Are buildings allowed to drift past their property lines? 7

[KootK]

In a perfect world, I would like it if no building ever drifted (seismic or wind) beyond it's bounding property lines. That would keep things simple. So long as future buildings followed the same requirement, we'd have no issues with pounding etc.

I have, however, worked on numerous, neighbor-less buildings where the edge of structure was literally right at the property line. That obviously implies that these buildings would drift over their property lines under wind and seismic actions.

So my questions are:

1) Should building drift be limited such that buildings do no cross their bounding property lines under wind and seismic?

2) Do we typically assume that existing, neighboring buildings have been designed not to cross the property line under wind/seismic? Typically, I have not.

3) Do we typically assume that future, neighboring buildings will be designed not to cross the property line under wind/seismic? Typically, I have not. Rather, I have assumed that future building designers would have to estimate the drift in my building and deal with it regardless of the property line location. I haven't liked this assumption.

4) When considering the pounding of neighboring buildings, you're allowed to pull the SRSS statistical voodoo to minimize the required gap. How would one adapt that provision to a case where the goal is not avoiding contact with an adjacent, mobile structure but, rather, contact with a stationary, vertical plane?

Yes, it is shameful that I've gotten this far without ever having resolved these issues.

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.

 

[msquared48]

If we allow adequate gaps between the buildings, the firefighters will be rescuing more stuck cats.

Mike McCann, PE, SE (WA)

 

[DETstru]

Structural separation was first addressed (I believe) in the 97 UBC. It was then incorporated into the following IBC's

Today, ASCE 7-10 addresses it in section 12.12.3 Structural Separation.

1) Yes, building drift should be limited to avoid crossing a property line.
2) If the building is already on the property line, it will definitely cross in a seismic/wind event. Theoretically, buildings designed after 2000 or so (in the US) shouldn't cross that line. I don't think this is really true in practice, especially for shorter structures. Fortunately it's not as big of a deal for those. Most tall buildings I've come across have accounted for this appropriately (especially west coast).
3) Your buildings should be placed to avoid crossing the line (it's code required), so should theirs.
4) I believe you're only allowed to do the SRSS "voodoo" if it's two structures on the same property. Structures on a shared property line must be set back by the full inelastic displacement.

 

[jakordas]

Buildings should not drift over the property lines. That line not only is on the ground but it technically goes infinitely upward.

 

[appot]

goes infinitely upward

Shouldn't the property line perimeter get larger as you go up?

 

[Teguci]

Add to the list of questions - When designing an adjacent structure, how do you calculate the seismic deflection of a building that wasn't designed for seismic loads?

This was the case for a parking garage expansion at an airport. The old garages were BOCA wind load only with the new garages designed to seismic. Only expansion joints separated the structures.

 

[MotorCity]

Although I can't speak to the legality of whether or not a property line extends infinitely upward, that has always been my assumption.

If your building is shorter than both adjacent buildings, I would imagine there would be no drift from your building due to wind. Only seismic drift may be an issue.

If there is an existing building at or near the property line, in my opinion, you should be entitled to construct your building within the same distance to the property line. Then, it may become an issue of which building "reaches further" across the property line.

If both owners were also structural guys, they would probably consider connecting their buildings and reducing the drift of both buildings.....alas, a perfect harmony.

 

[DETstru]

Teguci,
I would probably just assume the max allowable inelastic drift from ASCE 7.

MotorCity,
FYI- ASCE 7 explicitly does not allow shielding by other structures or topography to be utilized to reduce wind pressures or drifts (unless you go through the wind tunnel procedure).

 

[KootK]

Add to the list of questions - When designing an adjacent structure, how do you calculate the seismic deflection of a building that wasn't designed for seismic loads?

ASCE7... one of the things that I miss most about the good old USofA. I've no wish to restrict the discussion to analogous Canadian provisions. However, if anyone can point me to any, I would be grateful.

Add to the list of questions - When designing an adjacent structure, how do you calculate the seismic deflection of a building that wasn't designed for seismic loads?

You've beaten me to the punch here. When I posted this, I had it in mind that I would create a seperate thread regarding the level of detail that one should go to in evaluating the drift for existing structures. I'll still do that.

Another issue, that also deserves a separate thread, is the role that erection tolerances play. A building will have a "baked in the cake" drift as a result of erection tolerances. Technically, to stay on side, you'd have to add that to whatever seismic/wind drift is estimated.

I would probably just assume the max allowable inelastic drift from ASCE 7.

This can be impractical in low to moderate seismic zones. H/40 seismic is obviously a lot more than H/500 wind. In my usual area of practice, most buildings have been designed for the H/500. As a consequence of that occurring in a low seismic demand location, seismic drift is often much less than H/40.

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.

 

[KootK]

Shouldn't the property line perimeter get larger as you go up?

Should it really?? No joke? I've never heard of this but two people have starred it so far so I'm starting to think that you might be on to something. If property lines are concurrent at grade, doesn't this mean that they'd overlap at higher elevations.

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.

 

[Archie264]

I assumed that was in reference to the earth's curvature. If so, interesting as an academic point but not relevant to building design.

 

[jayrod12]

That's how I imagined the "joke" as well.

 

[KootK]

Phew... Pretty funny in retrospect. Unfortunately, my sense of humor goes out the window when I'm in serious information collection mode.

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.

 

[JStephen]

Now I understand why they have to make those skyscrapers tapered, it's just drift clearance driving that.

 

[msquared48]

Yea... It's cause of all the big fat cats at the higher levels.

Mike McCann, PE, SE (WA)

 

[DETstru]

Yea... It's cause of all the big fat cats at the higher levels.

Those are just the tuned mass dampers.

 

[bookowski]

Not relevant to you but to add to the discussion - NYC Building Code only makes you provide 1" per 50ft of vertical height - which is just about never sufficient to limit your drift to within your property. It's an interesting concession to developers worried about every lost inch. Otherwise it's what DETStru said for in the US.

 

[Agent666]

In this part of the world we are only allowed to drift over the street boundaries and not adjacent properties.

 

[kingnero]

Shouldn't the property line perimeter get larger as you go up?

This would also imply that for basements or underground floors, your property line gets smaller.
As demonstrated by the upside down skyscraper project in Mexico City...

 

[MrHershey]

Actually discussed this on a recent job of ours. Nothing next door now but developer of course wants to use every square foot of the small site that they can. We concluded that it would be prudent to stay on our side of the property line even during earthquakes. Technically the limitations are in the seismic chapters, but we decided it would likely be prudent to apply the same limitations to hurricanes even though you wouldn't think too buildings would be blowing in exact opposite directions.

IBC 2009 included some language specific to this for the first time:
'Where a structure adjoins a property line not common to a public way, the structure shall also be set back from the property line by not less than the maximum inelastic response displacement of that structure.

Exceptions:
1. Smaller separations or property line setbacks shall be permitted when justified by rational analyses.
2. Buildings and structures assigned to Seismic Design Category A, B, or C'

ASCE 7-10 then picked up this same language, but removed exception 2 and IBC versions since 2009 don't include this portion and just reference ASCE 7-10. So as of right now in the US you need to be set back from building line at least your inelastic displacement unless you can convince the building official (and any potential future judges and/or juries) otherwise.