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Re: Seismology Opinion - Cantilever Columns

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I wrote the Opinion and will be more than happy to discuss the decision
process that was involved.  Our meeting for the March 25 seminar will be
Green River golf course 3:00 PM, 2/8/00.  I sent you an e-mail about it last
week but have not heard back yet... can you be there?

Bill Nelson


----- Original Message -----
From: SEConsultant <seconsultant(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Saturday, February 05, 2000 1:27 PM
Subject: Seismology Opinion - Cantilever Columns

> I just read the Seismology Opinion for the use of Cantilever Columns. The
> provisions are not too bad until you get to issue number 5 which state:
> "The total shear in all columns combined shall be less than 15% of the
> shear based on tributary area."
> I interpreted the document to read:
> You may design by factoring up the loads to the embedded columns only IF
> only if these provisions are met - otherwise, you must factor up the
> structure in the direction of force to match the lower R.
> Let's look at a couple of conditions to see how restrictive this actually
> is:
> 1. It eliminates most conditions where embedded columns are used between
> lines of adjacent shear (assumed 50% of the tributary force) or at either
> end of a series of diaphragms (assumed to be one) resisted by three lines
> shear (approximately 25% of the load at the ends) or any condition that
> not result in a very small tributary width totaling no more than 15% of
> story shear.
> This is a very restrictive clause which will require most structures to be
> penalized by forming them into compliance with a total lowered R in the
> direction of applied load.
> In my opinion, limiting the rule by a percentage of the base shear is too
> generic a method of determining appropriate application because it tends
> eliminate most conditions other than the edge of a patio structure. If
> was the intention, why not just restrict it this way rather than trying to
> do it mathematically?
> What appear rational, is the restriction of story drift to 0.005H or "the
> approximate deflection of the adjacent shear walls in the same orthogonal
> directing". I agree with this - it is rational and insures a uniform
> stiffness - it makes sense. 15% of total base shear makes no sense.
> The opinion also limits the column axial stress ratio based on a K=2.1
> (which is typical of a column allowed to rotate and translate at the top
> while fixed at the bottom) to be less than 10% of the allowable axial
> (fa/Fa). This does not makes sense, simply because the steel section
> required to resist the story drift restriction will make this a moot
> In most cases, in lightweight wood construction, the axial capacity of the
> column will be many time greater than the actual load.
> Let's look at a simple 10' Pipe column which is designed for a one kip
> lateral load. The factored load will be 2.5 kips (neglect axial load for
> moment). The minimum size of a pipe column to resist the load will be P8xs
> (8" diameter extra strong schedule 40 pipe column).  This is required to
> keep the story drift below 0.6" at 0.005H (a P8std will deflect about
> exceeding 0.005H). The allowable Axial Stress Fa is around 22.7ksi for
> DL+Short Term loading. This is where the axial ratio gets kind of
> because the columns would be allowed up to approximately 24.3 kips just to
> meet 10% of their capacity.
> I'm trying to understand how these restrictions were arrived at - maybe
> Seismology committee can provide notes as to what rationale led to these
> restrictions so, as designers, we can understand what the group is
> attempting to protect. This example uses a relatively small lateral load
> only 1000 pounds. The load to a front of a 22' deep garage caused by wind
> loads is closer to 2000 pounds of shear or 5 kips applied to two columns
> (factored 2.5 times).
> Now here is the next part that confuses me:
> In my mind the stiffness of the resisting element has less to do with it's
> material than the ability to calculate the actual deflection under a given
> load. Therefore, why does it matter whether we are dealing with an
> steel column, a plywood shearwall or a masonry wall. As long as we can
> calculate the deflection on each element we can compare performance under
> load. The stiffness or rigidity becomes a comparison between calculated
> deflections in each line of shear.
> If the goal is to balance deflection - essentially equalizing stiffness
> between lines of shear - why would we want to factor uniformly through the
> structure. I would think that we would want only to factor up the loads to
> elements that are normally more flexible in order to make them converge on
> the same stiffness as the more rigid elements.
> I would like to read the rationale on how this Opinion was formulated. Any
> suggestions on how this can be accomplished?
> Regards,
> Dennis S. Wish, PE
> Structural Engineering Consultant
> (208) 361-5447 E-Fax