Return to index: [Subject] [Thread] [Date] [Author]

RE: storage racks with partially restrained moment connections - wind frame analysis in high seismic zones

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Message text written by INTERNET:seaint(--nospam--at)seaint.org
>The finite element programs we have at my work do not have rotational
spring
elements for beam to column connections.  I have seen ways that beam
moments
of inertia can be adjusted and ways to put in flimsy dummy members to
account for the rotational stiffness but these do not account for a plastic
plateau when the connection does not provide any additional rotational
resistance. 

This is common to all buildings too. They plateau out, but our design is
based on the initial tangent modulus anyway. Given that, why should be
treat racks any differently?

>I have heard 500 kip*in/rad is a typical rotational stiffness for most
partially restrained storage rack moment connections.  Assuming the
connectors do not fail what is a typical limiting rotation when a joint
goes
plastic and does not provide any additional rotational stiffness? <

500 is actually a bit high for many types, but depends on the type of
connector.  However the initial tangent stiffness (i.e. before any
yielding) is typically over 1000. Our office uses an average stiffness
which is curve fit to the hysteretic curve which we determine by test. By
the way, the ductility of these connectors is superb, usually exceeding
0.3-0.4 radians (!). If we had buildings like these we'd be happy indeed.
This behavior is very likely the reason for the excellent performance of
properly designed storage racks.

>1997 UBC 2222.5 requires racks to be installed with a maximum tolerance of
1" in 10 feet.  There are no storage rack drift limits in the UBC for high
seismic areas, has this been discussed for future codes?  This would make
wholesale retail areas safer.  Nobody may have been squished by storage
rack
related damage in Northridge, but when did the earthquake happen in the
morning and were the wholesale retail warehouses open yet?<

No. and all the code changes in the world wouldn't have saved the store
which had the wrong rack in it. It never had a chance. This is an
enforcement, not design issue. On the other hand all the properly designed
stores had NO damage in Northridge. Whether or not they were open is not
important. No one was or would have been hurt (by the racks at least).

>I would like to suggest a code change to limit the calculated  fundamental
period of vibration for racks, require the fundamental period to be
calculated with the Rayleigh method or a finite element model, and have
reasonable drift limits.  <

1) This is throwing the baby out with the bath water. The storage racks do
have these drifts and periods. They have been experimentally verified.
Changing the code to give a different period is flying in the face of
reality.
2) The large drifts and superb ductility are what make racks work.
Tightening up the drift limits would destroy this behavior. What's wrong
with high drifts? They're good for the safety of the structure. Think of it
as base isolation taken a bit further. No one lives in them, so they can
have unrestricted drifts without difficulty and enjoy all the benefits
associated with it.
3) Agreed, and a code change is not required. Calculation methods are
already restricted to the Rayleigh (or other rational) methods. You can't
use the empirical Method A stuff at all.

>It seems crazy to try to get fundamental periods out past 1.5, 2 or 3
seconds to lower the design lateral load on a heavily loaded non-building
structure in occupancies that people are congregating in.  It does not
matter if the rack does not collapse when people are squished by
refrigerators or are squashed by pallets full of 5 gallon laundry soap
buckets flying off the top of 16 foot storage racks.  I don't run when I
walk down an aisle in a wholesale retail warehouse, but I watch for ground
movements.<

Period if fundamentally related to the inverse square root of the period.
You can't have one without the other, We aren't "trying" to get it out
there, it simply is what it is. If you don't like stuff flying off the
racks, you certainly don't want to tighten up the drift limits which
increases the accelerations in the racks. The longer the period, the better
it is for the stuff staying on the racks. For what it's worth, we shook
actual merchandise. Very little of anything fell off, including 5 gallon
paint cans (the 1 gallons did come off).

I think we're in fair agreement. It's just that racks require a shift in
gears. Building engineers are taught to restrict drifts because high drifts
are "bad" in the sense that they make people seasick and crack walls and
windows in little shakers. The idea of allowing very high drifts is foreign
to their thinking. However, drift is actually a good thing. That's why all
drift limits are removed from the non building structures if they can be
accomodated. The ideal is the stationary pendulum. An earthquake doesn't
shake it at all, it simply stays put and the supports move. That's why we
took out the restraints on many suspended wlements in the 97 UBC.

Regards

Peter Higgins, SE