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RE: journalist query - factors of safety

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US Army Corps of Engineer's publication EM 1110-2-1913, Design and Construction of Levees, does in act show a minimum 1.3 factor of safety for slope stability at "end of construction" (Table 6-1b), and even permits factors of safety of 1.0 to 1.2 for rapid drawdown conditions (lower probability conditions).  For earth dams, EM 1110-2-1902, Slope Stability, also has a minimum required factor of safety of 1.3 for slope stability at "end of construction", and 1.1 to 1.3 for rapid drawdown.  Dams are required to have some "freeboard" above flood level, whereas levees are only required to account for long-term settlement (without added freeboard).
For concrete floodwalls, EM 1110-2-2502, Retaining and Flood Walls, requires a sliding factor of safety of 1.5 for the design flood condition and a reduced factor of safety of 1.33 for water to the top of wall (assuming some freeboard is provided). 
EM 1110-2-2200, Gravity Dam Design (for concrete dams), requires a sliding factor of safety of 1.7 for Unusual Load Conditions, including the Standard Project Flood.
I can't explain the reasons for the differences - just reporting the facts.
William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

From: John McQuaid [mailto:John.McQuaid(--nospam--at)]
Sent: Friday, November 04, 2005 8:34 AM
To: seaint(--nospam--at)
Subject: journalist query - factors of safety

Greetings. I’m trying to learn about factors of safety in general terms. In levees/floodwalls, documents show the standard FOS to be 1.3. We’ve had some engineers saying this was too low, at least for New Orleans, given the extreme risks involved with a breach during a hurricane. But I am trying to set this issue in context. What are typical safety factors for other kinds of structures – dams, highway overpasses, hospital buildings, etc. Are they prescribed by code or statute in some cases, or by some other means? How are extreme risks, loss of life, property, etc. taken into account? I won’t publish comments generated on this list. Just seeking input/perspective at this point.



John McQuaid

The Times Picayune, New Orleans

Washington bureau

1101 Connecticut Ave. NW s. 300

Washington, DC 20036