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Retaining wall sliding stability
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- Subject: Retaining wall sliding stability
- From: <William.Sherman(--nospam--at)CH2M.com>
- Date: Tue, 17 Jul 2007 17:14:53 -0600
Section 1806.1 of
the 2006 IBC requires retaining walls to be designed for a safety
factor of 1.5 against lateral sliding. This raises several questions:
1.
Section
1801.2 indicates that the allowable stress design load combinations in 1605.3
should be used with design formulas in this chapter. If section
1605.3.1 is used, must 0.6D be used in combination with the 1.5 safety
factor? (That would appear to be overkill!)
2. A literal
reading would also imply that this safety factor applies to load
combinations that include seismic forces. (However, there is a "Code
Interpretation" on the ICC website that says that the 1.5 safety factor does
not apply to seismic loads!)
3. The code
does not define how the safety factor is to be calculated. Many
have traditionally used the ratio of (resisting forces) / (driving forces) to
represent the safety factor. But EM 1110-2-2502, Retaining and Flood
Walls, by the USACE, defines the safety factor against sliding as the ratio of
(friction + cohesion) / (net sliding force at base). These are different
values when passive pressure is included. For example:
If H = driving
horizontal force = 100; F = friction = 100; and P = passive = 35:
Traditional SF = (F
+ P)/H = 135/100 = 1.35
USACE SF = F/(H-P) =
100/(100-35) = 1.54
Does the wall meet
the code or not?
The USACE method
seems more rational where a retaining wall extends some distance below grade but
has a more limited differential soil height across it. Take an extreme
case of a narrow wall placed in a trench below grade with only a few inches of
differential soil height. The soil would essentially be at rest with
nearly equal pressures on both sides - but it might be defined as non-code
compliant per the first method, since a resisting force of 1.5 times the
full driving force would be required; however, the second method would only
require 1.5 times the small differential force, a much more rational design.
Comments?
Bill
William Sherman -
CH2M HILL / Denver
Structural Technology
Discipline Leader (TeD)
Engineering Design Group -
Civil / Federal Engineering (EDG-CFE)
720.286.2792
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