Property Line Setback for tall buildings/Maximum inelastic response displacement

[leostr80]

Hello-
I am in charge of designing a hirg-hrise tower in California. The developer wants to use every square foot of the site. However, on one side of the site there is a restriction that a potential future development might happen. So as per ASCE 7-10, 12.12.3 the tower needs to set back by at least "Maximum inelastic response displacement δM from the property line. The current tower is 330 ft high, so allowing for max. 2ft elastic displacement (H/165) at top and considering Cd=5.0 and Ie=1.0, δM will be equal to 10ft. This is a substantial set back from the property line and conducive to loss of a lot of squad footage!!

The question is what else i can do to avoid this clause? Do you expect to achieve smaller values if you conduct an analysis based on inelastic response to design ground motions?

any help is greatly appreciated.

 

[KootK]

I'm surprised this didn't attract any responses. I'll do what I can.

The current tower is 330 ft high, so allowing for max. 2ft elastic displacement (H/165) at top and considering Cd=5.0 and Ie=1.0, δM will be equal to 10ft.

Per ASCE7, I believe that your maximum drift will limited to H/50 as opposed to H/33 as you've indicated. So that gets you down to 6.6' which is an improvement.

Do you expect to achieve smaller values if you conduct an analysis based on inelastic response to design ground motions?

This is often the case, yes.

The question is what else i can do to avoid this clause?

You can't avoid the clause but what you can easily do is simply design the structure to be stiff enough to control deflection as you see fit. del_M varies linearly with del_max, the elastic drift. And you can control your elastic drift by simply providing a stiffer structure. Therefore, you can control del_M. Stiffening the structure will cost the owner some money. It's up to the owner as to whether or not the extra SF is worth the trade. Even in quake country, H/50 is a lot of drift for a 330 ft building.




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.

 

[DaveAtkins]

I have never designed a high rise, but I thought elastic drift due to service loads should be limited to H/500.

DaveAtkins

 

[Retrograde]

I have never designed a high rise, but I thought elastic drift due to service loads should be limited to H/500.

You are likely correct but the maximum inelastic response displacement would be much larger.

 

[sandman21]

Where is the building located? For a building as tall as that you should be doing performance based design, giving the client a better building. You should start by going through the Tall Buildings Initiative/link]and [url=http://www.tallbuildings.org/PDFFiles/2014-LATBSDC-CRITERIA_with_2015_Supplements_Final_Rev2_08-14-2015.pdf]An alternative procedure for seismic analysis. The recommended drift limit is .5% of story height in any story, H/200. My guess is you are using Steel-SMF or Concrete-SMF so you have no limit on height and do not need to use any alternate guide procedures. If you have not done a PBD I would consider sub-contracting out the design to a firm that has done it before. You will have to pay, but the client gets a better prodcut and you can learn from them and do the next one without assistance.

 

[MrHershey]

Without addressing the actual question, would tend to recommend limiting deflection to less than that just for economic reasons. Your cladding is going to have to accommodate that deflection. If you're packing the floors in at 8-8.5 ft floor to floor you'd end up needing to accommodate an average (not max) of 3" of inleastic movement in the cladding from floor to floor if you're going to hit ten feet for a 330 ft building. For normal floor to floors, your deflection you'll have to accommodate in each connection is going to be even larger. While it can certainly be done (at least in theory), the cladding cost for deflections that large would be enormous.

If the architect (or whoever) is fighting the extra structure, may want to bring this up. Even if you can technically do it, it isn't free.

 

[thaidavid40]

You can also minimize the square footage lost by stepping back the building profile (building line) as you go higher, thus increasing the property line offset. Or you could slope the side gradually to match the required offset in a linear fashion.

Thaidavid