Difference in Moment and Reinforcement details using Track parameter.

Question

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.

Modestas · Answer

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.

Modestas · Answer

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.

Modestas · Answer

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.

Modestas · Answer

In the example which you attached, the maximum moment is calculated as wL^2/8, which is the formula used for simply supported beam. This is why I suggested to user member releases, to get the closer results to manual calculation. I agree that in most of the cases, member releases in concrete members are not used. It is up to an engineer to decide which way to go. This just illustrates the boundary conditions (in this case the assignment of member releases) has a great impact on the analysis results (in this case bending moments) and this is not a bug in the software.

Modestas · Answer

In the manual calculation which you attached it is considered that the beam is a simply supported - this is a simplified approach. If you want to get closer results to those manual and simplified calculations, then you should assign member releases at the ends of the beam. If you assign the member releases, the moments are not transferred to the columns, so the lower dimensions of the columns are sufficient. 
 In the real structures usually monolithical concrete beams and columns are connected rigidly, so no member releases should be assigned. In that case, the moment at the middle of the beams gets reduced (so differs more compared to manual calculation), but some moments are transferred to the columns and that's why a higher dimension column is required. 
 If your question is how to get the bending moments closer to the manual calculations which you attached - then the answer is to assign member releases at its end. If you question is how to model a connection between the monolithical concrete beam and column - then I believe member releases should not be assigned. Of course, you as an engineer should decide if the member releases should be used or not, depending on the actual structural condition.

Modestas · Answer

To make Forum posts more simple, it is better to keep one Forum post to one specific quetsion or issue. Different type of issues should be posted in a separate post. In this case, it would be easier for other users to find the answers if they would be facing the same issue. 
 Comming back to your question - yes, the slab design results are reported with respect to the local axis system. The reinforcement required to resist the local Mx moment is denoted as longitudinal reinforcement and the reinforcement required to resist the local My moment is denoted as transverse reinforcement. More details can be found in STAAD.Pro Help section Design > D. Design Codes > D1. American Codes > D1.F. American Codes - Concrete Design per ACI 318 > D1.F.6 Slab Design.