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RE: using existing structures

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Jason,

1. Is it possible to accurately determine the existing reinforcing in a
concrete beam?  I know concrete strength can be determined easily, but I
need the steel.

Yes.  Use a pachometer to get the general information but chip out a few
areas and measure the rebar directly.  Pachometers aren't as accurate as
advertised.  Also bear in mind that you need to determine the rebar steel
grade.  If you are lucky and chip out concrete that exposes the mill stamp,
you are home free.  If not, you need to pull a few coupons or assume 40
grade.  You can get an idea of the steel grade by the year of construction.
Older reinforced concrete beams used a lot of trussed rebar.  All the bottom
rebar will not run straight into the column.  Get an old CRSI manual to find
out what a truss rebar looks like.  

2. Is it possible to reinforce an existing concrete beam from ABOVE?  
Yes.  Remove the roofing and add concrete beams to the top of the existing
concrete beam above the roof thus increasing the depth of the newly created
composite concrete beam.  Connect the new concrete beam to the existing with
a properly scarified surface, and or drilled and epoxied rebar shear
connectors.  Calculate the horizontal shear at the interface to determine
how pushed you are in horizontal shear.

Regards,
Harold O. Sprague

-----Original Message-----
From: Jason Kilgore [mailto:jkilgore(--nospam--at)leok.com] 
Sent: Monday, July 07, 2003 6:02 PM
To: seaint(--nospam--at)seaint.org
Subject: using existing structures


I'm working with the renovation of an existing structure.  In general, it's
a 2-story concrete joist-beam-column building, constructed about 50 years
ago.  It is obvious that the building was originally designed for the
addition of at least one more floor.  I do not have any original drawings on
this building.

After finalizing all design drawings, the owner and architect have decided
that it would be "neat" to add light-frame 2-story penthouses to the roof. I
admit that it will look awesome after construction, however, the design has
now become a nightmare.

Setting aside all lateral upgrade problems (there will be several), my
current concern is supporting the gravity load from the new penthouses.  The
bearing walls cannot line up with the beams/joists, and one area has 34'
joist spans.  Because of the lack on information on the existing structure,
I was trying to avoid using the existing roof and relying completely on new
structure built on top of the existing columns.

The first floors of the penthouses have to be raised above the concrete deck
so the residents can see over the existing 8' parapets from their living
rooms.  The first plan was to build a new steel platform to both brace the
parapets and to support the penthouse floors.  The platform was supported
entirely by steel columns centered over the existing concrete columns.  Now
this will not work because the top of steel can't be higher than 1'-10"
above the top of concrete for various reasons (the residential floor level
is 2'-0" higher than this).  Some of the steel beams needed to be W21's to
span 34' with two stories of wall load, which obviously won't fit in 20".
I'm now looking at supporting the new structures entirely with the existing
concrete roof, with some possible reinforcing.

Finally, the questions:

1. Is it possible to accurately determine the existing reinforcing in a
concrete beam?  I know concrete strength can be determined easily, but I
need the steel.

2. Is it possible to reinforce an existing concrete beam from ABOVE?  There
is very limited headroom from the 2nd floor to the existing roof.  I was
thinking along the lines of adding a steel beam along the top of the
concrete beam and sharing the load based on relative stiffness.  Another
option would be to use epoxy/FRP reinforcing strips along the bottom of the
exposed roof beams, but this would be "ugly".

Any help would be greatly appreciated.  I'm going to call my contacts at a
few local testing agencies tomorrow.


----
Jason Kilgore
Leigh & O'Kane, L.L.C.
jkilgore(--nospam--at)leok.com
816-444-3144
816-444-9655 (FAX)

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