Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: SEAOC Blue Book C805.3 Lateral Force Distribution

[Subject Prev][Subject Next][Thread Prev][Thread Next]
I just finished reading the draft of the said document. Although I agree with 
most of what is presented, the last paragraph leaves me baffled:

"An example would be steel moment frames or cantilevered steel columns used 
in combination with wood shear walls.  If the steel elements are quite 
flexible, the shear walls are likely to carry a larger portion of the story 
shear than would be suggested by a tributary area analysis.  The relative 
flexibility of steel elements may have contributed to damage to tuck-under 
parking (soft story) buildings during the 1994 Northridge Earthquake [Mendes, 
1995].  A rigid diaphragm analysis or an envelope considering both rigid and 
flexible diaphragm behaviors is recommended. "

How would a rigid diaphragm analysis have helped alleviate the problem with 
this type of damage in Northridge earthquake? Assume that a rigidity analysis 
was performed and all the lateral load was resisted by the wall in the back 
of the garage. How would this have limited the excessive drift at the open 
front of the garage which was the main cause of the collapse at Northridge 
Meadows (there were no deformation compatibility requirements at the time 
these buildings were designed, even if there was, there is much debate as to 
how you apply that to a pin-pin one story steel column). 

The problem with these types of buildings was not the type of analysis of the 
diaphragm. In fact, had the structure been properly designed using the 
flexible diaphragm assumption it would have ended up with fixed base columns 
with substantial rigidity at the open front (designed for drift requirements) 
which would have limited the horizontal displacement at the top of the 
columns and may have prevented the collapse of the structure due the P-delta 
effect. As far as I remember, The Northridge Meadows had only flimsy 3" or 4" 
standard tubes with no substantial fixity at the base in front of the garage 
but long solid walls in the back.

Ben Yousefi, SE
San Jose, CA