Subject: Use of precast conc. columns to support steel beams and other iss ues
From: Albert Meyer <Ameyer(--nospam--at)martinaia.com>
Date: Thu, 31 Jan 2002 09:33:31 -0500
Fellow engineers -
Warning! Long message!
I am hoping that some of you with more experience than me can help educate
me here. I have a building that is a three / four story CMU bearing wall,
precast hollow core floor plank structure. It has a common area in the
center where steel tube columns support steel beams that the plank bears on.
This condition exists because of the openness of the plan. In the short
direction the CMU walls are shear walls and in the long direction cold
formed steel shear panels with diagonal straps are used. The floor levels
are topped with a 2" structural topping.
I have tried to logical provide columns which seem to me to be logical for
the material used, for example:
1) To support plank at the corridors I've typically used either a
double angle or steel beam, which bears on a masonry column integral with
the wall when possible, when this didn't work I used CIP concrete columns
placed integral with the CMU wall, this seemed logical to me.
2) I supported steel beams with steel columns whenever possible and
where steel beams were required to be supported at CMU wall locations, I
provided a concrete column with embed plates so that even when the
connection was eccentric you at least had two perpendicular CMU walls which
were tied to the CIP column.
The construction manager is now "value engineering" the project since all of
"this concrete floor topping and CIP concrete columns is way too expensive
and in New York City we've built hundreds of these kinds of buildings and
never use 2" concrete topping or precast concrete lintels over windows".
I'd like your opinions on a few issues.
a) I can look at the floor system without a structural topping because for
the design loads (this is primarily a residential building with 40 PSF live
load and a dead load of 100 PSF if the floor is topped with 2" concrete,
about 75 PSF dead load for untopped) the spans work without topping, I just
need to verify what changes if any I need to make to detailing for diaphragm
action of the plank to work since the prior assumption was that the
structural topped was used as the diaphragm. I know that topped plank will
distribute loads better and probably improve floor performance from a
vibration perspective, but maybe this isn't an much of an issue since the
plank is so massive any way. Any thoughts or red flags come to mind?
b) The PM wants to change columns to all steel or all PC concrete for
various reasons. If I were to go to PC concrete columns I'm not sure I
would be comfortable with an 8x16 profile, or if it is a profile a
manufacturer would make. I can't see a practical way to attach the PC column
to the CMU wall, but maybe I don't need to. Keep in mind that there are
also areas where the change of a column size impacts architectural,
especially since the interior designers seem to be dictating where I can
place my columns!
c) As a carryover from my old boss, I usually specify a two course bond beam
at the top of CMU walls. The PM wants to use a one course bond beam, and
I've seen many others do the same. Am I being too conservative?
d) The PM wants to use steel lintels at masonry openings instead of PC
concrete or CMU bond beam lintels. I've avoided steel lintels in masonry
(excluding brick veneer) because the connections seemed awkward and how can
you place reinforcing properly around openings with a steel lintel? When
the CMU wall is a shear wall how do you practically transfer shear at this
detail and account different material coefficients of expansion? If the
lintel span is relatively small maybe thermal expansion issues are
Sorry for the length of the message and my ignorance, but opinions are
Albert J. Meyer, Jr., P.E.
(215) 665-8570 Tel
(215) 561-5064 Fax
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