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97 ubc (soft story)

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Please understand first off that I am not picking on you.  I just think this
thing should be resolved.  I will submit this formally to the Seismology FAQ
list if it still holds water after I get jumped on by the rest of you.

There appears to be a problem with Section 1629.8.3, Item 3, if one assumes
your (and others') interpretation that the items in 1629.8.3 (where static
procedure is allowed) take precedence over those in 1629.8.4 (where dynamic
procedure is required), in the cases where they overlap.

My interpretation is that Section 1629.8.4, Items 2 and/or 4, would override
Section 1629.8.3, Item 3, where the two definitions overlap.  You and others
do not agree.

Section 1629.8.3, Item 3 reads as follows:

3.  Irregular structures not more than five stories
    or 65 feet in height.

I believe that this should be changed to read as follows:

3.  Irregular structures not more than five stories
    or 65 feet in height, except where Section 1629.8.4,
    Items 2 or 4, apply.

This change would simply codify what I already believe to be the intent of
the code as it stands now.  Similar wording already exists in Section
1629.8.3, Item 2.  As it stands now, it is ambiguous at best, and leads to
several interpretations that I believe are not the intent of the code.

Let me give an example.  For this example I will use your interpretation as
stated in the first paragraph above.

Consider a building five stories tall on a soft soil profile, S(F).
According to 1629.8.4, Item 4, a dynamic analysis is required for both
regular and irregular structures.  The exception noted in 1629.8.3, Item 2,
ensures that a dynamic analysis is still required even for regular buildings
under 240 ft tall on this soil profile.  However, since according to your
interpretation 1629.8.3 takes precedence over 1629.8.4, an irregular
structure ON THE SAME SOIL would only require a static analysis.

So, in summary,

A regular, 5-story bldg on soft soil requires a dynamic analysis.
An IRREGULAR, 5-story bldg on soft soil DOES NOT require a dynamic analysis.

Something seems to be wrong here.

Mark Swingle, SE
Oakland, CA


Martin W. Johnson wrote:

<<Section 1629.8.1 says that any structure MAY be, and certain structures
SHALL be, designed using the dynamic procedure. Section 1629.8.2 Lists
categories of structures which MAY be designed using the static procedure.
This includes any irregular structure less than 5 stories or 65 feet in
height.  The word MAY is used because, per section 1629.8.1, the engineer
has the choice of using either the static or dynamic method.  Section
1629.8.3 says that the dynamic procedure SHALL be used for all OTHER
structures.  This means all structures not meeting the limitations set forth
in Sectioon 1629.8.2 (irregular structures greater than 65 ft etc.).

<<My interpretation is that the static force method is permitted for a soft
story structure less than 65 ft in height, but a dynamic analysis is also
permitted, and is perhaps preferred. 

<<I recently had the chore of looking at a 3 story building where this same
thing occurred - the engineer used a steel moment frame at the first floor
to accomodate a large open lobby, and then used extensive pywood shearwalls
in upper stories where residences were located.  I didn't like it but could
only spell out what the code says.  Because there were also lateral offsets
between the shearwalls and the moment frames I pointed out several code
sections which had been overlooked in the design, but the soft story was
basically in
conformance with the code.