Difference between Seismic Weight and Gravity Load 2

[civilsf1]

Hello,

I have recently started working as a Structural Engineer, and I am sorry if this sounds as a really stupid question.
I am trying to put in the values of Seismic Weight in Etabs model for a 3-Story building. Can any one please guide me towards what is the difference between Seismic Weight and Gravity Weight?
Please correct me if I am using the wrong terminologies.

Regards

 

[NLefever]

Seismic mass would be dead load and tributary partition loading as applicable, gravity weight would include dead load and live load, and partition loading.

Depending on your type of design you may also need to consider portions of liveload if applicable to high density storage (think 125-250 psf storage loads, I've designed mezzanines where this LL storage load was required to be included in the seismic response since it is likely to be stationary and exist long term).

Your Deadload and corresponding seismic mass should be based on psf for your areas, and a line load of tributary wall weights (from above and below) at perimeters. Your partition seimic load will be an area load as well.

Just make sure to update all your load combos and seimic criteria in ETABs accuratley, as those can sometimes be overlooked. ETABS is not quite as automatic as one may think when first learning the program.

 

[civilsf1]

Hello all,
Any suggestions on, "How do you brace the top and bottom flange of the Beams/Girders in ETABS? For Girders I need to brace the top and bottom flange at the points of Beam.
For beam it's a composite beam, so just need to make sure that the top flange is braced."

 

[TDIengineer]

Mass is what ETABS is using for determining bldg mass and your seismic lateral load. Gravity is your dead load used for gravity design. Why would these be different you may ask yourself?
Well, for example partition is now a 15 psf live load but you also need to include 10 psf in your mass. So, if your floor was 50psf DL with a 15 PSF partition live+ 40 for classrooms (let's say) then DL=50, LL=55, and MASS=60.

Another example where you mass input may be different than gravity would be for including your exterior wall mass within the model. Let's say you have an exterior wall that is supported below, but runs by a raised slab with a slip connection. You would model a zero gravity line load for the exterior wall, but the mass would be the tributary wall height x its weight since that diaphragm will see that wall mass.

I hope this makes sense.

If you're having trouble setting your unbraced lengths in ETABS you really should talk to a more senior engineer. The members should have a parameter where you can set your own unbraced lengths like 0 feet for your purlins (since deck is there) and your bottom flange bracing for your moment frame beams will need to manually assigned.

Lastly, you may want to create just a model with your lateral elements only. I don't like how ETABS handles the gravity elements, especially for a composite floor system. Hand calc those beams or use Ramsteel. Then just include your gravity beam mass as a unit load per sqft across your floor to capture those masses.

Also, keep track of whether you have member self weight being included in the mass. Really double / triple check your calculated mass from etabs with a quick hand calc. You don't know how many times I have seen floor masses being double dipped, or frame beam and column masses not being included depending on how your model is set up.

contact CSI for your etabs questions unless your company didn't opt for that. It is really helpful to talk to their engineers about modeling tips etc..

Regards,