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Rigid wood diaphragms, more to disagree with

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>From the Merick Group:

UBC earthquake rigid wood diaphragm A plywood official reported that to
analytically test for a rigid diaphragm (94UBC, 1628.5 third paragraph) there
needs to be an equation for unblocked diaphragms. There is no equation for
such. He believes it is not then  intended for unblocked diaphragms to be
considered rigid. He then hints of existing fudge factors for blocked
diaphragm equations to represent the unblocked.

<Response> The design procedure in the '97 UBC considers the diaphragm to be
blocked in the deflection analysis. However, the calculated deflection is
increased by a factor of 2.5 to convert it to an unblocked deflection value
and then compared to the allowable story drift which, I believe, remains to be
0.005H. Therefore, unless the diaphragm deflection exceeds the allowable story
drift the diaphragm is considered rigid. 

The requirement for rigid diaphragm analysis and the subsequent increase in
shear to resisting walls to compensate for torsional effects is more likely to
occur in residential construction than in commercial structures. The reason, I
believe, is that commercial wood framed structures are more likely to transfer
shear to exterior walls in order to provide the greatest flexibility for
subdivision of leasable space. Therefore, greater chord forces and expected
deflections occur. 
Residential structures are more likely to have multiple diaphragms at
different levels with closer spaced shear resisting elements. Diaphragms are
generally deep enough in one direction to assure that seismic forces in zones
4 (and possibly 3) will govern in at least one direction. In addition, the
shape of many custom residential structures will displace the center of mass
farther away from the center of rotation and, if you agree with roofs as rigid
diaphragms, will introduce torsional forces into resisting shearwalls.

There is a certain beauty to the work that was put into this new methodology.
I understand the rational, but I don't believe that it addresses the true
issues. Engineers cannot educate contractors to understand how to develop load
paths. We can detail the hell out of our work, but cannot guarantee that it
will be built exactly as drawn UNLESS we spend more time in the field to
insure our design. Whether we want to admit it or not, we need to weigh the
economic effects of our presence in the field when we have other pressing
work. I realize this is a poor excuse, but many small independents such as
myself cannot financially afford to absorb the cost of consistent field
observation and meet the demands of other projects. If we were to seek
compensation for these services we would not be able to compete on work based
upon quality.  I believe that this is a bad precedence to set if we are to
provide the best package we can and still be able to maintain a realistic
living standard. However, scheduled observation is important which would help
mitigate the potential cost of repair that the insurance industry must bear.
The issue reduces to having engineers become inspectors to insure compliance
with there designs. If we police the construction phase, we would not need to
change the historic methods of design as long as we can insure that the load
path is constructed properly and not compromised in the course of
construction.  However, you need to seriously consider the liability placed
upon us because of this. Why is it necessary for engineers to become
responsible for the contractors quality of construction when we are not
responsible for choosing the framer?

I am opposed to these code changes, not because the Seismology committee has
created a more complicated design. On the contrary, the method is logical
albeit, in my opinion, unnecessary for adequate performance of wood
structures. I am opposed because we are overdesigning to compensate for
historically poor construction quality. Instead, we should be investing our
energy into uniting the construction and design industry to alleviate these
problems and get past the childish bantering of architect Vs engineer Vs
builders.  The day that our industry takes responsibility and weighs the needs
of our clients over the seemingly insurmountable task of ego battling, we will
not need to raise the cost of construction by overdesigning simply to provide
safe housing and mitigate unnecessary damages. There is abuse in this system
and we are skirting the issues if we believe that we can ignore each parties
responsibility to the whole package simply because they have lobby power to
avoid what is the responsible and morally correct action. We allow anyone who
has the strength of lift a hammer and low bid a service to call themselves
framers. Yet we rarely meet them in the field and even less often find them a
year or so down the road if something goes wrong. They need not be certified,
licensed or registered. They need not understand or have read the code that
defines conventional construction to be allowed to work. They need not
understand the provisions provided by prescriptive measures to be given
responsibility to build a home without an engineer or architect. The GC takes
some responsibility and the framer moves on -- job to job. This is wrong --
pure and simply wrong. 

Every time I go into the field to check the status of a large custom home I
find flagrant abuses of the structural system. Yesterday I went out to check a
10,000 square foot home worth over a million and a half and framed by one of
Southern California's largest and best known commercial framing company's. The
most critical shear wall at the interior of the home is designed for 8,000
pounds. The wall is 12.5 feet in width, 14' tall and braced by ceiling joists
at 10' above the slab. The crippled  area between 10' and 14' is a space above
the ceiling of the media room. The HVAC contractor, with the approval of the
Architect and the contractor (I was not consulted) installed two air plenums
each 12" tall by 48" wide -- leaving me a discontinuous shear wall except for
24" at each end to connect 8000 pounds of shear from the roof to the wall
below the plenums.

I lost my temper and demanded that they relocate the ducting to two diagonal
side walls which intersect with this one. This type of abuse is not the
exception to the rule, it is the rule and occurs on virtually every
residential project in this valley -- regardless of architect and engineer.
Commercial structures are much easier to build since shear walls are generally
placed at the exterior bearing and nonbearing walls. Residential construction
has, historically, been taken for granted by the structural engineering
community as a low risk system. Granted, few failures occur that endanger
life. I know that a few of you have argued vehemently as well that the code
only provides a minimum standard to protect life and anything more places
undue liability upon us.

This is simply no long the case. SEAOC has supported and published performance
based standards in the next code. The amount of damage incurred from natural
disasters has forced the hand of the insurance industry to demand that the
level of damage decrease or it will simply back out of offering policies. They
have done as much and simply left the damage that occurs to the responsibility
of the homeowner or federal disaster relief. If you think this is fair,
imagine the need to pay for your home twice if it gets damaged. No bank in
this country will forgive a loan because it was poorly constructed or suffered

Then, why the heck are we complicating the design process, penalizing the
homeowners and subjecting ourselves to greater liability when a weak link --
the improperly trained framer -- will do what they historically do and screw
up -- leaving us responsible for not only our initial design, but for their
errors. Those who are part of the process of building must take responsibility
for their actions. There are many abuses to this system which was documented
in the first issue of the CUREe Caltech newsletter. This does not reflect
statistics compiled by CUREe-Caltech but is a statistical statement of facts
by sources documented in this newsletter. If anyone wants me to list the
statistics I will do so. 

The following statistics were quoted from various sources and published in the
"CUREe-Caltech Woodframe Project Newsletter" Volume 1 Number 1 dated November,
1. "The CUREe estimate for the property damage total for the Northridge
Earthquake is $40 billion, over half of which was incurred by wood frame
2. "Insured losses for damages to buildings of all types from the Northridge
Earthquake currently total $11.25 billion." 
3. "Approximately 40% of the homeowners in the Los Angeles area had earthquake
insurance at the time of the Northridge Earthquake and of these, the policies
typically contained a deductible of 10%."
4. "Woodframe construction accounts for 75%, or $340 billion of the value of
all of the 2.3 million buildings in Los Angeles County."
5. "In California, 99% of the residences are of Woodframe construction?..The
"general range of the fraction of wood structures to total structures seems to
be between 80% and 90% in all regions of the US.."
6. "Of the 10,193 buildings posted as red (unsafe) or yellow (limited entry)
by building departments after the Northridge Earthquake, 91% (9,264) were of
woodframe construction."
7. "Of the 25 fatalities that occurred because of building damage in the
Northridge Earthquake, all but one occurred in woodframe construction."
8. "Prior to the Northridge Earthquake in 1993, USC professor G.G. Schierle
directed a survey of 143 woodframe buildings under construction in California.
Of the 332 specific seismic construction features inspected, one-third were
missing or improperly installed over 40% of the time. Depending on the
specific construction feature, the error rate ranged from 5% to over 60%.

My point is that the framer should be required to know better. He should be
trained to understand and protect all lateral load paths. He should understand
what the purpose of a shearwall  - in the most basic sense so that he can
protect the system from "bastardization" by other trades.  I had no prior
knowledge that the HVAC system intended to penetrate the wall and would never
have know until I did an observation at which time the damage would have cost
the owner a considerably greater amount of money to tear out and replace? 

I showed up weeks before the observation and caught the problem in the framing
stage -- before the ducts where fabricated and connected in place. In
addition, I discovered that the owners ordered the designer to install windows
in pony walls between different roof elevations to bring light into a large
space. However, this destroyed the shear transfer from high roof to low and
the capacity needed to be transferred to the shear elements below became less
than was needed. The wall capacity was in tact but the drag capacity was less
than the tributary distribution into the wall.

Building a home is complicated. Still, changes are decided in the field and
those who make the changes do not consider, for a moment, that the change may
have any affect upon the structural integrity of the building. No amount of
details, no amount of notes can force a guy with a hammer to pick up a phone
and call the engineer before making any changes. Changes are labor dollar
driven items and are typically done with maintaining a construction schedule.
I believe, many changes that do not consult with the engineer are deliberate
so as not to complicate the schedule or needs of the client.

I also believe that this is the reason so much money was spent to repair
damage to wood framed structures from natural disasters including seismic AND
wind related events. Past studies of damage in Florida from hurricane Andrew
concluded that the majority of damage could have been mitigated by better
construction practice and inspection.

Therefore, I am vehemently opposed to making the homeowner spend more money to
Band-Aid a serious problem we have in this industry. I believe strongly that
the CUREe-Caltech project will produce sufficient facts to support the
published statistic. I also believe, as I stated, that we do the entire
building industry a disservice by skirting the issues of individual training
and responsibility simply because the politics of change is so large and
difficult to transcend. SEAOC and the other structural engineering
organizations should demand autonomy to control structural issues and not
allow themselves to be overruled by non-engineering voting power on the floor
of ICBO. Contractors who choose to construct the structural elements of a
building (any building) should be educated, tested and certified to not only
build the system properly, but take full responsibility for their work.

I urge each of you to start speaking up on this issue, either on this list or
by sending email to your professional affiliations, board of directors and
representatives that foster code changes. You can obtain their email address
from the SEAINT website at <A HREF=" "></A> . This is the only way we can institute change.

Dennis S Wish PE