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  • 1.  Open web truss - design software

    Posted 08-04-2017 06:25 PM

    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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  • 2.  RE: Open web truss - design software

    Posted 08-07-2017 09:17 AM
    I can't vouch for its accuracy, but Redbuilt has some free proprietary software called Redspec that will design open web joists using various load conditions including concentrated loads.  You may already be aware that Redbuilt was the previous commercial side to Trusjoist.  Perhaps you could pick a joist in the software that has some equivalency to the one you are trying to analyze?  You should only use it to check ballpark feasibility, and I would not make any promises until the actual joist manufacture has provided you the final analysis (unless you are willing to assume the risk).  In fact, the software output has a disclaimer against using its results for existing joist applications.

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    Robert Fine P.E., S.E., M.ASCE
    Principal
    Fine Engineering
    Reno NV
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  • 3.  RE: Open web truss - design software

    Posted 08-07-2017 09:17 AM
    Edited by Miles Brumbaugh 08-07-2017 09:17 AM
    Hi Daniel,

    You can use Sap2000 for this sort of structure. But the problem is that, this software does not any default material like wood or timber! So you have to define one. By changing the elasticity and other mechanical properties, somehow, you could introduce a wooden section. I know that there are many software available for analysis and design of wooden structures rather than Sap2000, but I strongly recommend you do it in Sap2000 for its inevitable accuracy and reliability.

    regards,





  • 4.  RE: Open web truss - design software

    Posted 08-07-2017 09:26 AM
    Metal tubing webs with a Laminated Veneer Lumber chords would most likely be the old Trus-Joist Open Web Trusses.  I think that division was bought by Red Built.    If the chord is actually are 3.5" wide x 2.5" deep glue laminated lumber, that would most likely be from a company named Web Joist out of Chehalis, WA.  Both manufacturers would normally ask that you contact them for technical support when when you are relocating mechanical units on existing trusses.

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    MarkMyersP.E., M.ASCEGig HarborWA
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  • 5.  RE: Open web truss - design software

    Posted 08-07-2017 09:28 AM
    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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  • 6.  RE: Open web truss - design software

    Posted 08-08-2017 01:15 PM
    There are fairly simple equations for calculating equivalent uniform loads from point loads.  One convenient source for those equations is AISC Steel Construction Manual.  In the Thirteen Edition they are found in Tables 3-22a, b and c and in the 18-page Table 3-23.  Examples for a wide variety of loading and support conditions are addressed and the equations are applicable to any material, not just steel.  After identifying the set of equations that best fits your scenario, you could set up a spreadsheet to perform the calculations and allow you to vary the size and locations of the point loads.

    Next, there is an old, pre-finite element truss design procedure called "Method of Sections" which you could use to evaluate stresses at various points of concern along your trusses.  It works using pretty basic shear and moment calculations to estimate the forces in each member on opposite sides of the section cut.  I have seen engineers experienced with this member determine in a mater of a few minutes that a truss is over stressed at a particular location.  When I was in college, in the 70's that is how most trusses were designed.  Finite element analysis was only used for expensive ones.

    Hope this helps

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    Bruce Clarke P.E., M.ASCE
    Vice President
    Williams & Beck Inc
    Rockford MI
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  • 7.  RE: Open web truss - design software

    Posted 08-09-2017 06:14 PM
    If you do decide to handle this on your own, be sure to get a copy of the Evaluation Reports for the specific manufacturer of metal web pin connected wood truss you have.  This will give you the information on the variety of gauges used for metal webs (they usually have a paint stripe that corresponds to the gauge) and the capacities of the proprietary bearing hardware.

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    Mark Myers, P.E., M.ASCE
    Gig Harbor, WA
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  • 8.  RE: Open web truss - design software

    Posted 08-10-2017 04:00 PM

    Daniel,
    at the end of a group of SJI joists you may find a metal tag identifying the joist manufacturer with a code number for you to contact the manufacturer through a form sent to SJI.  The joists you are involved with may or may not be SJI, but they still may have a metal joist tag identifying them.  You will have to call various manufacturers with a photo and general dimensions of the joist material to get more load capacity information.



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    Martin Fradua P.E., F.ASCE
    Vice President
    Huntington NY
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