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# Re: "Big Column and small beam" Rule

• To: seaint-seaosc(--nospam--at)mail-list.com
• Subject: Re: "Big Column and small beam" Rule
• From: Rizwan Mirza <rizwanmirzapk(--nospam--at)gmail.com>
• Date: Sat, 28 Mar 2015 20:14:13 +0500
• List-subscribe: <mailto:SEAINT-SEAOSC-on@mail-list.com>

```Following is my humble opinion:

a) The presence of a strong diaphragm just means that all members of the
vertical structural sub-system (columns and shear walls) would undergo the
same deformation in the horizontal plane (the plane of the slab, or the
rigid diaphragm, as it would be called). The result would be that the
lateral load would be resisted by the various members of the vertical
structural sub-system in the proportion of their stiffness. The principle
that we must have strong columns and weak beams does not flow from here and
has no relation to the presence or absence of a rigid diaphragm.

b) For gravity loads, if you increase the stiffness of the columns the
support moments would increase while the span moments would decrease. This
would continue to happen till such time that the support moments are equal
to w L^2/12. But at this value of moment, the columns is likely to have a
fairly large cross-sectional depth for resisting the applicable bending
moments.

c) Suppose you do do not wish to achieve such large column cross-sections,
you would have end-moments less than w L^2/12, as computed from elastic
analysis.

d) With the gravity loads in place, we need to understand how the lateral
loads would be resisted when they appear.

e) One method of handling the lateral loads -- especially when they are
likely to be high as would be the case in a SMRF system -- would be to
resist the lateral loads in short-span peripheral frames. Let us assume
that you have a 60'-0" x 60'-0" plan, with 60'-0" span frames at every
10'-0" in X-direction. Keep the interior five frames with 60'-0" span and
sub-divide the two peripheral edge X-direction frames into say three spans
of 20'-0" each or four spans of 15'-0" each. Apply 50% of the X-direction
lateral load to each of these two frames and design for the applicable
forces. In the Y-direction you already have two peripheral frames of 6
spans of 10'-0" each. Apply 50% of the Y-direction lateral load to each of
these two frames and design for the applicable forces.

f) Let us now come to the concept of strong columns and weak beams. The
rule applies to all frames which are a part of the lateral load resisting
system, This only ensures that when flexural capacities are exhausted at
supports the hinges would form in beams and not in columns. I would be able
to further guide you on how to achieve this, if you find the need.

With kind regards,

Sincerely,

Rizwan Mirza, CEO
Rizwan Mirza, Consulting Engineers
Lahore, Pakistan

On 27 March 2015 at 10:02, Abolhassan ASTANEH-ASL <astaneh(--nospam--at)berkeley.edu>
wrote:

> My understanding is this is not related to rigidity of the diaphragm. It
> is a requirement for seismic design of moment frames.
> Best
>
> On Thursday, March 26, 2015, Alex C. Nacionales <acnacionales(--nospam--at)gmail.com>
> wrote:
>
>> Yes I meant "strong column and weak beam". I am a little rusty on
>> structural
>> systems, when analyzing Seismic loads, does this rule apply when you have
>> a rigid diaphragm?
>>
>>
>> Alex Nacionales
>> Vancouver, BC
>>
>> -----Original Message-----
>> From: seaint-seaosc(--nospam--at)mail-list.com <javascript:;> [mailto:
>> seaint-seaosc(--nospam--at)mail-list.com <javascript:;>] On

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