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RE: ICBO Seminar for 1997 UBC Earthquake Regulations[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaoc(--nospam--at)seaoc.org>
- Subject: RE: ICBO Seminar for 1997 UBC Earthquake Regulations
- From: "Dennis S. Wish" <wish(--nospam--at)cwia.com>
- Date: Tue, 21 Apr 1998 13:38:38 -0700
Bill, I think we are in agree on these issues. I have not considered the 94 ubc definition, since I too have not found a reliable diaphragm deflection criteria (although one exists in the 91 code, I have not gotten closely involved in this in the 94 code. I mentioned to Thor that evidence of damage to URM buildings after Northridge and Whittier Narrows pretty clearly showed that the greatest damage at floor level was a combination of the thrust of the diaphragm, the lack of appropriate tension connection and the increased slenderness ratio of the wall due to the lack of tension ties. However, the damage also would imply that the force originated in the diaphragm and that the this would be the result of the diaphragm deflection. I say this since the damage was greatest where chord force was large indicating a moment at the "mid-span" of the diaphragm - reducing as you get closer to the return walls where the reaction is expected to occur. there was corner damage as well, expected from the movement of the diaphragm in shear to the wall that should have been resisting this movement by shear anchors. The damage was evident at the floors where the stiffness of the diaphragm is increased (and at the roof which was common to fail a parapet due to a very flexible diaphragm). This may not be as evident in newer construction, however, with URM buildings the "punching" out of brick is pretty noticeable and typical where walls were not tensioned tied. Finally, I agree with you about the 3Rw/8 factor(or the new Omega sub naught) which is a "force amplification factor which takes into account material strengths and type of lateral-force-resisting system." (I took the seminar remember). The entire structure is not penalized by use of this factor as it would be if you needed to design an entire structure with an R of 2.2. I think we pretty much agree that this needs to be tweaked out depending upon the type of structure we are considering. I would tend to exclude single family dwellings from this criteria. One reason is that few of the custom homes that I design have exceptionally large diaphragms close to a 1:1 aspect ratio. Most have discontinuities - vertical and horizontal and some have much larger aspect ratio's. Therefore, I can see the possibility of a wood rigid diaphragm in a large tilt-up commercial structure but not in a custom home. I know you don't want to hear this, but I have to ask it... If we are to be this restrictive to all types of construction, how can ICBO still allow conventional framing provisions to be maintained? I understand that the '97 code has a revised statement of purpose which states in 1626.1 "The purpose of the earthquake provisions herein is primarily to safeguard against major structural failures and loss of life... not to limit damage or maintain function." Therefore, if by increasing the stiffness criteria and require the investigation of soil in relationship to the distance from active faults - how can the concept of conventional framing be justified. Dennis -----Original Message----- From: Bill Allen, S.E. [mailto:billallen(--nospam--at)earthlink.net] Sent: Tuesday, April 21, 1998 10:06 AM To: seaoc(--nospam--at)seaoc.org Subject: Re: ICBO Seminar for 1997 UBC Earthquake Regulations Dennis, my friend, you have confused me on a few issues. First of all, it is (was) very clear in the 1994 UBC regarding the definition of a flexible vs. rigid diaphragm (see section 1628.5). If the diaphragm deflection is greater than 2 times the deflection of the lateral resisting elements, then it is a flexible diaphragm. Otherwise, it is a rigid diaphragm. Unfortunately, I don't believe academia has given us the tools to calculate the deflection of an unblocked (or partially blocked) wood diaphragm, but it would be hard to imagine that a 20'-0" x 40'-0" wood diaphragm would deflect more than 2*0.005h. If I am correct in this assumption, the diaphragm is behaving more like a rigid diaphragm than a flexible one. Second, if the hypothetical structure has two "flagpole" columns at the garage entry and a full shear wall at the rear, I would suspect that the flagpoles at the front are less rigid than the shear wall at the rear. Outside of the structural mechanics, I agree with the potential soft story problem. Maybe this can be (should be?) compensated for by the 3Rw/8 feature of the code (or the ubiquitous Omega sub naught in the 1997 UBC). My $0.02 Regards, Bill Allen -----Original Message----- From: Dennis S. Wish <wish(--nospam--at)cwia.com> To: seaoc(--nospam--at)seaoc.org <seaoc(--nospam--at)seaoc.org> Date: Tuesday, April 21, 1998 8:48 AM Subject: RE: ICBO Seminar for 1997 UBC Earthquake Regulations >Ernie, >If the diaphragm were rigid, I would very much agree with you, however, the >structure is far too ductile to consider that because the elements on the >front of the garage are made stiffer than the back that they should take a >greater proportion of the total base shear. >The soft story ordinance in many cities (Santa Monica for one) is based upon >the fact that a box with one open side and a flexible diaphragm is not >structurally stable because the stiffness of the other three sides >compensates for the weak front. In fact, this has been the cause of many >failures in Los Angeles after Northridge and the need to stiffen the lateral >resisting elements in the soft-story. This is only done where there is a >living unit above since the effects are enhanced by the mass above. >My opinion for wood framed structures with stiff shear elements such as >steel (whether a moment frame or pendulum) is that the load from the >diaphragm to the stiff shear element is not increased - especially due to >the distance from the reactions - only the deflection is controlled and the >element can deflect only as much as the magnitude of the load will allow. >This also becomes a function of the diaphragm's capacity - which should fail >if exceeded or if the total lateral load is transferred into the stiffer >element. >Based upon your explanation, we would need to assume that no wood diaphragm >is flexible and provide stiffness calculations for all structures to >compensate for potential failures based upon rigidity failures of internal >shearwalls since they should not work based upon a proportional distribution >of loading. Yet, historically, buildings designed by proportional >distribution have done well - in fact, any building with a soft-story has >done well when the open front has been laterally secure, regardless of shear >element. Only buildings with "flexible" diaphragms designed by rotational >methods has been the concern. >Ernie, I don't mind changing my opinion if I have something conclusive to >prove it otherwise. Have any models been tested with this criteria? > >Dennis > >-----Original Message----- >From: ErnieNSE [mailto:ErnieNSE(--nospam--at)aol.com] >Sent: Tuesday, April 21, 1998 4:49 AM >To: seaoc(--nospam--at)seaoc.org >Subject: Re: ICBO Seminar for 1997 UBC Earthquake Regulations > > >Regarding the use of Rw=3 on one wall line only where the cantilever column >occurs, I'll be carefull about this. We have to use our judgement. > >For example, a 20 ft. by 20 ft. wood framed garage building with solid >plywood >shear walls on three sides and cantilevered steel columns on one side and >plywood roof diaphragm. Assuming flexible diaphragm, we distribute the >lateral >loads by tributary areas without regards to wall rigidities. Half the >lateral >load in one direction goes to the front cantilevered columns and the other >half goes to the solid plywood shear wall at the rear. Using Rw=3 for the >front wall only is the equivalent of doubling the lateral load at the >front(or >using 100% of the building lateral load at the front wall)) causing the >columns to be stiffer due to the bigger load. > >This is not the usual way I design this type of building. The garage was >just >an example, but on similar buildings of this type, I use judgement. My >feeling >is that the roof diaphragm is not 100% flexible and does not distribute the >lateral loads by tributary width without regard to relative wall rigidity. >Depending on how rigid the rear wall is compared to the front wall, building >dimensions, and other factors that affect lateral load distribution, I use >50% >building lateral load to the front and 80% to 100% building lateral load to >the rear. Now, using Rw=3 at the front, I'll use 100% building lateral load >at the front. > >Another example, a 20 ft. by 40 ft. building similar to the first example >with >two cantileverd steel columns on the front 20 ft. side, a 20 ft. long >plywood >shear wall on the interior wall 20 ft behind the front wall and another 20 >ft >long shear wall at the rear, 40 ft behind the front wall. The side walls >are solid 40 ft. long plywood shear walls. I'll distrubute the loads by >tributary width, but I'll double the loads on my front wall(equivalentg to >Rw=3) and multiply the interior wall load by 1.5 (the equivalent of the >interior wall carrying the building lateral load from the front to the >interior). > >My point is USE YOUR JUDGEMENT. Keeping in mind the intent or interpretation >of a code requirement, do not just follow specific code section application >instructions blindly. Try to imagine how building lateral loads act on the >building based on physical and structural characteristics of the building >and >lateral loading conditions. > >Just my opinion. > >Ernie Natividad > > > > > >
- Re: ICBO Seminar for 1997 UBC Earthquake Regulations
- From: Bill Allen, S.E.
- Re: ICBO Seminar for 1997 UBC Earthquake Regulations
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