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# Moment of Inertia in Truss Design

## Moment of Inertia in Truss Design

(OP)
I am designing a simple horizontal truss beam and am wondering how you relate the "shape" of the strut or cord into the design. All the calcultions I find only refer to the area of the members. Is this assuming a square? and you can then relate that needed area into a differnt shape?

### RE: Moment of Inertia in Truss Design

This is because you will be modeling only axial stiffness in the truss, and loaded at joints. For tha analysis you only will be using E and A. But for checking the member you will need to know the actual shape in order to properly ascertain how buckling affects each member.

### RE: Moment of Inertia in Truss Design

(OP)
ishvaag, thanks. So does this mean that I CAN equate the "square" shape and I value needed to another more efficient shape for the deflection claculation? My biggest question here is , Is it assuming a square to begin with?

### RE: Moment of Inertia in Truss Design

Yes.  If you want to think of the shape as a square, you can equate its area A to a more efficient shape for bending (or buckling) calculations.  But for your biggest question, a truss member just uses area A regardless of shape.  Shape isn't specified (except in a buckling analysis).  So it doesn't assume a square.  But if you're applying any transverse load to your member, then it's a beam member, not a truss member.  Simply-supported beams (or pin-connected beams, neglecting axial shortening) just need any reasonable I value, regardless of shape, to get end reactions and internal forces.  But to get beam stresses, the shape must be specified because you must use the correct moment of inertia I and extreme fiber distances (stress recovery points) for the stress calculations.  Hope this helps.

### RE: Moment of Inertia in Truss Design

(OP)
Vonlueke, thanks for confirming this. I am applying the shape values to the top and bottom chord because of the deflection and stresses allowed. For the struts a simple area calculation is used. It made sence to me, but I couldn't find a written referance to confirm it. Again thanks to both of you and this web site is GREAT!!

### RE: Moment of Inertia in Truss Design

But you can only substitute shapes as long you model the members having pins as joints. Otherwise the bending stiffness starts to play part in your solution and deflections vary, normally not much if you have deep trusses and moderate size of the assumed shapes but much if the memebers are assumed deep for the truss depth.

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