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RE: Retaining Wall Design Practice (Seismic)[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Retaining Wall Design Practice (Seismic)
- From: Harold Sprague <spraguehope(--nospam--at)hotmail.com>
- Date: Tue, 11 Nov 2008 16:03:19 +0000
The evidence was presented in the development of the NEHRP Provisions and Commentary. There were no serious retaining wall failures in the Northridge event, nor was Northridge the design earthquake. Building code should not be predicated solely on smaller events. We need to learn from larger events and advances in engineering study and predicate what we learn on what is considered to be the design event. Otherwise there is no need to develop any building code. |
There were several failures in the Kobe earthquake which was still not the design event, but it was closer to the design event in magnitude and effected many soil types. The nature of the soil failures was studied and equations were developed to characterize seismic induced soil pressures. Some of the equations were from studies many years prior to Kobe. Some geotech engineers used them and some ignored them.
The nonbuilding structures committee asked for a more consensus guidance for the determination of retaining wall pressures from the geotech committee. The geotech committee developed a very good commentary which was the genesis of the provisions that became the code.
The NEHRP Provisions are the traditional beginnings of what will become code for seismic. Anyone can access the Provisions and provide comments through the member organizations. The Provisions and Commentary are accessible for free from http://www.bssconline.org/.
"Where are the bodies?" is an unfortunate phrase indicating "where is the evidence?". Prior to the work of the BSSC, there was wide diversity of opinion about the magnitude and nature of the applied seismic loads and the response of earth retaining structures, but there was no consensus. The BSSC process and its evolution to the ASCE 7, IBC, and CBC was not considered without vetting to some of the leading seismic engineers in the country and predicated on a significant body of international work.
I had several teleconferences involving the leading geotechnical engineers in the country prior to moving forward on this. It was not developed in a vacuum, nor without many years of proper vetting.
This process started almost 10 years ago. And was developed under my watch as BSSC PUC TS chair. Arrows and bullets may be directed my way.
Regards, Harold Sprague
Subject: Retaining Wall Design Practice (Seismic)
Date: Mon, 10 Nov 2008 14:31:05 -0800
The inclusion of seismic is actually driving the design on our subterranean projects now. My question, "where is the justification for this"; or as Frank Lew used to say, where are the bodies?
Is anyone aware of a bunch of basement wall failures or retaining wall failures due to seismic? How about any failures? I would really like to be able to point to something to explain the added cost to the client besides, "it's the code now".
Paul Feather PE, SE
From: Phil Doody [mailto:phil(--nospam--at)m-me.com]
Sent: Monday, November 10, 2008 4:01 PM
Subject: Soldier Beam Retaining Wall Design Practice
Is there a consensus on the design of wood lagging for soldier beam retaining walls? Several design methods appear to be in use and each gives very different answers.
Basically, there appears to be at least three ways of designing wood lagging that I have encountered:
#1 Determine the uniformly applied load on the lagging due to the soil pressure at a given depth. Select the lagging based on the required section modulus, S = (Moment)/(Allowable bending stress).
#2 Same process as above but instead apply only 60% of the theoretical uniform load. The justification for the 0 .6 reduction factor is that soil movement causes the lagging to flex outward, and induces a redistribution of soil pressure away from the center of the lagging thus reducing the bending moment. This method is described in the Caltrans Trenching and Shoring Manual.
#3 The third method of sizing lagging is based on FHWA recommendations contained in Federal highway Administration Report Number FHWA-RD-75-130. A table in this report provides the minimum thickness lagging for various soil conditions, soldier pile spacings and excavation depths. This method cannot be used if there are surcharges behind the wall.
The most conservative method is the first method and the one which I am most accustomed. However, I would like to know if others use methods #1 and #2 and if anyone has observed lagging failures using these methods?
Thanks for your input, Phil Doody
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- Retaining Wall Design Practice (Seismic)
- From: Paul Feather
- Retaining Wall Design Practice (Seismic)
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