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Re: IRC Braced Panels

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Dennis,

What happened to the wind up-lift on the roof? Shouldn't this be combined with the lateral wind and dead load? If you do, you get very little holdown contribution from the roof dead load, making the lack of a mechanical holdown an even larger problem.

Ted,

So you are saying that where IRC designed buildings would perform well, they should be designed by IRC? While an obviously true statement, the core of the debate is determining where the IRC methodology applies, and then enforcing it.

I very much agree with your statement that IRC design has it's place. I think if it were applied to the type of buildings it was originally intended for (rectangular, 8ft plate heights, few windows, lots of interior walls), that it is a useful tool for home builders. The problem is that over the years, home design styles have evolved to asymmetric, 10ft plate heights, all windows, & no interior walls. And through political and economic pressure, the home builders have kept the existing code virtually unchanged, and relied on ambiguity and ignorance to get nearly any residential design that someone could dream up approved under IRC.

but there were obvious problems with a lot of residential construction due to
>"operator" error, that is, misapplication of the prescriptive code.

My contention is not that the IRC method should be scrapped. Rather, as I have noted in previous posts, it should be updated to reflect current building practices, and the advances in building technology for wind and seismic loadings. And, I think most importantly, the scope of applicability should be clearly defined, and an effort made to educate code officials (most of whom do not have engineering backgrounds, and lack "sound engineering judgment") as to when the IRC can be applied. The method of parts allowed in the IRC should be restricted to elements that are structurally independent. Designing one wall line as an IRC braced wall line, and the next wall line as an IBC shear wall should not be allowed.

On a related subject, are licensed engineers stamping prescriptive designs? To my understanding, this is overkill, and an unnecessary extension of liability. The whole point of the prescriptive method is to get the engineer out of the loop. It seems to me that when you stamp a prescriptive design, you are saying that it meets current engineering standards and codes (IBC). And as Dennis and Ed Tornberg showed, it does not. So you are now taking responsibility for whatever shortcomings there are in the prescriptive code. Just curious how other engineers are handling this situation.

Dmitri Wright
Portland, OR





Subject: Re: IRC Braced Panels


Ted Ryan wrote:

>I do believe that used with an appropriate amount of engineering judgment
as
>to the applicability of the IRC a house can perform well in a high
loading
>event.  Much of the destruction observed due to the Northridge quake was
>either poor construction or lack of engineering where engineering was
>warranted within the prescriptive code.  I am not advocating IRC for all
>houses.  It simply does not apply to all residential construction, but it
>does have it's place.  You are right to make clear the distinction
between
>life safety and performance.  Confusing the two could result in lots of
>trouble for sure.  However, I due think that the IRC can be used (again,
>with sound engineering judgment) in high risk regions and the building
would
>perform well.  Many papers that came out of the CUREE project concluded
that
>much of the residential construction did well in the Northridge quake,
but
>there were obvious problems with a lot of residential construction due to
>"operator" error, that is, misapplication of the prescriptive code.
>Properly applied and constructed, prescriptive construction did well.
>
>Ted Ryan
>
>
Ted,
We are making some progress here and I'm glad that you are beginning to
see another perspective to the problem. Let me concede too that any home
that is not tied together properly or engineered homes that are not
constructed to either the design details or in a manner consistent with
good construction methods will not perform well and are indeed a problem.
So lets move on to the next issue. Lets assume that you live in a region
subject to a 70-mph wind load with an exposure C rating. Also lets
assume that you decide to construct at simple home 40' x 50' with roof
trusses spanning the 40' at 24-inch on center. Let's look at one wall -
the long wall bearing the weight of the roof trusses (tributary) and
assume a roof dead load of 20-psf and a live load of 16-psf (reduced by
slope). Assume you use the UBC wind load criteria on a one story
building you should come up with a wind pressure of about 18-psf
(rounded). If applied to the 40-foot wall with a tributary height of
8'-0" (assume 1/2 the height of the wall from slab to plate plus 1/2 the
total peak height for a 4:1 pitch roof. So you will have a shear from
wind at one line of wall that bears the trusses equivalent to 18-psf x
8-ft x 40/2-ft = 2880-lbs.
The code says that you must have three braced panels in this line since
each can not exceed 25-feet; so one at each end and one toward the
center. Each braced panel must have a ratio of 2:1 or less, so for an 8'
plate you need a 4x8' plywood panel (or OSB). Now we have 2880-lbs
divided by three panels or 960-lbs per panel.
The overturning moment is 960-lb times 8-feet or 7,680 - ft-lb.
The tributary roof dead load is assumed to be 20-feet x 20-psf or 400-lb
per foot. Assume the weight of the wall at 20-psf  and the resisting
moment is:
Mr=400-lb x 4-ft x 2ft + 20-psf x 8-ft x 4-ft x 2-ft = 4,880-ft-lb. <<
7,680 ft-lb.

This shows that there is an upload force on the braced panel of ; O.T.
1,066 (assuming the holddowns to be less by 6-inches at each end giving
3-ft from center to center on the holddown anchors).

There is no requirement in the UBC Section 2320 (I assume the same in
the IRC) to provide a mechanical holddown to resist uplift or
overturning of the braced frame unless it is an alternate braced panel
2'-8" in width. Worse, in greater areas around the country the roof is
sheathed either asphalt shingle or the newer lightweight anodized metal
roofing that reduces the dead load of the materials and increases the
uplift force. If the plate heights are extended to 10-feet, the
overturning moment increases while the resisting moment is increased by
only 160-lbs from the taller wall.

If there is an uplift that is not resisted, then something must absorb
the force. You discussed the redundancy in wood design and I agree that
it exists. It is unlikely that the wall panels will actually lift, but
it can happen as the plate splits down the center line of the anchor
bolts. The panel will most likely rock, until stable again, although
rocking is a dynamic action in earthquakes and wind on shearwalls is
treated more as a static load.

My point is that something will give - plaster cracks, mud-sills crack,
gypsum cracks, etc. The cost of repair is more likely within the
homeowners deductible (in California it is 15% of the replacement cost
or the insured value of the home) before the insurance company touches
cent one. However, if the $50.00 holddown (labor and materials) were
added on each end the damage might have been minimized.

The IRC is a basic construction methodology that dates back to the
change from log cabins to platform and balloon framing due to the
arrival of the saw mill. It has changed little in ideology but the code
has expanded its use to larger and taller structures than was originally
intended. This was not a developed evolution within the guidelines of
engineering practice, but one of politics to keep engineers out of
traditional framing. The structural engineering community used to
believe that homes were not part of the structural engineering field
UNTIL the cost to repair or replace these homes broke the bank with
insurance companies after Hurricane Andrew, Loma Prieta, Northridge, and
other moderate earthquakes that did so much damage.

You mentioned the opinions of CUREE, but just after the adoption of the
1997 UBC in California, but I think you got this a bit wrong. Kelly
Cobeen and the others on the committee after the first shake test in San
Diego (or was it Irvine) decided that California faced a real potential
problem with the performance of conventional prescriptive construction
as they believed the damage in a large scale earthquake would bring loss
of life. It was published and submitted to SEAOC around 1999 if I recall.

One of the weak links is the compliance to a decent construction
standard. As the use of mechanical connectors made construction much
faster and easier, it also induced framers to modify the connectors and
to use them improperly. I have three calls this week from real estate
agents to do an inspection of a home (in three local cities) where the
roof truss was reported to be modified by the home inspector. My
experience with this is that a framer will cut a truss chord to working
something in that they need and do a field repair without drawing
attention of the building inspector.

I designed a million dollar home and went out to do my first structural
observation to find out that the framer "did it his way" and not as I
detailed it. He claimed that he could not find my detail references on
my shearwall plan. I showed him where the detail references were on the
foundation and framing plans and indicated that I also provide a
shearwall plan independent of framing plans to make it clear where
shearwalls are to be placed and where holddowns (also referenced on the
foundation plan) are to be installed. I could have had the home ripped
down, but I worked with the GC who fired the framer after this job and
we finished the home. The owner refused to pay me for the additional
redesign time and threatened to sue everyone if I proceeded to try and
collect the money. Since he had the financial means to drag me into
court I gave up an additional few thousand dollars in time to make sure
he had a safe home to live in.

Here is the punch line - anyone who can lift a hammer can call himself
(or herself) a framer. As often as I go to a job site, the framers who
are inexperienced will screw up more times, but even the seasoned framer
who is not used to working on custom homes will screw up. If you can't
get it right under the guidance of the engineer of record, how can you
get it right if simply left alone to do it the only way the framer knows
how. This is why you can't control framers in prescriptive design unless
you have a strong knowledge of the most important part of the
construction job - framing.

Now , if you require additional training and certification of a framer
before he can step on the site and frame a home, we might have a
different matter and we might not be so far apart in our arguments. If
this were true, we might not need to make the engineering design so
restrictive that we, as a professional community, have to debate
constantly over every ounce of induced shear that might be derived from
the rotation of the diaphragm (rigid analysis vs. flexible analysis).

CUREE discovered as many of the professional community had, that a text
book construction such as what existed on the shake tables did not
accurately represent what was done in the field and what was left that
people lived in scared many of those on CUREE as to what will happen to
homes in high risk areas of the country.

We have a real problem when the basic numbers show you that even on a
simple 2000 square foot (40x50) single family home will still have over
1,000-lbs of uplift on a braced panel that the code does not require to
be held down.

Dennis

The only mention



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