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Re: Retrofit existing flat slab

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Stevenson:

A. I don't know if this will work for a zone 4 application or not, or even
if the as constructed details would allow it, but one possibility if for
the forces from the diaphragm to be transfered at the columns by bearing
of the concrete on the steel wide flange column.  This option may not be
feasible with the potentially high forces that you might be dealing with
from a zone 4 seismic design.

You other option, which would be rather expensive and a pain in rear, that
I can think of is to add headed studs by coring a hole, welding the stud,
and then grouting the hole.

Another possibility is to connect the beams to the underside of the
concrete slab with some post-installed anchors.  This would require either
drilling thru the flange of the beams from the underside (the contractor
will love you for that) or welding "connector" plates to the flanges that
already have some holes in them.

B.  One way to provide lateral support for out of plane loads (I assume at
the top) is to provide intermediate plates (or continuous if required)
bent plates or angles on one or both sides of the block wall.  Obviously,
if a bent plate or angles is supplied only on one side, then it needs to
be connected to the wall in some manner.  If two bent plates or angles are
provided, then they only really need to be attached to the floor above
(i.e. one plate or angle on each side of the wall).  Obviously, be sure to
allow for the vertical deflection of the floor above so that loads are not
taken into the wall.

I would take a hard look at the 6" dimension.  Even for an interior wall,
6" CMU spanning 40 ft vertically sends up a flare for me, especially in a
zone 4 application.  It might still work if I interpret things
correctly...that is it appears that your columns are spaced at 20
ft...assuming that the wall can be tied to the columns (be sure to allow
movement of the frame system relative to the wall...otherwise the wall
will likely attract force and act like a shearwall), then you could span
the wall horizontally to the columns (the wall will likely naturally take
the out of plane load this way anyway [if tied to the columns] since it
will be stiffer in the horizontal direction).

For anchorage for out of plane loads at the base of the wall, you best bet
is to epoxy rebar dowels into the slab or something similar.  All you will
really need is shear transfer, so the potentially limited embedment depth
should not be a overwhelming issue.

HTH,

Scott

On Mon, 30 Apr 2001, Stevenson Lim wrote:

> I have a project to retrofit a three story mezzanine
> concrete floor building that was built in 1954. Metal
> roof deck and walls. Lateral resisting elements are
> angle cross bracing at perimeter. Building located in
> Zone 4 and close to fault line. The square area is
> 21,600 square. ft each floor (60' x 360').Second floor
> elevation at 20'-10" and 32'-10" for third floor.
> These 5" thick concrete two-way slabs were supported
> by 14WF beams spaced at 10' o/c across 60' span and
> 24WF girders at 20' o/c. Two ends and one intermediate
> columns at every 20' bay carry the loads down to the
> footing.
> From the existing drawing, there is no welded stud on
> any top flange of the beams/girders nor metal deck for
> diaphragm shear transfer. The top of beam/girder
> flange was embedded 1/2" into the 5" thick concrete
> slab below. My main concern areas are as follow:
> 
> A. Shear transfer from these flat slabs to the steel
> beam and column bracing without any shear transfer
> elements. Any suggestion?
> 
> B. 40 feet high, 6" CMU min. reinforced in-filled wall
> between 24WF steel columns spaced at 20' o/c. Any
> suggestion for out-of plane anchorage?
> 
> I would appreciate if someone can share with me
> similar experience on this retrofit project or point
> me to the proper source of information that I can be
> enlighten on.
> 
> 
> Stevenson Lim, P.E.
>  
> 
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