RE: Seismic load combos and IBC

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Seismic load combos and IBC
• From: "Wesley Werner" <wwerner(--nospam--at)conewago.com>
• Date: Fri, 17 Jun 2005 13:06:22 -0400
```Jim,

Qe is just V distributed to the various lateral force resisting
members based on height and stiffness of the members and diaphragm. Em
is only required when you are using the special load cases Eq 16-19 and
Eq 16-20. These equations are used to design certain components such as
diaphragm collector elements 1620.2.6. Read through the requirements in
1620 for SDC B. There may be other elements that require the special
load cases. Another situation where you may have to use these equations
is when you use a special moment frame or some other exotic (for our
area) lateral load resisting system. Around here (PA & MD, the only
lateral load system that I typically use that requires some special
detailing is "Intermediate reinforced masonry shear walls." The
detailing requirements for this system is not too difficult.

Here are some things that I have found out as I have gone
through the seismic portion of the code.

1. When using ASD remember to use 0.7E. I often calculate my
forces and then start applying them to diaphragms, etc without using
0.7E.
2. When Em is used the allowable stress may be increased by 1.7
(1617.1.1.2).
3. When choosing a lateral force resisting system remember to
check into the detailing requirements for the system. It may be more
economical to choose a system with a higher R than to meet the detailing
requirements.
4. For a rectangular building sometimes wind will control when
it is perpendicular to the long side of the building, and seismic will
control the direction parallel to the long walls.

Wesley C. Werner, EIT

-----Original Message-----
From: Jim Wilson [mailto:wilsonengineers(--nospam--at)yahoo.com]
Sent: Friday, June 17, 2005 12:04 PM
To: seaint(--nospam--at)seaint.org
Subject: Seismic load combos and IBC

Could someone please confirm the following?  I'm still
playing catch-up with the whole seismic code stuff.

Where Qe = "the effect of horizontal seismic forces"
as defined by the IBC, is that simply = the value for
V or Fp as determined per the code?  This doesn't seem
to be specifically stated.

And when are the special seismic load combinations for
combinations for E?  The description in 1605.4 Special
seismic load combinations includes "where specifically
required by Sections 1613 through 1622..." yet I can't
put my fingers on where it is actually required.

FWIW, This is mostly in SDC B area with the equivalent
lateral force procedure.

Jim Wilson, PE
Stroudsburg, PA

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