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RE: I am beginning to think that those of you using the IBC are notas swift as I thought :)

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Dennis:

The trendy thing in Alaska right now is in-floor gypcrete topping slabs.  My
big question on ASCE 7-05 for houses is for the simplified method section
12.14.5 reads "...steel decking, (untopped), wood structural panels, or
similar panelized construction are permitted to be assumed flexible."  This
can be read that steel deck must be untopped to be considered flexible, but
that wood panel may be topped and be considered flexible? C12 is of no help.
IBC 1613.6.1 - amends 12.3.1.1: toppings <1.5" can be considered flexible if
each line of bracing complies with drift per Table 12.12-1, per ASCE 7-05
12.14.8.5 says drift for a simplified method building can be assumed as 1%.
My assumption is for a house when there is a topping slab it is considered
flexible per 06 IBC 1613.6.1 and ASCE 7-05 12.14.8.5.

There were requirements for overstrength load combos on framing under
in-plane and out-of-plane offsets since the 1994 UBC. I haven't bought
Breyers new book yet but my old green Breyer book had 3/8*Rw*E for this
stuff.  The requirement is in ASCE 7-05 for both the simplified method for
two story houses and ELF [the simplified method kicks you into ELF for a
three story house] with the irregularity. This only for framing not straps or
anchors.  Yes it sucks.

Rho isn't always going to be 1 in a one story structure because if you have a
three sided diaphragm, you don't have two bays on each side and you have a
torsional irregularity.

It seems good to try to do the simplified method for houses to begin with so
you don't have to check story drift.  If you end up with a three sided
diaphragm or offset for a three-story or something that kicks you into the
equivalent lateral force procedure you are still stuck with Rho =1.3 and you
get to check drift.  

It sounds like you are doing a lot of value engineering.  I want to hire you
to design my house if you don't charge for the value engineering.  Don't let
these regal commercial guys get you down, they are just belittling your forte
to mask their own insecurities.

Scott.



-----Original Message-----
From: Phuong Nguyen [mailto:PNguyen(--nospam--at)portla.org] 
Sent: Wednesday, May 07, 2008 3:00 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: I am beginning to think that those of you using the IBC are
notas swift as I thought :)

...and the plan checkers at the counters are not the engineers running the
calc, and are just started learning too, and maybe not even a licensed
engineer.

>>> Scott Maxwell <smaxwell(--nospam--at)umich.edu> 5/6/2008 3:44 PM >>>

Dennis,

My friend, keep in mind that for the vast majority of the country, seismic is
a relatively non-issue.  The biggest area that deals with seismic
(California) has only recently started dealing with the IBC.  Then factor in
that most of the country allows the use of the IRC for single residence home
design.  And then factor in that in many places in the country, an engineer
is not required for single residence home design.  The point is that number
of engineers that actively use the IBC for seismic design of wood-framed
single residence homes is rather limited.  Then further reduce that number as
only a small portion of those likely participate on this email list.  That is
the unfortunately reality.

Regards,

Scott
Adrian, MI


On 5/4/08 10:40 PM, "Dennis Wish" <dennis.wish(--nospam--at)verizon.net> wrote:



	Seriously, this is not an insult. I posted some questions a few weeks
ago and expected to get at least on if not more opinions on the matter of Rho
and basically received two that were more questions than advice. So let me go
through this once more as it is becoming more important in the design of
low-rise light-framed structures using flexible diaphragm assumptions. 
	 
	After reading Breyer's latest edition of "Wood Structural Design"
Chapter 16, the issue of plan irregularity seems much more important in this
code cycle than in pasts. There are a number of horizontal and vertical plan
irregularities that occur in most residential structures that trigger
penalties - even if the code exempts most penalties from the assumption that
2-story structures or less and in some cases 3-story or lower light framed
wood structures have exceptions to those irregularities that assume large
torsional irregularities. However, a simple vertical irregularity where the
interior shear walls that do not stack can force a penalty requiring the use
of Eh +/- Ev; where Eh = Rho*QE (12.4-3) and Ev=0.2SdsD (12.4-4). These are
triggered mostly in SDC of C, D, E & F. 
	 
	QUESTON #1
	 
	This brings me to the same question related to Rho. In a one story
structure, Rho is always going to be 1.0, but in a multi-story single family
structure, the condition must be that each level carries more than 35% of the
base shear. Depending on the height of the levels it is possible but many
times unlikely that the percentage of shear in the diaphragm that is
transferred to Lateral (vertical) load resisting system is often less than
35%. 
	 
	I need to know if my assumption that the percentage of distributed
shear is taken from the lateral distribution of forces typical to each
diaphragm Level based on the results of Fx - where wx*hx/Sum(Wi*hi)
represents the percentage of vertical shear distribution as indicated in
formula 12.8-11 and 12.8-12. In other words is it these lower levels where
Rho can increase to 1.3 while at the higher levels or in a one story building
Rho can be assumed 1.0?
	 
	Since no one has addressed this question directly other than to offer
their own interpretation, am I to assume that no one really knows or that I
am so inferior to others that it is simply a dumb question that does not
require a response :-) I fail to believe that any of you would believe any
question seeking knowledge is a dumb question and therefore must assume that
most do not know the answer?
	 
	QUESTION #2
	 
	The second question has to do with using the Overstrength factor
Omega when a vertical or horizontal irregularity occurs in structures that
use the Overstrength factor as a penalty over using a rigid or dynamic
analysis to solve the load distribution. This is triggered in section
12.4.3.1 and is relevant in almost any structure in which a vertical or
horizontal irregularity exists and flexible analysis is still allowed in lieu
of a rigid analysis and/or a dynamic analysis.
	 
	QUESTION #3
	 
	The horizontal Plan irregularities for structures with reentrant
corners triggers the above penalties if the offset divided by the overall
length of the side exceeds 15% (as it would in most "L" shaped or "U" shaped
structures. However, is the same penalty added if the irregular geometry with
reentrant corners is broken and analyzed in rectangular blocks where each
block is assumed to have two parallel shear walls at each exterior side (in
both orthogonal directions) even if the shear needs to be dragged into the
assumed shear wall though the collector?
	 
	The above penalties occur mostly in Seismic Design Categories D
through F and sometimes in B and C. My problem is that in residential
construction these common plan irregularities when considering the entire
geometry rather than the component blocks occurs most often than not. It is
rare to have all walls stack and because of this our analysis must take the
penalty into consideration.
	 
	Can anyone offer me some advice on this. I own the 2006 IBC, ASCE
7-05 and Commentary as well as Breyer's current edition of Wood Structural
Design (6th Edition) and none of these resources offer a clear explanation of
what to do. In fact, it appears that the Seismic Design manual's Examples 1a
and 1b are also incorrect in the assumption of Fx and Vx when irregularities
exist. The only time that they do not seem to apply is when the entire
geometry is broken into blocks and joined at common lines of shear but I
don't believe that this is what the authors of the code intended.
	 
	How about some help from all of you who have been using the IBC since
2000 and seem to be so much more advanced than us lonely engineers here in
California :-) We really need some straight answers.
	 
	Dennis S. Wish, PE
	
	Dennis S. Wish, PE
	California Professional Engineer
	Structural Engineering Consultant
	La Quinta, CA 92253
	760.564.0884 (Phone, Fax and Answering Machine)
	dennis.wish(--nospam--at)verizon.net
	http://structuralist.wordpress.com
	http://www.structuralist.net
	 
	
	




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