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  • 1.  Initial Structural Frame Softness

    Posted 01-21-2019 03:18 PM
    Edited by Tirza Austin 01-21-2019 03:17 PM
    We have a project consisting of a 300 foot span, all bolted, box truss girder, connected through moment connections to columns. We are measuring the mid-span deflection as load is added to the girder. We are in the initial stages of adding load and have seen some un-expected initial softness in the system followed by much more rigid and as expected behavior. My theory is that the frame has had to "settle" into its connections, foundations have had to engage the soil in resisting the "kick out" forces, loads have had to distribute to both sides of the box girder, etc before getting the expected stiffness. I am wondering if others have experienced this and/or if anyone has any other thoughts on this. Thanks. 

    Colton Simmons P.E., M.ASCE


  • 2.  RE: Initial Structural Frame Softness

    Posted 01-22-2019 12:16 PM
    Edited by Tirza Austin 01-22-2019 12:16 PM

    I have a couple of questions: 

    1. Please clarify what you mean by "initial softness is more than expected". Is the entire frame settling? Is the truss deflecting more than expected? Both? Can you give us an idea of the magnitude of the difference between expected versus actual (less than 5%, more?)

    2. If the entire frame is settling, your supposition may be correct. If it's more localized, can you tell if it is localized to a truss member or if the connections are allowing the most movement?

    3. Have you received the 3rd party testing reports indicating all connections, and especially the moment connections, have been fully welded? (I once received a phone call that a connection I had designed was "failing", only to discover that the designed moment connection had not yet been welded in the field.) Similarly, do the foundation reports confirm bearing capacity was met or exceeded?

    4. Is the connection as detailed and the connection behavior assumed via modeling match? For example, are the truss connections in your analysis model designed as fully pinned (and the moment connections as fully fixed), and were the connections confirmed to be detailed this way, especially if this was a delegated design (common in USA East Coast)? Since we have to idealize connections in our models, I usually find that the actual "real" behavior in both to be somewhere in between, especially when it comes to deflection.  

    5. Was the construction sequencing/loading on the truss as expected? Are there any splices in the truss that could be contributing or unusual sequencing that could affect behavior? (For an example, if your truss is supporting a 4 story transfer column and the contractor decides to pour concrete on top floors first, this means the composite action you may have been counting on for the truss member is not in place when the upper floor concrete loads are applied and would result in higher deflections that if the <g class="gr_ gr_2652 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del" id="2652" data-gr-id="2652">truss</g> level was poured first.)

    Stephanie Slocum P.E., M.ASCE
    Engineers Rising LLC

  • 3.  RE: Initial Structural Frame Softness

    Posted 01-23-2019 02:32 PM
    Edited by Tirza Austin 01-23-2019 02:32 PM
    Hi, Colton.

    Are the loads expected service loads?

    I would check the fabricators pre-camber too, if it wasn't cambered enough it could cause higher deflections and higher stresses on the bolted connections.  My two cents. 

    Elliot House P.E., S.M.ASCE
    W.B. Clausen Structural Engineers
    Emeryville CA

  • 4.  RE: Initial Structural Frame Softness

    Posted 01-28-2019 04:07 PM
    Edited by Tirza Austin 01-28-2019 04:07 PM
    There was temporary bracing tied to the frames that was pulling it down and pre-loading the frame. As more load was placed on the frame, it was not deflecting much, if at all. This was because all we were doing was reducing tension in the temporary bracing. Most of the load is on the frame and the deflections are in line now. Thanks for all the thoughts everyone.

    Colton Simmons P.E., M.ASCE
    Nucor Building Systems, TX
    Terrell TX