Difference in Moment and Reinforcement details using Track parameter.

Dear All,

I am performing beam analysis result and getting difference in max moment value and reinforcement detail when analyzing is performed using Track 0 and Track 1&2. Kindly look  into the case and help me to resolve it.

Query 1 - In Beam no 36 analysis, max moment is coming at section 5 m is 483 KN-m when checked in post-processing mode  and at same beam it is coming max  at section 4.17m ( 467.58 Kn-m) when checked in output viewer using Track 1. 

P.S. -  Max moment value is observed at Load Combination 3 in both the cases.

Query 2 - At same beam i.e Beam no 36, at sec 5 m, reinforcement is coming (3 -25D) and at sec 4.17 m reinforcement is coming ( 5-25D) at bottom face. however, as per my understanding max reinforcement should come at section 5m. 

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  • Hi Modestas,

    Please help me to clear few doubts - 

    1. Is there any way to check the moment value of slab in STAAD ?

    2.  What should the most practical approach to design beam Case 1 or Case 3 ?

    Also please comment on Query 2 asked earlier which is related to Case 1 design. Is it correct way to design beam to avoid sub-division of elements of beam by meshing of plates ?

    3. Manual calculation of moment in beam no 36 (IN STAAD) which is denoted by Beam no 19 in manual calculation is 625 KN-m, which is much higher from both the cases i.e Case 1 and Case 2. 

    Here my question is - Does STAAD transfer one-way slab load action to beam from right and left same (Rectangular load pattern) as the design steps mentioned in the attached pdf below.  If yes, I am not able to understand the huge difference in moment value under the same loading condition in STAAD as of manual calculation ?PDF

  • 1) You can check bending moment in the slab by checking the Plate Stress graph (Mx and My local moments) or by going to Results Along Line option.

    2) If the slab exists in the real structure, then I think Case 3 is more accurate to design a beam, because slab stiffness is accounted for. If you model the loading without the slab and by using the floor load, the slab stiffness is not accounted, so the results are not so accurate.

    3) The force distribution on the members depends on various parameters, like member stiffness, boundary conditions, etc. So distribution of bending moments of such 3D model is much more complex (and accurate) compared to this manual calculation.



  • Hi Modestas,

    I agree that result can vary and the variation upto 5-10 % is considerable. Here in our case, variation is more than 25% compare to manual calculation is not considerable (after meshing of slab which accounts stiffness, boundary conditions factors also).

    Please help me to understand the role of stiffness and boundary condition while calculating the moment in Beam due to slab load as my understanding says - Stiffness is Moment of Inertia and it will not affect beam design whether we model plate or not (as primary function of slab is to transfer total load coming on the slab to beam) . If it was the case of slab design, I could agree with your statement of stiffness. 

    Further, to check the software viability, I model same structure  with different -different condition's (Floor Loading and Plate pressure on slab by meshing) and analyse 4-5 beams at different -2 location and observed difference in the value of moment of beam at same location under different conditions and under same loading conditions. Hence, My conclusions states bugs in the software.

  • In your attached document, the bending moment is calculated considering a simply supported beam. If you would model a single beam in STAAD.Pro and assign pinned supports at its ends, you would get the same bending moment for the applied loading as in the manual calculation.

    Moreover, this is a 3D model where the forces are being distributed between the members depending on their stiffness. If you change a stiffness (section properties) of one member, it will have an effect of he forces distribution for all structure. This is expected in the statically indeterminate structures.

    To better match the results with manual calculations, you need to assign member releases for the beams in question. Also, you should use floor load and not model plates (as manual calculation does not consider plate). The bending moment will be much closer to manual calculation. Again, the difference will be, because STAAD is calculating this as a 3D structure where forces are distributed depending on their stiffness and this is much more accurate than in manual calculation.

    If the structure would be modeled with a slab, then some of the applied forces would be resisted by the slab as well, hence the moments in the beams would be lower.



  • Hi Modestas,

    To better match the results with manual calculations, you need to assign member releases for the beams in question.

    Member release is the concept applicable only in steel design, as the degree of freedom varies whereas degree of freedom in RCC in "0". Therefore, we can't consider the member release on the ends.

    Further, I performed column design in two cases -

    Case 1 - When member were not released 

    Observation - Column is failed under section 230 x 230 mm and passed under 230 x 380 mm.

    Case 2 - When member were released 

    Observation - Column is passed under section 230 x 230 mm (Manual calculation adopts column size 230 x 230 mm)

    Kindly help me to elaborate the different section design in two above mentioned cases as in practical schenerio Case 1 is more accurate (as member release will not applicable in practical situation specially in RCC).

    Under Case 1 (There will be transfer of forces and moment on Column due to beam which seems correct for practical point of view) whereas in Case 2 (By releasing Beam, we are not allowing the transfer of moments due to beam on column, hence column is passed under 230 x 230 mm) which seems incorrect approach from practical point of view.