with Basic Beam Design

01/21/2022 9:14 PM

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#5
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Re: Some Help with Basic Beam Design

01/21/2022 9:41 PM
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#8
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Re: Some Help with Basic Beam Design

01/24/2022 10:55 AM

Simple statics:

All the forces around that bottom center node have to sum to zero. The horizontal chord does not resist any vertical force at this node. Assuming the one vertical cord is in compression, the two diagonal chords have to resist the vertical force. Therefore, the y component of their force is up. That means they also have an x component of force that pulls horizontally away from the node. This horizontal chord has to resist the horizontal forces that are created by the diagonal chords resisting the compression force of the vertical chord. This force horizontal force cannot be resisted by the vertical chord. In fact, alignment could result in the vertical chord being pulled apart.

So, alignment A is the correct method of connecting the chords at the center joint.

I do totally agree that adding chords to assist the bottom chord in bearing on the supports at either end is totally called for. Otherwise, that chord not only has a horizontal component induced by the center node, but now also has a bending force. The bending force adds tension to the top of the chord and compression to the bottom of the chord. And the forces are additive to the forces that exist due to the center node. Thus, if the horizontal chords have compression induced by the center node, this compression adds to the compression created by adding a bending force at the supports near the ends of the horizontal chord.

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#6

Re: Some Help with Basic Beam Design

01/22/2022 12:01 AM

Make the centerlines of the members intersect at a common point. That would reduce secondary stresses and make the structure able to support more load. "A" seems to do that.

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#9

Re: Some Help with Basic Beam Design

01/25/2022 2:19 PM

I thought that this would be a very easy and basic question. But we have several answers on both sides.

CCOOP609 has gotten his answer voted as a Good Answer, though. And frankly, he most sounds like he knows what he's talking about. I do however appreciate everyone's input. I still would like some feedback though... if any would like to chime in.

Meanwhile, I'm going to go ahead and design my structure using model "A" , as CCOOP suggested... with the added struts on each end, as SolarEagle suggested in his reconfiguration of my original model "B". The one that looks similar to an M, although with all the radial struts resting on the horizontal beam.

Comments are welcome.

I will, in the end, be sending my drawings to a PE in order to get the drawing certified. So, he'll tell me if it's fine before we build it, but I'd rather send him the proper configuration to start with, rather than have to change it around after he looks at it.

Thanx again for everyone's help.

FYI, this is going to be an awning and the structural members are going to be aluminum rectangular tubing... probably 1" x 2"

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#10

Re: Some Help with Basic Beam Design

01/27/2022 8:27 AM

Thanx all, for all the reference material and suggestions. Here is what I came up with and turned in. Different project than my first example, but I have a number of arches to design. This truss configuration is called "Bowstring".

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#11
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Re: Some Help with Basic Beam Design

01/27/2022 9:21 AM

REV: 001

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#12
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Re: Some Help with Basic Beam Design

01/27/2022 10:01 AM

You will probably find out that just like the Romans in stone, that you won’t need any of those truss elements, unless you are running heavy truck traffic across the bottom.

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#13
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Re: Some Help with Basic Beam Design

01/27/2022 11:46 AM

You might also need some plan for holding the roof down in high winds...such as a tie down assy....here we just use like 1/8"-3/16" stainless wire anchored to the floor with eyebolts run across the panels...

https://www.metalconstructionnews.com/articles/top-mistakes-by-metal-roof-installers

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#14

Re: Some Help with Basic Beam Design

01/27/2022 12:11 PM

Has anyone noticed that this is not a beam? It's an arch truss.

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#15
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Re: Some Help with Basic Beam Design

01/27/2022 12:40 PM

Has anyone noticed that it is only 10 ft across? ...and the truss is made of small aluminum tubing?

https://www.mbci.com/products/roof/curved-roof-systems/curved-battenlok-/

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#16
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Re: Some Help with Basic Beam Design

01/27/2022 1:20 PM

Correct. It's the snow load that is the issue. That, combined with the fact that I'm trying to keep that small aluminum tubing as small as possible.

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#17
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Re: Some Help with Basic Beam Design

01/28/2022 12:31 AM

Roger that....maybe you could add some heat..?

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#19
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Re: Some Help with Basic Beam Design

03/03/2022 3:17 PM

What zone are you located... I was just working on something similar,...

For the live load requirements, What zone are you located?...

for the horizontal I normally use this calculation for deflection... L/360 or L/180.. depending.

Unfortunately, this doesn't have a arc...

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#18

Re: Some Help with Basic Beam Design

01/29/2022 4:33 AM

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