Hi all,1) Is there a way to set the ground level in STAAD Pro or is the y=0 level the permanent ground level? This is for the purpose of computing the seismic forces because, from what I know, distribution of the computed base shear per level of a structure is related to the distance of a level from the ground surface.For example, I have to design a 4-storey steel structure. The pedestal protrudes above the ground slab by a 0.5m and in modelling the the steel structure in STAAD, the support, which is the base plate connection, is above the ground slab by 0.5m.Let's say that the distance from the 2nd floor to the ground level is 3.3m so the distance that will be used in modelling the steel from the 2nd floor to the base plate is 2.8m.2) Do you neglect the 0.5m distance and just proceed in the modelling and assume the base plate connection as located in the ground?3) If yes, will this not have any effect in the seismic forces?4) If no, what do you do for this case?5) Do I miss something?Hoping to have some clarity regarding the matter.Thanks in advance everyone!
I would also be curious about this as it would be nice to model mat foundations in the same model as the structure above grade without having the mat foundation mass considered in the generated seismic loads and have the height considered for vertical distribution from ground level and not the mat foundation level.
In STAAD the ground level is not at Y = 0 but rather at the elevation corresponding to the lowest node (i.e. node with the smallest Y coordinate.) The program recognizes this ground level and uses it during seismic load generation when distributing the base shear to each floor of the structure according to the code specification.
In regards to the example you brought up, up you can really model the base nodes at either 0m elevation or 0.5m elevation and the seismic load generation will handle the vertical base shear distribution accordingly. In either scenario, the program will identify the first floor by virtue of the fact that there are nodes there. In other words, it doesn’t rely on a distance or height designation to find a floor, but rather identifies any floor by the presence of nodes.
So, whether you neglect the 0.5m distance and define the base nodes at Y = 0.5m, or you decide to account for the pedestal and put the base nodes at Y = 0.5m, in both cases the total seismic force will be the same. Similarly, the amount of force distributed to each floor will be equal.
If you wish to set up your model with both the structure and supporting mat foundation you can do so in such a way as to omit the weight of the mat from the calculation of seismic base shear. Simply set up the seismic definition so that the SELFWEIGHT is assigned to all the structural elements but not the mat foundation elements. In such a scenario the ground level will be at the elevation of the mat nodes, however as long as the seismic weight is accounted for correctly in the seismic definition the base shear calculation and vertical distribution will be correct.
Sorry for the late response priegel.
Thank you for your explanation. It really cleared things up. Just another question. You said
priegel said:In other words, it doesn’t rely on a distance or height designation to find a floor, but rather identifies any floor by the presence of nodes.
I just wonder if I can make the seismic load generation of STAAD skip some nodes for the vertical base shear distribution for reasons such as no actual floor exists at that point? or does STAAD distribute the base shear to any node it detects vertically?
priegel said:In such a scenario the ground level will be at the elevation of the mat nodes, however as long as the seismic weight is accounted for correctly in the seismic definition the base shear calculation and vertical distribution will be correct.
This is an interesting concept priegel. I would like to try to study this. Hoping I can get back to you if I have future queries. Thanks!