ELF Procedure Period Question 1

[Respawn]

I ran a ELF analysis on a model in SAP 2000(Special Moment Frame, h= 60ft). The base shear and global z reaction for each of the loadings is near what I calculated on my Excel. However, My period is different under the mode where I get 90% mass participation in each direction. The period calculated by the program is T= 0.39s whereas my excel calculations got Ta= 0.64s. Can I accept this?

 

[Agent666]

It should be fairly close for a concrete frame I would have thought, not double the period of some estimate, that's a significantly different location on a design spectrum in terms of the resulting seismic load.

I've seen people inadvertently double some component of the mass source and come out with weird answers that don't agree with hand calcs for example, or incorrectly entering E (out by a factor of 10) or stiffness modifiers. Hard to know basically with little tid-bits of information being provided. OP seems to have at least gotten rid of all the spurious modes at least.

Again though, I'm not sure how exactly the OP is estimating the period in excel. Is it simply using an empirical equation based on structure height, or something more detailed that looks at the actual stiffness. I didn't exactly get an explanation except it was done in excel, whatever that actually means only the OIP can elaborate on.

I'd also question the use of fully fixed restraints at the base (as appears to be the case form the latest screenshots), that's rarely achievable nor realistic.

 

[JoshPlumSE]

Is this a parking structure with inclined ramps? That's what it looked like in one of the images.

If so, then have you considered that the ramps might be acting as extremely rigid braces between floors. If you hand calc is assuming only moment frame stiffness at each floor, then this could easily explain the discrepancy.

 

[Agent666]

Josh, as I interpreted it, the discrepancy is the other way.

The model is more flexible than the hand checks, not stiffer due to building in any ramps without modelling any sliding joints.

 

[Respawn]

I think the discrepancy lies within the ramps. My excel sheet period was calculated using eq. 12.8-7. I attached my mass source inputs.