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RE: EQ resistant Design

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I agree with Mark that the intent of the code is fairly simple and basic. As
per IBC 1616.1, "The design ground motions shall be assumed to occur along
any horizontal direction of a structure."

Per IBC 1620.1.10 for Seismic Category B, "The direction of application of
seismic forces used in design shall be that which will produce the most
critical load effect in each component. The requirement will be deemed
satisfied if the design seismic forces are applied separately and
independently in each of the two orthogonal directions." Per IBC 1620.2.2
for certain structures in Seismic Category C and per IBC 1620.3.5 for all
structures in Seismic Category D, design must be based on the following
loads: "One hundred percent of the forces for one direction plus 30 percent
of the forces for the perpendicular direction" or alternatively, "on a
square root of the sum of the squares (SRSS) basis".

My understanding is that the 30-percent method is considered to be a
reasonable approximation of the SRSS method (but not exact). The code is
simply providing alternate methods to account for ground motions to occur in
any horizontal direction. I do not agree that designing for full seismic
forces in two orthogonal directions independently is equivalent to designing
a structure to resist full seismic loads in both directions simultaneously;
and it may not be conservative for some structures where seismic motion is
applied at an angle to the orthogonal axes of the structure. Nevertheless,
the code allows design only along the orthogonal axes for lower seismic
performance categories.


> -----Original Message-----
> From: Mark Gilligan [mailto:MarkKGilligan(--nospam--at)compuserve.com]
> Sent: Thursday, March 21, 2002 12:50 AM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: EQ resistant Design
>
>
> This whole topic has been made much more complex than it really is.
>
> The basic provision is that you must consider the effect of
> the earthquake
> acting in the most critical direction for each member.
>
> For many structures this requirement is satisfied by analyzing the
> structure in the principle directions.  This assumes that the lateral
> resisting elements are parallel to the principle directions.
> When dealing
> with non perpendicular systems or when the same element
> participates in
> non-parallel walls, braced frames, or moment frames then you need to
> consider the effects of most critical direction.
>
> In order to simplify this process the  code provides several
> methods by
> which you can approximate the expected forces using the
> results of analyses
> in two perpendicular directions.
>
> The extreme situation is when you have 4 columns in a square
> with braced
> frames on each of the sides.  In this situation when the direction of
> seismic forces is at 45 degrees with respect to any of the
> sides two of the
> columns will see a reaction 1.414 times that that you would
> obtain when the
> forces are parallel to one of the sides.  The critical
> direction for the
> braces would probably be one of the principle directions.  Each of the
> approximations is based on this relationship.
>
> If the force in each direction is the same (fx=fy) the SRSS
> method gives
> you (fx^2+fy^2)^0.5=1.414*fx
>
> The 100% and 30% gives you fx*1.0+fy*0.3= 1.3*fx.  The
> difference between
> 1.414 and 1.3 results from the fact that some SEAOC
> Seismology Committee
> members believed that 1.3  was conservative for most
> situations and that
> there was little reason for concern in those situations where
> it wasn't.
>
> Some others will use 0.707*fx+0.707*fy= 1.414**fx.
>
> This is as simple as it gets.  Forget about fancy dynamic
> effects.  Their
> is no difference between the concept of orthogonal effects and most
> critical direction.
>
> Mark Gilligan
>
>
>
>
>
>
>
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