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RE: Concrete slab retrofit (fwd)

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Do what I have done in the exact same situation (the details are so
close it is almost scary). Assuming you have head room, construct a
frame on top of the slab that distributes the weight to three of the
joist. I used SS tubular steel members to construct a "mat foundation"
that went under the MRI and spanned 3 joists. The stiffness was chosen
to ensure that the load was actually spread out to all three members,
although the center joist did end up with a bit more than the outside
joists. Much, much cheaper in our case than working underneath b/c the
space below was a clean room.

Eric Green

-----Original Message-----
From: Scott Maxwell [mailto:smaxwell(--nospam--at)engin.umich.edu] 
Sent: Friday, December 20, 2002 11:50 AM
To: seaint(--nospam--at)seaint.org
Subject: Concrete slab retrofit (fwd)

Greetings!

I am forwarding a message to the list for someone that has had some
difficulty in getting it to post.  So here it is...


---------- Forwarded message ----------
Date: Thu, 19 Dec 2002 13:35:43 -0500
From: Andrew D. Kester <andrew(--nospam--at)baeonline.com>
To: 'Scott Maxwell' <smaxwell(--nospam--at)engin.umich.edu>

I have a questioning regarding the strengthening of concrete T-beams or
ribbed slabs.

Specifically, I have an old hospital building, and on the second floor
(3
with a basement) they want to put some MRI type equipment that ways
around
5000lb. Worse case is all the equipment will be lined up on one such
T-beam.
This currently stresses the beam to a level of 287%! So I am telling the
client the only way to strengthen this system is from underneath. I have
already checked the girders and there is lots of extra capacity there,
so
the beams are the main issue.

Here are the parameters and several ideas, but I would like to hear from
others on their experience. Also any specific "don't forgets" would be
welcome.

Span= 13.5'
Beam depth= 8"+ 3-1/2" slab/flange
Beam width= 6"
Spacing= 30" o.c. (24" gap in between each beam)

Load= Point/dynamic load = 5000lb, uniform LL, uniform DL (slab wt, MEP)
Seismic non issue.

We can and will put a steel plate underneath the equipment, on top of
the
floor slab, to spread out the load and to bridge over the flanges, on
top of
the flange.

Some of my ideas, in the order of best to worse (in my opinion):

1. Bolt channels to the sides of the beams, creating a sandwich.
-Should I then design it as a composite section, or could I assume
something
very safe load sharing between the channels and the concrete? Or will
the
large difference of stiffnesses make this incompatible?

My quick calcs show that if I assumed that the concrete beam carried
absolutely NO LOAD, that I could easily design 2 channels to carry all
of
the current and additional loads. I like this idea because this building
was
built in the 30s and I would rather bypass the current slab. This is my
favorite option. I would only have to reinforce 2 or 3 beams, so this
would
be rather cost efficient and easy to construct.

This means that the concrete beam must deflect before engaging the
steel,
correct? If I did my calcs right, the EI of the concrete beam is nearly
equal. I could oversize the channels to make the EIs about equal. Or
would I
oversize the channels to make the EI of the channels greater then that
of
the beam, thus ensuring a deflection  of the beam that would then engage
the
channels. I would ignore the concrete beams and composite action, they
would
merely be carrying the load to the channels.

2. Go in the space between concrete T-beams with W-beams, flush against
the
bottom of the concrete slab. But here we run into problems again with
load
sharing, and making sure that the steel beams are flush against the slab
and
are actually carrying part of the load. I do not want to jack the beams
into
place, so the best I could do is remove the LL from the slab prior to
installing them.

3. Composite steel plates- bolt and epoxy to beam to add additional
flexural
steel. I do not like this idea because I do not know all the properties
of
the existing concrete beams. It also makes limited improvements for
greater
or equal cost to above mentioned ideas.

4. Least favorite and most costly to construct- dowel into existing
beams
perpendicular to webs, forming and pouring beams in the gaps of the
existing
beams. This sounds like a construction nightmare.


I thank all of you in advance for your insight and suggestions, and hope
all
of you have a safe and happy holidays.

Sincerely,

Andrew Kester, EI
Longwood, FL





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