Additional Storeys

[countervail]

Hopefully just a post to confirm I do know what I'm doing.

We have a project where a client has fit out a seven storey building. He now wants to pop two additional storeys on top.

We have been engaged to look at feasibility and have done some quick numbers and are happy with vertical load but lateral load looks like a problem, 22m high to 29m high, increases lateral load by 30% and overturning forces on foundations by 70% (unless I have badly erred).

We have gone back to client and said we know nothing about the foundations but this is likely to be an issue with the potential for remedial foundation work (just as an aside client says no records of the original construction are available, but I think he just can't be bother to look).

Client has come back and said he has two reports from other engineers who say it should be fine and don't mention lateral loads at all and he thinks we are just over emphasising minor concerns.

Have I gone bonkers (mad if you are in the US), but turning a seven storey building into a nine storey building is likely to give you an issue with lateral wind loads?

 

[JAE]

Yes

You should run away.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[POPEYES]

it should be fine and don’t mention lateral loads at all

Concerning... I suppose if you don’t look you can’t find anything wrong

 

[Agent666]

I've seen cases where for seismic load the loads actually get less due to the increase in period with increasing the height, but for wind you really cannot get away from the increase as the lateral load is proportional to the frontal area.

No one back in the day is logically going to over-design the original foundations for 70% extra load, so I'd say your calculation is correct (29^2/22^2=1.73 = ~70% increase) and the opinion that significant foundation work would be warranted is also correct, and much of this is not very practical to achieve in practice. I'd bet there are also many other areas of the building that would no longer work, not just the foundations. Addressing these would likely involve significant investigation/strengthening work, which when added up starts to make the additional two storeys less desirable from an economical perspective.

I'd state your concerns, and make a point of making sure the client understands the concerns are based on the lateral loads which is not considered by the other reports and ask that he reviews this with the other reports authors before politely declining any further involvement and walking away (no need to waste a run on these types). You got to ask why this client had 3 people look at the same thing, I suspect they are playing the field/numbers so to speak until someone agrees with their uneducated view of the world.

Another approach is to counter with what you believe can be achieved, a single extra lightweight level might be achievable for example combined with a site specific wind assessment, and may show no net increase in wind.

 

[POPEYES]

How cost prohibitive is wind tunnel testing? The couple tests I’ve seen have shown wind loads far under code. Perhaps a test would prove overturning loads aren’t much higher with the extra stories than what was originally designed for. Just an idea

 

[countervail]

Thanks gentlemen, very helpful to know I'm not mad and it is something to be considered¡