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RE: Braced Steel Frame System Question

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Alan,
Let me respond to a portion of the questions you ask. From your explanation,
the lateral restraining system ends at the second floor diaphragm. If this
were, in fact correct, the first story would be unstable. However, if the
structure were correctly designed, then it might indicate a different
lateral restraining system at the first level, such as a moment frame, or
other shearwall system. The braced frame can terminate at an upper level
diaphragm so long as the reactions at the base of the braced column
appropriately transfer the shear and axial forces to another system. While
it may not be easily apparent, the designer may have transferred the braced
frame reactions in some manner through the second floor diaphragm (though a
girder for example) to another type of system below.
Another example, considering a shearwall system, might terminate a shearwall
at the upper level diaphragm and be able to transfer shear through the
diaphragm to an adjacent wall or frame below - the eccentricity of the
discontinuous restraints depends upon the diaphragms capacity to transfer
the demand from above. You can see this when looking at a multi-story
building where the upper level cantilevers over the lower level and where
the shear is connected through the diaphragm from the end of the cantilever,
to the first floor shear elements.

If you are plan checking the project, the lateral load path should be
clearly defined in the calculations as well as in the drawings and details.
If it is not clear to you, then it will most likely be even less clear to
the person who is going to build it.

Assume that your instinct is correct because the original designer has not
clearly shown you the load path to the foundation. Question the softstory
and require the appropriate supporting analysis that will justify the
accumulation of loads from the roof to an adequate foundation.

Dennis S. Wish, PE


-----Original Message-----
From: Efren Allan Yango [mailto:engreay(--nospam--at)pacific.net.ph]
Sent: Sunday, March 11, 2001 6:55 AM
To: seaint(--nospam--at)seaint.org
Subject: Braced Steel Frame System Question


To Engineers with braced frame experience :

I am doing a design check on a mall-type structure with some portions
3-storey high and others 2-storey high. All beams and girders were
assumed simply supported by the original designers (no moment frame
construction). The columns were designed for simple axial load and
minimal moment usually resulting from the beam connection eccentricity.
The lateral force resisting system adopted was an X-braced frame system.
The size of the whole structure is around 15,000 sq. m with beam spans
as long as 12m.

I have checked the path for the gravity loads and it met all the
criteria of our codes here. My questions are now related to the braced
frames.

    1)    Can the braced frame be located at the 2nd floor to 3rd floor
only? (One end of the digonal at the 2nd floor and the other at the
third floor). No braces
            at the first floor were located. I believe that this would
create a soft storey. In terms of member forces, this would pass but I
have doubt on this type of
            design. That's like terminating your shearwall at the second
floor instead at the foundation level.
    2)    Can flat bars (designed as tension-only members) be used as a
bracing system? I believe that tension-only design is discouraged by the
governing codes.
            How about for low-rise structures? Is the slenderness limit
of 720/sqrt(fy) strictly followed?

For now, I don't want to conclude that the building has been improperly
designed. I would like to know first if I'm missing out on something. I
have other questions but I would like to limit them to these 2 for the
meantime. I hope engineers with experience in this type of structures
could help.
Thanks

AllanYango