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Re: Period Calculation for One Story Building with Flexible Diaphragm, FE

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Frank, the subject at hand is taking on as many permutations as the menu in
a Chinese restaurant.

Returning to the original concern about the chord component of diaphragm
deflection, I doubt that any consistency can be had from one engineer to
another in modeling what the chord is and what it does, if any discretion is
left to the engineer. If I evaluated the same building on three different
occasions, with a spell of amnesia in between, surely three different
results would come out.

And I am not yet allowing for variations in objectivity. Usually there will
be a client party who hopes fervently for an overall outcome that minimizes
the cost or scope of strengthening work. They will want their engineer to
represent their interests in the way their lawyer does: candidly on private
matters but putting the best face on public ones. Maybe an engineer will be
pressed to retrofit-in a set of bungee cords and call them the diaphragm
chords, to take advantage of their beneficial "effect" on calculated
building period.  

In other words, leaving discretion to the engineer is to tempt bias and
reliance on loopholes.  But to eliminate this discretion, and cover every
contingency so the engineer can only plug in the numbers in a ministerial
way, throws all the discretionary burden onto the code writers. Are they
good enough to cover everything accurately, in advance, in their formulas?

At heart, it seems once again that the issue is in how much control to
exercise in advance in code provisions, how much discretion in evaluating
the particulars to leave to individual engineers, and how much
check-and-balance control over those engineers and their discretion to
maintain in the plan review stage.

As for this FEMA document, I haven't seen it, but I already feel uneasy
about its reliance on calculated building period as being decisive in
determining the lateral forces a one-story building will experience. I think
there is a scale effect being overlooked somehow, where actual displacements
are more than can be easily swallowed up in the limited dimensions of a
single story. Look at what has been done in the 97 code for upwardly
cantilevered steel columns that laterally brace apartment floors above. R
factors got severe. Softness, lateral drift, and lengthened period have
become penalties, not benefits, in making lateral force determinations. The
larger tilt-up buildings have been found to flap sideways more violently
than expected at diaphragm midspans, due to period-reducing diaphragm
flexibility, than near to the cross walls. Is this period business in FEMA
310 sound or humbug?  Are we in the right restaurant?      

Apologies for begging the questions more than answering them.
Best Regards
Charles O. Greenlaw, SE     Sacramento CA


At 08:04 AM 6/12/98 EDT, you wrote:
>Again, I raise the question: "Using the FEMA 310, Equation (4-1) to calculate
>the period of a single story building with a single span flexible diaphragm,
>which is subject to many interpretations and possible misunderstandings, are
>different engineers able calculate the "Pseudo Lateral Forces", using the
>period, T, from this equation, on a reasonably consistent and uniform basis
>that it is good public policy to put such an equation in a national guideline
>for the seismic evaluation and retrofit of an existing building?
>Frank E. McClure    FEMCCLURE(--nospam--at)aol.com