Earthquake design 3

[CCEV]

when designing a structure for earthquake, would the earthquake forces be higher at the top of the structure or at the base? or would the earthquake loads be dependent of the dead loads on the specific floor? thanks

 

[Qshake]

The story forces are dependent on the story mass and coincident acceleration that is driven by ground acceleration. However, when ascertaining base shear or the overall overturning moment, we are looking at the total effect of those individual forces. As such bottom stories see the largest force response while the upper stories see the largest displacement response. This assumes, of course, that there exist a linear elastic force displacement relationship.

 

[NigelM]

You can see it yourself when you calculate the forces following the UBC procedure to calculate the seismic forces of a building. You will see that the forces in the bottom stories will be higher than those in the top stories, and just like Qshake mentioned, the top stories will have larger displacements.

Nigel

 

[NigelM]

Oops!!!! SORRY, I WROTE UPSIDE DOWN!!!
THE SEISMIC FORCES WILL BE HIGHER IN THE TOP STORIES!!

NIGEL

 

[Qshake]

Well, the point to make here is that when summing for the base shear, the forces will be larger in the bottom stories. Additionally, since the top stories are transferring the force to the base each downward story will have to transmit its force plus the one above it, etc.

When we look at the story shear that is influenced only by two items mass and acceleration. Atop a building, with smaller members as building design practice dictates, the displacements are larger as are the accelerations owing to the less stiff arrangment. Less mass more acceleration. Near the bottom, more mass, less acceleration.

But because load path is important and the forces are transmitted to the foundation the ultimate seismic forces at the bottom of a structure are larger.

 

[CCEV]

thank you all for your suggestions. they have been very helpful. i was just wondering though, how would a foundation take the horizontal forces? aren't foundations designed in such a manner in where it only takes vertical forces?

 

[Qshake]

Abosolutely not! Foundations must be designed to resist vertical, lateral, and any overturning forces. In short, the foundation must be designed to resist all of the forces acting on the structure.

 

[PBroad]

Following the Northridge Earthquake in 1994 work at UofC Jacob's school of Engineering has studied post-tensioning to produce weld-free steel frame buildings. This would significantly reduce surge transference and dampen the potential damage. Contact Andre Filiatrault for detals at University of California San Diego .

 

[revank]

In the last month or so I saw an article someplace that discussed the Statistical Analysis resuls of seismic events in a certain area. It was thought the analysis might lead to a way to make a slight guess on the possibilities of future quakes in that area.
Now I cannot find that article. I thought I had saved it, but if so it is so well saved it cannot be uncovered. Has anyone else seen that article and if so where in the world is it??
Thanks
RevanK

 

[pigdog]

RevanK,
Check out this site for earthquake design.

http://www.bssconline.org/

What you may be remembering is that the USGS has published maps of probabilities of lateral accelerations. FEMA has incorporated this information into their design reccommendations, and it also shows up in the Army Corp site, and the ATC-49-1 report:

http://www.atcouncil.org/

The end result is that bridges with piles in soft silt and clay will be designed differently than bridges on rock, even though they may be in the same seismic zone.