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Re: IRC Braced Panels[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: IRC Braced Panels
- From: "Dennis S. Wish, PE" <dennis.wish(--nospam--at)verizon.net>
- Date: Wed, 12 Jan 2005 20:37:25 -0800
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 ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp* * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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