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RE: Cantilevered Steel Column Additions

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Karen,
I think you are asking for a lot of trouble if you try to keep the
structures independent as you then have to make sure there is sufficient
clearance between cantilevered columns (deflection or story drift) tha
the walls of the one story structure.

One way to address the problem is to support the gravity loads of the
upper two stories on a post and beam construction concealed in the walls
of the first story portion. This MAY reduce any foundation upgrades
needed other than the column footings. 

Next construct the second floor diaphragm independent from the first
floor ceiling/roof. I've done this on residential jobs were we were
putting in a second story and did not want to remove the original roof.
This assumes that your shear transfer is at the exterior walls of the
building which appears possible based on your aspect ratio of 20:50 or a
little greater than 2:1. You can pony wall the second floor diaphragm
above the first floor roof framing and transfer shear through the pony
walls (assuming a plywood shearwall system). You still need to assume
that there are two diaphragms at the second floor but the lower
diaphragm is already connected to the structure below. Be careful when
accumulating shear from the new roof down as you need to take into
consideration the mass of the existing room. As an alternative, you can
consider removal of the lower roof once the upper stories are in place -
raising the ceiling hight in the commercial area which may be desireable
- especially for plumbing and mechanical runs.

Finally, use the existing structure to transfer shear. If there is a
soft-story condition at the 20-foot side (I assume the 20-foot elevation
is the storefront) than you will need to add a frame or some other
lateral resisting system here. I don't think you will have much luck
with cantilevered columns as the first story shear from a three story
building will require a pretty large piece of steel unless you have
interior shearwalls that will help distribute shear away from the
open-front.

This is more conventional than trying to keep the structures separate.
The one thing I never asked is how would you gain access to the second
and third floor if not through a stair in the first story. If this were
the case, how to you make a positive tie to the two diaphragms to keep
the stairs in place during lateral motion? I think this is a nightmare.

Hope this helps - I think it is more ecconomical in the long run.

Dennis S. Wish, PE

> -----Original Message-----
> From: Casano, Karen [mailto:Karen.Casano(--nospam--at)dgs.ca.gov] 
> Sent: Thursday, July 05, 2001 12:15 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: Cantilevered Steel Column Additions
> 
> 
> I'm not a frequent contributor to this list, but I hope I can 
> still get some input from some of you:
> 
> I was approached by a contractor about helping him design a 
> second and third story residential addition over top of an 
> existing single story small office (20' x 50', standard wood 
> construction).  Because the existing structure and foundation 
> are of unknown (and questionable) quality, he wants to 
> consider designing the new structure up on cantilevered steel 
> tube columns, outside and structurally separate from the 
> existing building (leaving the existing flat roof in place).  
> I don't have experience with this type of system for an 
> addition, though I know it's used.  It seems viable from an 
> engineering standpoint, but I was warned by an architect 
> friend that this type of construction could create 
> moisture-proofing problems because the upper structure is so 
> much more flexible than the lower structure.  Even light 
> winds, or a heavy truck passing,  can induce differential 
> movements and break waterproofing seals. 
> 
> I'm wondering how this is handled normally.  Can separate 
> building membranes be installed without creating an 
> architectural monstrosity?
> 
> If I were to design the steel columns to take all the design 
> lateral loads, but combine the systems(locate the columns 
> within the walls and tie together), this would lessen the 
> impact to the building membrane under light lateral loads, 
> however, there would be some stiffness compatibility issues 
> between the columns and the existing walls.  The existing 
> walls would have to rack a certain amount before the columns 
> would even be contributing to the lateral force resisting 
> system.  How far can I expect that a wood wall could rack 
> before it has lost vertical load carrying integrity?
> 
> This is San Diego, zone 4, 70mph wind speeds.  
> 
> Karen Casano. P.E.
> San Diego, CA
> 
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