Pier Stiffness for High Rise Buildings 2

[theshearstud]

Just out of curiosity...

When modeling concrete core wall for a tall building, I would assume you would need to model the foundation stiffness, pier stiffness in compression, and pier stiffness in tension when checking serviceability requirements such as deflection and accelerations.

Is it typical to assume that the entire pier length is cracked under tension loads? So your axial pier stiffness in tension is effectively the stiffness contribution from the pier steel alone over its length? This seems to be quite conservative since by the numbers, large piers with much reinforcement will have a large effective area and can support quite a bit of tension load before cracking. Any thoughts?

 

[bookowski]

In my experience it is not typical to model soil-structure interaction for checking serviceability. The foundations are modeled as reaction points. If you include the spring stiffness of your foundation elements you will have an extremely difficult time meeting standard service requirements for drift and accelerations. The taller the building the more amplified the effect will be.

Otherwise yes, column type elements should be substantially reduced in stiffness for tension cases to account for cracking. I don't think there is a rule but I have seen numbers usually a bit higher than the effective area of steel alone but still highly reduced.

 

[KootK]

For typical midrise to commonplace high rise, I've seen I've mostly seen soil structure interaction ignored and concrete cores modeled simply as elastic sections with their stiffness modified in an attempt to account for cracking.

I've never been involved in the design of a super tall building. Based on the case studies that I've read, however, it seems common to include soil stiffness interaction. It's also common to have something akin to an outrigger system to increase the effective depth of the building and address the concerns that Bookowski mentioned.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[theshearstud]

bookowski and KootK

I appreciate the feedback.