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Re: "Big Column and small beam" Rule

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In my humble opinion, the rule for seismic design is not big column and
small beam; it is strong column and weak beam. There is a difference
between the two.The former deals with relative stiffness while the latter
with relative strength.

Relative Stiffness
If you increase the stiffness of column, its capacity to restrain the beam
end, against rotation, increases and under the gravity loading, the support
moments of beam increase while the span moment falls. There is no rule that
governs this stiffness ratio. It is, however, true that increasing the
column stiffness has a snowball effect and the support moment may in fact
become so large that it actually 'requires' a columns with large depth for
strength reasons.

Relative Capacity
The strong column and weak beam theory is based on the location of hinges
when the flexural capacities of the various members, at the connections,
are exhausted, as they ultimately would under seismic action. The intent of
good design is to ensure that the hinges are formed in beams and not in
columns. The former does not form a mechanism , leading to a collapse,
while the latter does. May I repeat that achieving this has nothing to do
with relative stiffness. All one needs to do is to ensure that if the
column exhibits its worst probable behaviour while the beam exhibits its
best probable behaviour, the hinge is still formed within the beam. This is
the conceptual framework. In order to achieve this we need to recheck the
capacities of both columns and beams, at supports, under the adopted
reinforcement and perform a final check using certain modified parameters
for computing capacities. If you require I might be able to furnish further
assistance in this regard.

I hope it helps.

Rizwan Mirza, Consulting Engineer

Lahore, Pakistan

On 20 March 2015 at 19:25, Alex C. Nacionales <acnacionales(--nospam--at)gmail.com>
wrote:

> Hello List,
> 
> The " Big Column and small beam rule" for SMRF gave me problems when I was 
> designing
> 
> because it made the columns very big for low rise long span structures. I
> was worried when
> 
> my former boss who is a senior engineer increased a beam depth  on a
> structure because
> 
> I initially thought this  violated the rule.
> 
> I realized later the building with RC columns, beams and slabs has a rigid 
> diaphragm so the rule might not apply in that case.
> 
> I have read from other books in Seismic design that this rule may not be 
> a good theory. Your comments please.
> 
> Thanks in advance.

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