Weight of Truss

[cedriklevesque]

I'm working with the version 8.3.7 of Etabs.

When I write de weight of my joist (in kg/m as asked by Etabs) (take 1 by example) and i go to see the members forces, the forces are 15.17 kN.

If a change my steel properties, the force change (in the members force option). Does someone know how Etabs calculate the dead load of a truss ???

thanks

 

[masomenos]

If ETABS is like SAP2000, it will automatically compute the self weight of members based on section properties and material. If you want to assign your own dead weight without the self weight added in, you will need to go to Define>Load cases and change the Dead selfweight multiplier from 1 to 0

 

[cedriklevesque]

Thank,

But for a truss (joist), i only have to enter Height, Weight and Inertia. So Etabs can't calcule the weight of the truss.

Do you have another idea ?

thanks

 

[masomenos]

I am reminded of the famous statement from the prison guard in the movie 'Cool Hand Luke'.

I don't get your point that ETABS "can't" calculate the weight of your truss. Since you did not give us many details, I will assume based on what you've written that you are defining your joist by Defining/adding a general frame section instead of modeling the truss in more detail. Is that correct? I read that ETABS has a joist library, although I'm not sure if it is applicable to your project.

If you defined/added your joist as a 'general section', ETABS will assume a material (steel?) unless you tell it otherwise. Based on your section and material, it will assign selfweight unless you go to the Define menu>Loads and change the DEAD selfweight multiplier to zero as I suggested in my first reply. Did you do that? You did not say.

Alternatively, if you do not want ETABS to use selfweight for joists only, but still have ETABS calculate selfweight for the rest of your model, you will need to define a new material with zero weight per unit volume and make sure that when defining joists you use that zero weight material instead of steel. That way, you can turn DEAD selfweight multiplier to 1 in your Define menu>Load cases and have ETABS automatically calculate selfweight for the rest of your model if you are analyzing more than just joists.

Another possibility why you are getting unusual results: SAP has the habit of changing units from whatever you have assigned.. it will convert back to Kip-in default for analysis results. I will bet that ETABS does the same thing. I suspect that after assigning a distributed Frame/line load, you may not be adjusting the units before reviewing output. Before checking output you will need to go to the bottom right hand corner of your screen and change to whatever units you want to see before you generate your output report

Let us know if this helps. If not, please give some more detail as to how you have modeled your joists