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

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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

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? 

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?

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.  

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


Scott M Haan  P.E.
Plan Review Engineer
Building Safety Division, 
Development Services Department,
Municipality of Anchorage
phone: 907-343-8183   fax: 907-249-7399

> -----Original Message-----
> From:	Peter Higgins [SMTP:76573.2107(--nospam--at)]
> Sent:	Saturday, March 17, 2001 9:23 AM
> To:	INTERNET:seaint(--nospam--at)
> Subject:	storage racks with partially restrained moment connections -
> wind frame analysis in high seismic zones
> With regard to the partially restrained moment connections, the basic
> theory requires that:
> 1) The beam be able to support the load without any support from the end
> connections (i.e. be designed as simply supported)
> 2) The connection have sufficient ductility to rotate more than the end
> angle the beam will have if simply supported without loss of strength
> 3) The connector have sufficient strength to resist the lateral load.
> If the above are satisfied, then the structure will simply distribute out
> any fixed end moments under design lateral loads into the beams, leaving
> the connections to resist lateral loads alone. While not convenient, you
> can verify this in the computer by computing the end moment rotations in
> the
> beams and input them as initial displacements before running the lateral
> loads analysis. You will see that the connections then resist lateral
> loads
> alone.
> This is a very neat analysis trick which greatly simplifies the design of
> a
> partially restrained frame. It has been used for decades, and was
> particularly
> popular in the pre computer days. [The first time I saw such a building, I
> panicked and ran to my chief engineer (an old salt if ever there was
> one)who was
> very understanding about it, gave me the theoretical references, and then
> gently reminded me that the building I was so worried about had been there
> over
> 30 years, was clad in "Barker Board" (a very brittle ceramic tile which
> some of you may remember), and was showing no signs of displacement, let
> alone
> distress under some very harsh environmental conditions in northern
> Alberta]
> As for Home Depot racks, I am biased since I serve as engineer of record
> for virtually all of them in the higher seismic zones. There was no
> Northridge damage in the
> properly loaded racks. One old store did have about 30% of its racks
> damaged, sometimes severely, but it had been overloaded due to an
> oversight
> (the merchandising
> scheme changed, but they forgot to retrofit the racks). For an unbiased
> view, you might contact Andre Filiatrault of the UCSD Powell Laboratories.
> He has shaken fully
> loaded Home Depot racks on his table. At first he was extremely skeptical,
> but has since changed his mind I believe (last statement to me was that in
> an earthquake he's
> getting under the racks). However, he should speak for himself, and you
> might give him a call.
> In Seattle, there was essentially no damage, even in Olympia where the
> ground did shake very hard (probably exceeding 0.4g PGA). We inspected all
> of the stores within 30 hours
> of the event. Same was true for Costco, Sam's Clubs, and Office Depots
> (which I also design). I'd like to think we're doing at least some things
> right.
> Regards,
> Peter Higgins