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Re: 2nd Call - UBC 97 Section 1630

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Personnal interpretation and not that of City of LA - Struct. Div.

Mr. Tan, as you noted correctly, equation 30-1 and 30-2 are to be used for
completely different things.  I grant you that the  words "an element" for E
and "in the structure" can lead to confusion.  

If you look at the rest of the seismic provisions, you will note that Em is has
a limited use: Discontinuities (1630.8.2) and Collectors(1630.2.6).  They are
to be used in load combinations 12-17 and 12-18 per Section 1612.4.  The
concept behind this seems to actually be the inverse of its definition.  That
is, Em will give you the maximum force that can be developed in the structure
SUCH THAT these critical element(s) can be analyzed/design based on worst
conditions possible.  In addition, note that Eh is multiplied by OMEGAo, the
overstrength factor.  Again, the idea being that you are trying to create the
worst loading condition of the structure SUCH that these critical elements are
properly design.  It is a similar concept to FEMA 310's Force Controlled Action
(the maximum force that can be delivered by deformation controlled action to
the element or connection).

E is the general seismic load used for the rest of the structural provisions
and most of the load combinations.

Finally, I believe there is a limitation when used with convential wood framing
but I am not 100% sure.  I  may be thinking of something else.  

Eh is just the horizontal force.

Regards,
ed gonzalez

>>> <jerome.tan(--nospam--at)PAREURO.COM> 03/11/99 12:25AM >>>
No takers ???

______________________________________________________________________

E is defined as the earthquake load *on an ELEMENT of the structure* resulting
from the combination of Eh and Ev. Eh is derived from 1630.2 in Eqn 30-4 and
limited between Eqn 30-5 & 6. Ev is the equivalent vert. eqk component added to
the DL and is zero in ASD.

Em is the max eqk load *on the STRUCTURE* resulting from the product of the
amplification factor and Eh of Section 1630.2.

Kindly enlighten me if E differs with Em in that the former is applied to
elements while Em is for the entire structure. And if E is just a horizontal
load and not a vector (i.e. rho * Eh - applied horizontally and Ev - applied
vertically).

I interpret it right now in that E is also applied to *the ENTIRE STRUCTURE*
and Em is the maximum that can be applied. For example, if my computations say
that E is a horizontal 100 tons, and Em is 125 tons, I use E as my design eqk
load. But if say E is 150 tons and Em is still 125 tons, I will use Em for E
because E > Em.


TIA,

Jerome A.M. Tan
Makati City, Philippines

Jerome.Tan(--nospam--at)Pareuro.com