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Re: 97 ubc (soft story)

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Thanks for the response.  It seems to me that this is an example of a
conflict or ambiguity in the code, and that you have chosen one side, while
I have chosen the other.  1629.8.3 Item 3 and 1629.8.4 Item 2 have an
overlap in their definitions of building type, and that is where the
ambiguity lies.

A building less than 5 stories or 65 feet in height, having a vertical
irregularity of Type 1, 2, or 3 (Table 16-L), would fall into both
definitions (1629.8.3 Item 3 and 1629.8.4 Item 2).  Therefore it is up to
interpretation (rather than clearly stated) as to whether a static or
dynamic analysis is required for such a building.

Here is my interpretation:

Section 1629.8.3 uses the word "may" in the first sentence, however 1629.8.4
uses the word "shall" in the first sentence.

Section 1629.8.3 Item 3 applies to ALL irregular structures less than 5
stories or 65 feet in height, regardless of whether the irregularity is a
vertical or plan irregularity, and even includes irregularities not defined
in Table 16-L and 16-M (see 1629.5.3).

Section 1629.8.4 Item 2 applies only to SPECIFIC types of irregular
structures, rather than to all irregular structures.

Considering that the dynamic requirements use the word "shall", and that
Section 1629.8.4 Item 2 is a SUBSET of the more general definition above it,
it seems to me that this requirement takes precedence over the other.

If this were not the intention of the authors, then Section 1629.8.4 Item 2
would have the further restriction that it applies ONLY to structures
GREATER than 5 stories or 65 feet in height.  The following paragraph,
1629.8.4 Item 3, has such a restriction for clarification.

Based on the reasons given above (derived only from the language of the
code), as the code is now written (and has been since the 1988 UBC), it
seems clear to me that ALL buildings with soft stories, mass irregularities,
and vertical geometric irregularities (Items 1, 2, and 3 in Table 16-L) are
required to have a dynamic analysis.

Mark Swingle, SE
Oakland, CA

These are my own opinions.

PS  Also note that before the 1988 UBC, the only requirement was that the
dynamic characteristics be "considered" for buildings with structural

PPS  Perhaps this is a good candidate for the Seismology FAQ list.


On 8 Jan 00, Ben Yousefi wrote :

<<In a message dated 1/7/2000 4:17:18 PM Pacific Standard Time,
  Mark.Swingle(--nospam--at) writes:

<<  << The code DOES allow a soft story.  However, the penalty
       is severe.  A soft story is a.k.a. a vertical 
       irregularity of Type 1 (Table 16-L), and you must
       therefore perform a dynamic analysis per 1629.8.4.   >>


<<This, IMHO, is not completely accurate. Although as a general rule
table 16-L references section 1629.8.4, Item 2 (Dynamic procedure) for
structures with stiffness irregularity, Section 1629.8.3, Item 3
specifically allows the use of static procedure for any irregular
structure less than 6 stories or 65 ft in height. There may be other
ramifications for the design based on the type of irregularity,
but dynamic procedure is not required.

<<Ben Yousefi, SE
<<San Jose, CA>>