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Seismic design of pole building - HELP!
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: seaint <seaint(--nospam--at)seaint.org>
- Subject: Seismic design of pole building - HELP!
- From: "Mark T. Swingle" <mswingle(--nospam--at)earthlink.net>
- Date: Tue, 18 May 1999 23:39:39 -0700
Dear fine people, I sent this message 2 weeks ago and only received TWO (that's 2) replies!! It is full of very interesting code issues, at least IMHO. Please read again and comment on anything you can think of. You can send it privately instead of to the server if you wish. Thanks!! Mark Swingle Mark T. Swingle wrote: > -------------------------------------------------------------------------- > Your following message has been delivered to the list > seaint(--nospam--at)seaint.org at 00:54:06 on 5 May 1999. > -------------------------------------------------------------------------- > > I am designing a pole building in northern California (Mendocino > county, near Hwy 101). The poles are already in place, but the client > would like me to design the structural system for gravity loads and > lateral loads. > > The poles are tapered wood poles, approx 12"-14" in diameter at the > base, and are embedded 8 feet in the ground. The structure is two > stories above the ground (raised floor diaphragm and roof diaphragm) > with floor-to-floor heights approx 10 feet. There are 3 rows of 8 > poles each, with the rows 16 feet oc and the poles 8 feet oc in each > row. > > The client would like to keep the interior spaces as open as possible, > and therefore would like to avoid shear walls. The proposal is to use > knee braces at each pole in each direction. If the braces are at 45 > degrees, then each leg would be approx 2.5 feet long to keep the > braces above 7.5 for adequate headroom. Whether wood or steel braces > are to be used is an open question (see below). > > I WOULD APPRECIATE SUGGESTIONS, COMMENTS, AND CORRECTIONS ON THE > FOLLOWING QUESTIONS AND THE LINE OF REASONING: > > 1. WHAT IS Rw? > > This building does not seem to fit into one of the accepted > structural systems listed in 1994 UBC table 16-N. > > Strictly speaking, this is a moment frame system since the poles > are required to resist lateral forces through bending, shear, > and axial forces. However, Item 3.4 lists only steel or concrete > ordinary moment-resisting frames (not timber). If steel braces > are used, then this would be some sort of mixed system anyway. > > Item 2.4.c. (Ordinary braced frame of timber in a building frame > system) seems like a close match, but Rw=8 seems too high for the > actual lateral system proposed (see below for discussion). > > Item 1.4.a. or 1.4.c. (Braced frame of where bracing carries > gravity load) seems like a close match too, but this is for a << > Bearing wall system>>. However, using Rw=4 (for timber) or Rw=6 > (for steel) seems like a good idea (again, see below). > > 2. HOW DO I GET DUCTILE BEHAVIOR? > > This lateral system seems to have little ductility if > constructed with wood braces. It may have some with steel braces > (see below). > > I do not have any experience with wood braced frames or knee > braces resisting lateral loads as the primary LFRS. If a wood > brace were to buckle, it seems that the connections at each end > would fail if the connection were comprised of more than one bolt, > due to the rotation at the ends. > > Steel knee braces may perhaps be made to work, using connections > at each end similar to that used in special concentric braced > frames where a gusset plate is allowed yield when the brace > buckles (in the plane formed by the brace and pole). However, I > don't think <<exposed>> steel braces are what the client had in > mind for a pole house in the country! > > In any case, it seems prudent to use a shear demand on the pole > that would correspond to yielding of the brace, using conservative > material properties. > > 3. HOW DO I CALCULATE DRIFT, or, HOW DO I MODEL THE FOUNDATION? > > How do I account for drift without a soils report? How do I know > how much the building will deflect under <<real(?)>> earthquake > loads with such a "flexible" foundation. The pole formula in > Chapter 18 (1994 UBC 1806.7) will give me the moment capacity of > the pole at the foundation (assuming the worst soil) but it does > not say what the rotation is under that load. I realize > determining the rotation is complex (read: non-linear), but I > don't know where to begin (other than hiring a soils engineer). > Perhaps some of you will think I'm making this too complicated, > but I don't want to ignore the drift requirements. The whole > intent is to avoid collapse due to P-delta effects, isn't that > right? > > Put another way, how can I model the superstructure reasonably > accurately without knowing how much the pole will rotate at the > base? > > Based on the above discussion, my inclination is to use Rw=4, C=2, > and to design as much ductility into the knee braces as possible > (whether wood or steel). I am tempted to use Rw=3 for an inverted > pendulum, but the "real" C value would then most likely be lower > than 2 anyway, due to the long period of the structure. By using this > approach, most of the elements will remain elastic anyway, assuming a > built-in factor of safety of at least 3 or 4 anyway (for the > connections and for the pole values for axial, shear, and moment). > Remember, this is relatively far from active faults. > > There are some other aspects to this this building (that I have not > described) that make it even more complicated. I will mention those > only if it seems necessary to do so. I think I have described enough > to generate some comments. > > I would also appreciate any references (Internet or otherwise) > regarding this type of construction. I know this type of construction > is used all over the US, but I am particularly concerned with adequate > seismic performance. > > HELP!! > > Mark Swingle, SE > Oakland, CA >
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