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Re: IRC Braced Panels

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I personally do not stamp IRC houses.  I only stamp if there is engineering
involved and I produced the plans.  I have reviewed plans and written memos
describing whether I thought that a house met the IRC, but I would not stamp
the plans since in most instances I did not produce the plans, nor were they
produced under my supervision, a requirement for stamping here in
Washington.

Ted Ryan


----- Original Message ----- 
From: "Dmitri Wright" <dmitri(--nospam--at)pciengineers.com>
To: "Seaint" <seaint(--nospam--at)seaint.org>
Sent: Thursday, January 13, 2005 8:17 AM
Subject: Re: IRC Braced Panels


> 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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