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. 

Parents
  • Hi Modestas,

    As earlier issue is solved by restarting the software, hence please ignore earlier mentioned case.

    Query 1

    Further for the same project and at same beam i.e. (Beam no 36) I did analysis of beam by creating 3 cases and I observed different results in all the 3 cases. Kindly help me to analyse it correctly for practical application purposes.

    Case 1 - STAAD model is analyse using Floor Loading option (Here plate is not modeled, however Dead Load, Floor Finish Load and Live Load of Slab is included in Floor loading)

    Observation - Max sagging moment on Beam no 36 is - 454 KN-m ( Load case 3)

    Case 2 - STAAD model is analyse by modelling plate (Here meshing was not done and Dead Load of plate is not applied separately since it  will be included by applying self weight of the structure, Floor Finish Load and Live Load of Slab is applied  by plate pressure option)

    Observation - Max sagging moment on Beam no 36 is - 52.6 KN-m ( Load case 3)

    Case 3 - STAAD model is analyse by modelling plate (Here meshing was done and Dead Load of plate is not applied separately since it will be included by applying self weight of the structure, Floor Finish Load and Live Load of Slab is applied by plate pressure option).

    Observation - Max sagging moment on Beam no 679 (mid-span beam) is - 334 KN-m ( Load case 3)

    P.S - Loading condition is same in all the 3 cases mentioned above. Also, I have attached 3 model for your ease of working.\ purpose.

    Query 2

    Since, In case 3, after meshing of plate, beams are sub-divided into "n" no of elements which results in facing difficulty to analyse the each beam individually. Please help here to suggest how to overcome this situation.

    Assembly Hall Project 1 with floor loading.stdAssembly Hall Project 3 with meshing.stdAssembly Hall Project without meshing.std

Reply
  • Hi Modestas,

    As earlier issue is solved by restarting the software, hence please ignore earlier mentioned case.

    Query 1

    Further for the same project and at same beam i.e. (Beam no 36) I did analysis of beam by creating 3 cases and I observed different results in all the 3 cases. Kindly help me to analyse it correctly for practical application purposes.

    Case 1 - STAAD model is analyse using Floor Loading option (Here plate is not modeled, however Dead Load, Floor Finish Load and Live Load of Slab is included in Floor loading)

    Observation - Max sagging moment on Beam no 36 is - 454 KN-m ( Load case 3)

    Case 2 - STAAD model is analyse by modelling plate (Here meshing was not done and Dead Load of plate is not applied separately since it  will be included by applying self weight of the structure, Floor Finish Load and Live Load of Slab is applied  by plate pressure option)

    Observation - Max sagging moment on Beam no 36 is - 52.6 KN-m ( Load case 3)

    Case 3 - STAAD model is analyse by modelling plate (Here meshing was done and Dead Load of plate is not applied separately since it will be included by applying self weight of the structure, Floor Finish Load and Live Load of Slab is applied by plate pressure option).

    Observation - Max sagging moment on Beam no 679 (mid-span beam) is - 334 KN-m ( Load case 3)

    P.S - Loading condition is same in all the 3 cases mentioned above. Also, I have attached 3 model for your ease of working.\ purpose.

    Query 2

    Since, In case 3, after meshing of plate, beams are sub-divided into "n" no of elements which results in facing difficulty to analyse the each beam individually. Please help here to suggest how to overcome this situation.

    Assembly Hall Project 1 with floor loading.stdAssembly Hall Project 3 with meshing.stdAssembly Hall Project without meshing.std

Children
  • Case 2 is not correct at all, as plates have to be meshed. If the slab exists in a real structure, then I would go with Case 3. The bending moment in the beams in case 3 is smaller, because slab is taking some moment too.



  • 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.