Daniel, First you need to identify which type of truss it is and the manufacturer, so that you could look into their electronic catalogs to see which truss it is for material types, member cross section properties, etc. One Option could be that you look into one of most commonly used General Purpose Structural Analysis software to check an existing truss by modeling the truss as a 2D Plane member with the appropriate cross section properties and material properties for top and bottom chords, diagonals,support conditions, and member end connectivity (either fixed-beam action or pinned-truss action). Once you have the model set up then you could input the applicable design loads, gravity dead and live, snow, wind, and the point loads from the mechanical HVAC roof top unit at the proper locations to get the bending moments, axial loads, shear, deflection, etc. There are several commercially available software such as STAAD-Pro, RISA, SAAP2000, Dlubal, Visual Analysis, and some others .... depending on your budget you may have to see which one suit your needs.
Another Option if you do not have any software available and would like to do a quick check of the existing truss, then will need to go back to the basics of Strength of Materials and Elementary Structural Analysis to identify and check mainly few controlling design parameters: SHEAR, BENDING, DEFLECTION, AND LOCAL BUCKLING. Set up your simple supported existing truss with the appropriate loads and find: a) Maximum Shear Force at the two ends, b) Maximum Bending Moment along the span, magnitude and location. c) Maximum Deflection along the span.
From the maximum shear force you could check one of the most critical parameters for wood-steel composite trusses, sometimes the controlling parameter for the design of the truss, which is the shear stress on the wood members which may be a low value compared to the compression and tension stresses of wood. From the maximum bending moment, M you could then use the classical beam formula of M = w.L^2/8 to find the maximum allowable equivalent uniform load as: w = 8.M/L^2 which could give you an indication of the level of acceptability of the existing truss from the maximum bending moment. For deflection check you may to go to the truss manufacturer catalog and get one of those formulas that they suggest for the type of existing truss to then check the estimated approximate deflection for the equivalent uniform load from above. You will also need to check axial loads for top and bottom chord members for compression and tension and axial loads in the diagonal members to check also compression and tension in the web members.
After you have done the checks above, then you will need to go back and look at the location of the HVAC unit for the point loads on the bottom chord and then check the top chord member for bending as a simple supported beam between panel points. This to make sure that the top chord member will not break under the point load from the HVAC unit. If top chord does not work then you may need to add an extra diagonal member from top to bottom chord panel points to make sure that the loads from the HVAC unit are properly transferred to the existing truss.
I know the above may sound like a lengthy process but if you master it, you could do it in short period of time and you may not need a software, but simple a calculator and the existing truss electronic or paper catalog from the truss manufacturer to run the calculations suggested above. Give it a try and let us know what you find and which method you ended up using or what else you may be able to find out there.
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Pedro Munoz Ph.D., P.E., M.ASCE
Principal
PRM Engineering, LLC
Methuen MA
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Original Message:
Sent: 08-04-2017 15:51
From: Daniel Joita
Subject: Open web truss - design software
Hello,
I'm checking the suitability of placing various mechanical units on top of existing open web trusses. Web is made of tubular members while the top and bottom chords are wood members / laminated.
I found some design tables with allowable uniform load in PSF but it does not address concentrated loads.
I'm looking for design software to check existing trusses, where I can input the truss geometry, member sections and material property and come up with a quick check.
I know, I could do it in RISA or other FEA but I need a tool that provides a quick check specifically designed for open web trusses. This would help to quickly estimate truss capacity and suitability for various distributed and concentrated loads.
Any suggestions on how to make my life easier as far as using existing software, app, MathCAD routine or even a spreadsheet?
Thank you
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Daniel Joita P.E., M.ASCE
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