lateral deflection

how can i check whether my column or beam has passed deflection check. for instance if span/180 is check for beam deflection how can i enter? should i use DFF command?
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  • Actually, deflection checking is based on the resultant Beam Relative Displacement values, so the use of DFF will correctly evaluate lateral deflections in addition to vertical deflections.  I am jumping into a class right now, but I will post an example after I get out.

    Cheers,

    Chris



  • Here is a simple model to demonstrate that deflection checking is basing its result on the resultant deflection.

    STAAD SPACE
    START JOB INFORMATION
    ENGINEER DATE 22-Dec-09
    END JOB INFORMATION
    INPUT WIDTH 79
    UNIT FEET KIP
    JOINT COORDINATES
    1 0 0 0; 2 30 0 0;
    MEMBER INCIDENCES
    1 1 2;
    DEFINE MATERIAL START
    ISOTROPIC STEEL
    E 4.176e+006
    POISSON 0.3
    DENSITY 0.489024
    ALPHA 6e-006
    DAMP 0.03
    END DEFINE MATERIAL
    MEMBER PROPERTY AMERICAN
    1 TABLE ST W21X48
    CONSTANTS
    MATERIAL STEEL ALL
    SUPPORTS
    1 FIXED
    2 FIXED BUT FX
    LOAD 1 LOADTYPE None TITLE LOAD CASE 1
    MEMBER LOAD
    1 CON Z 6 15
    UNIT INCHES KIP
    1 CON Y -10 180
    UNIT FEET KIP
    PERFORM ANALYSIS
    UNIT INCHES KIP
    PARAMETER 1
    CODE AISC UNIFIED
    FYLD 50 ALL
    DFF 400 ALL
    CHECK CODE ALL
    FINISH 

    The output file shows a dff value of 276.  The ratio of DFF/dff = 400/276 = 1.449 (the controlling ratio).

    The span of the beam is 360 inches.  360/276 = 1.304 inches.

    Check the Beam Relative Displacement Detail table, and note that the Resultant displacement at midspan is 1.302 inches.

    So to summarize, thedff value is being calculated based on the Resultant displacement, and the deflection check will work for lateral deflections as well as for vertical deflections.

    Cheers,

    Chris 

     

     



  • what is the dff value? and which factors will it govern.
  • The value of dff (lowercase letters) is a deflection result that is calculated by "Deflection Length"/Max Deflection. 

    • "Deflection Length" defaults to member length, so in the case of members that consist of multiple analytical members connected by intermediate nodes, it becomes necessary to define a "Deflection Length" by defining a DJ1 and DJ2 parameter to identify the two ends of the "Deflection Length".  In my example in the previous post, "Deflection Length" = member length = 360 inches.
    • Max Deflection is the maximum resultant deflection found to occur anywhere along the length of the member.  Whether studying a single member or a multi-segment member (using DJ1 and DJ2), the Max Deflection value will be normalized to remove the component of deflection that is attributable to the displacement of the end nodes of the member.  In my example in the previous post, the maximum resultant deflection = 1.304 inches.
    • In my example in the previous post, dff = 276 = 360 inches / 1.304 inches.
    • DFF is the user specifed limiting value of dff.  In my example in the previous post, DFF= 400.
    • For the purpose of a unity check, the ratio is calculated as DFF/dff.   In my example in the previous post, DFF/dff = 400/276 = 1.449

    When a code check is performed, the ratio of DFF/dff is calculated just as many other ratios are calculated for evaluating all of the applicable code clauses.  If the value of DFF/dff is found to be the controlling ratio, then it will be indicated as the controlling condition for that member.  If the value of DFF/dff is NOT found to be the controlling ratio, then STAAD.Pro will indicate something OTHER than deflection as being the controlling condition.  (Note that just because a member doesn't indicate deflection as its controlling condition, it does not necessarily mean that the member passes the deflection check.  It just means that something else had a higher ratio than deflection.)

    I hope this helps.

    Cheers,

    Chris



  • Thank you so much for your response, it really cleared up the air. one thing i wanted to ask is usually we simple compare the deflection with span/(some constant value), cant we just do this. for example i have 20 meter span. i put DFF = 180 so the value becomes 20/180 = 0.111m or 111 mm. if deflection exceeds this value the rafter or column fails?