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# RE: Static vs. Simplified Static design

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Static vs. Simplified Static design
• From: "Haan, Scott M." <HaanSM(--nospam--at)ci.anchorage.ak.us>
• Date: Tue, 13 Jul 2004 06:43:35 -0800

Dennis:

I heard about some building department policy in Los Angeles or
somewhere that allowed wood diaphragms to be assumed to be flexible in
wood buildings if the simplified static method was used for design.

I agree with Paul, but want to add my cynicism: assuming rigid
diaphragms when designing lateral systems in wood buildings is crazy.
The wall stiffnesses are not linear, residential structures change
during construction frequently, the analysis takes longer with dubious
results, and people don't want to pay the extra money for the extra
time.  Wall stiffness depends on nail slip which changes with nailing
patterns which depend on the loads which means iteration.  Not easy to
do by hand. Is there software that does a rigid diaphragm analysis for
wood buildings without making oversimplifying assumptions just to do the
load distribution? If so does everybody have to buy it to design houses?
The people writing the code need to consider these factors.   I would
leave whether to design wood diaphragms rigid or flexible for wood
buildings to the designers discretion.

Scott Haan

-----Original Message-----
From: Paul Feather [mailto:pfeather(--nospam--at)SE-Solutions.net]
Sent: Monday, July 12, 2004 4:41 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Static vs. Simplified Static design

Dennis,

First, you are still confused on drift.  There is a calculated drift, a
maximum inelastic drift, and an allowable drift.  Calculated is from
analysis, maximum is an amplified form of the calculated, and allowable
is based on story height.  Maximum must be less than allowable.

As far as the rigid diaphragm for wood framed structures is concerned, I
will go out on a limb and state that I think rigid diaphragm analysis
for wood structures is simply un-realistic.  Unless you have a clear
case of a rigid blocked diaphragm on flexible cantilevered columns or
something similar it is a complete crap shoot to think you can perform a
rigid diaphragm analysis with any hope of accuracy.  Why do you think
there are strict limitations on when a wood structure can be designed
using rotation? And yet on the other hand we are supposed to analyze the
structure for rotation?  Calculating the stiffness of wood shear walls
with the code formulas and the multitude of out dated assumptions
inherent in their use is a waste of time.  If you end up within 20 or 30
percent of the actual behavior I would be impressed.

A PROPERLY DESIGNED AND DETAILED wood system based on flexible analysis
is perfectly acceptable and will in my opinion perform better than the
most rigorous supposed rigid analysis solutions.  I could force a
structure to "distribute" load to areas I have adequate wall and go
light on areas where I do not have adequate walls using a rigid
analysis, but I would not design a wood structure this way for the same
reasons I don't believe a rigid diaphragm analysis is valid.  Using a
rigid model I could direct virtually all the load to a three story hall
line and not worry about all those pesky openings on the exterior walls,
but in my own mind I could never justify such a design.

The static design approach has no dependence on a flexible analysis.
Paul Feather PE, SE pfeather(--nospam--at)SE-Solutions.net www.SE-Solutions.net
----- Original Message -----
From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, July 12, 2004 4:16 PM
Subject: RE: Static vs. Simplified Static design

> 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
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
>
>
>
>
>
>
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