Modeling a Building Slab With SAP2000 - Property Stiffness Modification Factors 1

[CWEngineer]

To model the floor and roof in a building I was looking at the SAP2000 tutorials for guidance. The tutorial mentioned to set the f22 modifier = 0 in the Set Modifiers (please see attachment) because this prohibits in plane axial loads. Trying to get a better understanding of why this is required, appreciate your assistance in helping me clarify why this is required.

Also, there is no need to set any other modifiers to 0?

Thanks

 

[CWEngineer]

Forgot to mention that I am using the "Shell - Thin" type for the floors and roof.

 

[stressed]

Putting a f22 modifier of 0 makes no sense to me. Especially since f11 in the in-plane perpendicular direction has no modification factor. If the slab really acts in-plane like a diaphragm then why would you want to ignore that? Is there something about the detailing of slab connections that the tutorial is trying to explain?

Where did the tutorial come from? Was the tutorial created by CSI? If so, do you have a link?

If you want to design reinforcement for the slabs, you'll need to manually draw or define section cuts in order to obtain integrated shears and moments.. that, or export the slab into SAFE for design

 

[CWEngineer]

Here is the link, the discussion regarding the modifiers is in minute 21 of the video.

http://www.csiamerica.com/products/sap2000/watch-and-learn

Thanks

 

[stressed]

In the video tutorial from your link it's a sloped roof not a floor slab, and the person narrating the video explains that he uses a modifier so that the roof doesn't act like a flange with the top chord beams of a truss. Presumably he wants the roof to transfer gravity load but no in-plane load.

Normally in designing slabs you would not use modifiers unless you want to account for cracking by assigning modifiers to the moments of inertia. In-plane stiffness may also be modified, but typically it's the same modifier in both in-plane directions if the engineer for some reason doesn't want to fully take credit for the slab's in-plane stiffness.

Use Edit menu mesh or assign automesh options to control load distribution along with assign area load options to make the load act one-way or two-way. Section cut tool is useful for obtaining integrated shears and moments for design of reinforcement. SAFE is a great tool for slab design since it will automatically design top and bottom reinforcement as well as do the punching shear checks.

 

[CWEngineer]

Thanks stressed,

Just looked at the video and noticed, and as you mentioned it was for a slab on a truss. In my case, the floor slab is for a building and its on wide flange beams.

In the video the Membrane f22 Modifier is set equal to 0 to prohibit in plane axial loads and the stiffness factors of the roof area objects is modified so that they will not act as flange for the top chord of the trusses.

Just want to confirm, so for my case I will keep all the Property/Stiffness modification factors equal to 1?

Thanks

 

[stressed]

Since you mentioned that your slab is on top of I beams, is the slab connected to those beams via shear studs? Is it a metal deck filled with concrete, or is it concrete only?

 

[CWEngineer]

I am planning to use a composite beam design (shear studs with metal deck filled with concrete).

Would really appreciate it if you can provide some guidance regarding the Property/Stiffness modification factors for both the composite beam and concrete only, for my information.

Thanks!

 

[stressed]

Well if you're going to do composite design than the concrete filled deck should probably be modeled as "membrane" only without out-of-plane stiffness. SAP2000 does not offer composite beam design but ETABS does.

 

[CWEngineer]

So if I am planning to do a composite beam design with SAP (hand calculations since SAP does not do this), I would just set the Membrane f22 Modifier equal to 0 and all the other modifiers equal to 1? And if I am planning to just use a concrete slab, then the Membrane f22 Modifier along with other modifiers are equal to 1?

Really appreciate your help

 

[stressed]

As I mentioned before, a membrane area element has in-plane stiffness only. F22 is in-plane so unless you have something specific in mind, I would keep f11 and f22 modifiers as 1 or close to 1, not 0. You could use a membrane element, but since membranes have no out-of-plane stiffness, if you mesh them such that there is no support on any edge you can potentially encounter instabilities. Alternatively you can assign property modifier to "shell" type area elements in order to make them act like membranes assigning tiny multiplication factors (.0001) to all three bending directions. That tiny remaining stiffness often can prevent instabilities. Model the thickness to an equivalent uniform thickness to account for the deck ribs.

SAP will automatically account for weight and mass of elements modeled including your deck, so if you want to assign weights from a vendor table for the filled deck, you might consider assigning property modifiers to your filled decks to zero out the weight and mass which you substitute with assigned area load from vendor. You can have the SAP mass model based on loads instead of modeled elements and assigned masses.

SAP has some steel design assignments for camber and camber limits that you might be able to utilize for your design.

 

[CWEngineer]

Thanks so much your help stressed.