SAP2000: Modeling Roof and Floor in Building (SMRF) 1

[CWEngineer]

I am designing a special moment resisting frame and I am using the SAP2000 - 01 Introductory Tutorial (

https://www.csiamerica.com/sap2000/watch-and-learn#page=page-1)

as a guide. Based on this tutorial, it mentioned to create the roof of the building as a shell.

However, when I run my model it does not look right (please see attached). Basically, only the girder on one side of the building is deflecting, while the girder on the other side is not deflecting at all.

Could this be because I am using am modeling the roof and floor as a shell? Should I be modeling it as a plate or rigid diaphragm?

Appreciate your help

 

[CWEngineer]

I divided the shell elements in the roof into small elements using the procedures provided and I still getting the same results. I divided shell into 9 by 9, so the element could coincide with the composite beams. I am attaching the file, hopefully you have time to help me figure this out.

Thanks

 

[IngDod]

What version of SAP2000 are you using? And what you want to do with the roof and floor? you want them only to transfer loads to the beams or you want them to design, say a concrete slab? If you want them only for load transfer I would recommend using the "none" area section then you apply the loads to the area as one way and they get transfered to the beams.

 

[CWEngineer]

The version of SAP I am using is 15.0. My understanding is that SAP does not perform composite beam design, so I was planning on performing independent composite beam calculations.

I plan on designing the beams, hence I want to transfer the loads appropriately to the beams.

I am also planning on obtaining the story drifts by inputting earthquake motions, so I want the roof and floors to behave properly, and not like they are currently shown on my model.

Thanks

 

[bodi79]

I ran your model, and I see all beams deform well. Before I ran it I merged all the joints. I don’t know if that did difference or not, but you can try it.
On the menu bar, go to Edit, select edit points, then select merge joints. Finally enter a small value (for example 0.01).
I hope this will help

 

[IngDod]

Sorry it took me so long to reply. For your two concerns:

1. Load transfer: You can use an area of type "none", this will give you no load on the beams since the area has no material. After you draw the are you assign a load to it.. the trick is to assign the load as one-way distributed to frames, this way the loads will only transfer to beams which are oriented in a certain direction with relation to the area object... In the case of steel decks you want the area to load only beams that are perpendicular to the deck. After you assign the loads you can check how they are distributed by going to "display" and then area and then load assigns, on the window choose the load case to display and then check the option for one-way loads. In a 3D view you should now be able to see the loads distributed over the frames. If the loads are not distributed to the frames you want, you will have to rotate the area object.. you do this by selecting it and going to assign and then local axes.. assign a 90 degree angle... check again to see if the loads are being distributed properly. I suggest you verify this with some hand calcs to make sure you got it right... this usually quick and easy for simply supported beams. This will save you the hassle of having to distributes the loads manually to each beam. Again, I stress the need to double and triple check that the loads are being distributed correctly.

2. If I understand correctly, you need to assign a diaphragm constraint to the joints... to do this you select all the joint in a floor (make sure they are on the same floor.. dont add joints from different floors to the same constraint) you go to assign... then joints and then constraints... here select diaphragm and then create a new one... repeat this for every floor. You must make sure that this assumption is valid for your particular structure... I have used this in the past for steel decks. For flexible diaphragms, such as roofing and such, this is not valid as this systems are always considered to be too flexible. By doing this you are assuming that all joints in the same floor will move in X or Y together... so you will get equal deflections in X or Y for all joints within the same constraint. Check the mode shapes to see if they are as they should be... you should be seeing movement of the structure as a whole on the firsts shapes. Again, you should validate your results... Its easy to make mistakes when using computers.. and after you run it it all looks nice and pretty...

What kind of earthquake loads are you applying? response spectrum?

 

[CWEngineer]

bodi79,

I tried following your directions and I am still getting and awkward deflection. This is what I did, can you let me know if I missed something:
1. Showed the 2d-view of the building (roof)
2. I selected all the joints of the roof.
3. On the menu bar, went to Edit, select edit points, then select merge joints. Finally enter a small value (for example 0.01).

Ingdod,

I tried following your directions and I am still getting and awkward deflection. This is what I did, can you let me know if I missed something:
1. Showed the 2d-view of the building (roof)
2. I selected all the joints of the roof.
3. Assign/Joint/Constraints, selected diaphragm then click on "Add New Constraint", Constraint Name = Diaph1, Coordinate System = Global, Constraint Axis = Z and did not select "Assign a different diaphragm constraint to each different selected Z level"
Note: That I did these procedures on the roof in which I had already made the roof as a shell.

Right now I am just running the case with the dead an live loads.

Appreciate your help.

 

[IngDod]

Don't use a shell, unless this shell has the actual properties of the real roof... which is hard to achieve for a steel deck. Remove the shell and add a null area object like I mentioned in the previous post... Having a diaphragm and a shell is redundant... if you have a shell it will work as a diaphragm for the structure. Also the diaphragm is not going to have any effect on deflection due to live and dead load.. it will have an effect when you do lateral loads.

 

[CWEngineer]

My floors and roofs are going to be concrete with Verco Metal Decking, that is why I was using shells because I was have the concrete strength and thickness. Also wanted to apply the uniform dead and live loads to area section.

I deleted the shell on the roof and followed Steps 1, 2 and 3 in my thread above. When I do this, there is no more roof shown (please see attached picture). I don't see how I will be able to add the uniform dead and live loads to the roof.

Really appreciate your help, in trying to figure this out.

 

[IngDod]

Strange... Can you attach the .sdb file so i take a look? It seems to me the area was not defined for some reason. Metal decks usually are designed and work assuming that the deck acts as positive reinforcement for the slab.... If you define the shell using only concrete thickness its not going to behave according to reality.. You would have to find the stiffness of the deck in each direction (good luck with that..) and modify your shell properties to match.

 

[CWEngineer]

Attached is the .sdb file.

I was planning to model the concrete diaphragm in SAP, to transfer the loadings and design the beams and girders. Then was planning to perform the composite beam design using Enercals. Is using SAP, not really appropriate for designing my building? Or do I really need to use ETABS?

THANKS

 

[bodi79]

I meant to merge all the joints of the structure, not the roof only. But this problem usually happens when you import part of the structure from other file or from the sap existing files. Such as when you import spherical dome to existing cylindrical tank. In that case the two parts may not not merge together, though the joints in the same locations.
I feel this is not your problem. I don't know why when I ran your model, it seemed well to me. I'll try to look at it again and if I catch the problem, I'll let you know.

 

[CWEngineer]

Thanks for your help.