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RE: Static vs. Simplified Static design

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Ben,
You misunderstood or I was not clear on my comment. What I was trying to say
was that out of 37 plans I've checked in the last three weeks, none of them
have complied with a rigid diaphragm analysis even though the EOR used the
Static method of analysis. Their assumption was that the diaphragm was to be
designed as flexible since the structure was less than two stories and it
was residential. Essentially, they ignored the rigid analysis. 

I think that we are on new territory here - a standard of professional
practice that does not comply with the letter of the code by ignoring the
diaphragm deflection calculations.

I know Santa Monica's rigid (no pun intended) standards for compliance to
the letter of the code (you even made CNN for the Schreiber family hedge
heights), but for the most part I have not found a city in Riverside County
that won't accept whatever the engineer presents.

My question (which you answered part of) has to do with the use of the
Static Method and the Simplified Static Methods. My interpretation has been
as follows:

Static,
1. Check for diaphragm deflection and determine if diaphragm is rigid or
flexible.
2. If rigid, design with consideration for rotational shear through the
horizontal diaphragm based on wall relative rigidity.
3. Design to the worst case for wind or seismic and it is left to the
discretion of the engineer as to whether or not rigid analysis is used, or
if an envelope solution combining the worst case for rigid and flexible is
used.
4. If the diaphragm is rigid by deflection analysis, then the flexible
diaphragm design is not allowed without consideration for rotational forces.

Simplified Static
1. May be designed using flexible analysis as a substitute to design
consideration for rigid analysis by nature of the 20% arbitrary increase 2.5
Ca to 3.0 Ca as long as;
a) The building contains no vertical and horizontal irregularities,
b) The building is a light framed residential one or two family structure
c) And that the building is 2-stories or less in height (with or without a
basement).

What have I missed here and is this a correct assumption? The Simplified
Static design follows your original Tri-County design that many building
officials are accepting. 

I am trying to come up with a sufficient explanation of these two sections
of the code for light-frame residential structures that the local engineers
and building officials can agree upon that would constitute a professional
standard of practice. So far - I see designs all across the board and few
responses to my questions like this from anyone other than those quoting the
code sections.

This is the same problem I am having with Delta-sub-S for the Static
Deflection analysis used in shearwalls and embedded column (flagpole)
design. I recently wrote up a correction based on my interpretation of Delta
Sub-S and I am now not sure I was correct. I don't want to do an unjust act
against the EOR by making him comply to my own incorrect interpretation, but
the code is written very ambiguously on this subject and the ICBO Seismic
Design Manual II for the wood design problems are equally ambiguous without
explaining how Delta Sub-S is calculated.

Maybe you can explain his one to me as well. So far I see a Maximum Drift,
an Allowable Drift, and an Actual drift. The Allowable may not necessarily
be equal at any one time to the Maximum drift. An example of this is the
maximum bending capacity of a flag pole column that takes lateral load. The
Maximum story drift is based on the story height, the Allowable is based on
the Steel's calculated drift based on the steel's capacity and the Actual
drift based on the stiffness of the steel column and the applied demand.

I hope you can help me here - since there were so few replies to my e-mail I
have the feeling that others are as confused on this issue as I am.

Thanks Ben,
Dennis

Dennis S. Wish, PE


California Professional Engineer

Structural Engineering Consultant

dennis.wish(--nospam--at)verizon.net

http://www.structuralist.net

 


-----Original Message-----
From: Ben Yousefi [mailto:Ben-Yousefi(--nospam--at)ci.santa-monica.ca.us] 
Sent: Monday, July 12, 2004 11:55 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Static vs. Simplified Static design

Dennis

I am not sure where you got the idea that the static method is only for
flexible diaphragms. The guidelines for using the static method are spelled
out in section 1629.8.3. For all structures less than 65 feet or 5 stories
one can use the static method.

The use of simplified method is more restrictive and the criteria is spelled
out in 1629.8.2. Basically I see the advantage in mainly not having to
calculate the drift, which is tedious for wood shear walls. You also don't
have to distribute the forces along the height by the triangular method. The
20% conservatism is sufficient for making up any inaccuracies that may occur
as a result these simplifications. 

Ben Yousefi, SE
Santa Monica, CA

>>> dennis.wish(--nospam--at)verizon.net 07/08/04 07:28PM >>>
This next question is to settle a debate (or to start one). For the majority
of plan check that I run across, engineers and architects are disregarding
the full-compliance methods and are assuming a flexible diaphragm design on
less than three story light-framed structures made of wood with a wood
diaphragm. However, the majority of submittals use the Static design methods
per UBC or CBC Section 1630.2. While most calculate Rho, the results are
almost always less than 1.0 so the default Rho value is taken as 1.0.

Then there are those who have chosen (for some reason) to use the Simplified
static design which adds approximately 20% to the base shear value. I happen
to fall into this category, but my rationale is that the conservative nature
of the Simplified static design compensates for construction flaws.

 

My understanding is that the engineer can only use the Static design of
1630.2 if he first determines by analysis that the diaphragm is flexible. If
not, he must follow the full-compliance method and balance or chose the
worst case wind / seismic; (flexible/rigid) for each line of shear.

 

Now if he wants to use the Simplified Static design of 1630.2.3 he must
first check the structure for irregularities and that the engineer must use
the default values for soil properties for SD in zone 4 and a near source
factor not exceeding 1.3. The comparative formulas that tend to yield the
lowest shear for each type is;

 

V=(2.5*I*Ca/(1.4*R))*W (Static) Formula 30-5

V=(3.0*I*Ca/(1.4*R))*W (Simplified Static) 30-11

 

I typically beef up the near source values and place my site closer to the
active fault in order to have a more conservative base shear if I don't know
where the site location is or a soils report is not provided or required by
the city.

 

The question is; What is the correct interpretation for the use of Static
Methods and when should one consider using the Simplified Static design? The
obvious outcome of this is to determine what the existing standard of
professional practice is regardless of whether or not it complies with the
letter of the code which is not (in 100% of the 35 designs I've reviewed)
being followed.

 

Your help in this matter would be greatly appreciated.

 

TIA

Dennis

 


Dennis S. Wish, PE


California Professional Engineer

Structural Engineering Consultant

HYPERLINK "mailto:dennis.wish(--nospam--at)verizon.net"dennis.wish(--nospam--at)verizon.net 

HYPERLINK "http://www.structuralist.net/"http://www.structuralist.net 

 


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