IBC 2009 1807.2.1 Retaining Wall Keyway
IBC 2009 1807.2.1 Retaining Wall Keyway
(OP)
I noticed IBC 2009 is requiring active earth pressure be applied to the keyway in addition to passive resistance (last sentence of 1807.2.1). Any thoughts from retaining wall designers on this topic? This was not in IBC 2006 or 2003.






RE: IBC 2009 1807.2.1 Retaining Wall Keyway
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
EIT
www.HowToEngineer.com
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
Exactly, as all loads, to include lateral, must be traced to the foundation, of which, in this case, the keyway is an integral part.
Sorry for the excessive punctuation... :)
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
So are we saying walls design in 2005 with are underdesigned because active pressure was not imposed?
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
The attached design example #2 picture from Hugh Brooks' Basics of Retaining Wall Design, 9th edition, shows the active pressure going only to the bottom of the footing, not to the bottom of the keyway. However, Figure 8.1 in Brooks' design mauual shows pictorially and states "For sliding force, extend (active pressure diagram) to bottom of key."
Brooks also states, "When using a key to increase sliding resistance, both IBC '09 and the 2010 California Building Code have added Section 1807.2.1 which reads: 'When a keyway is extended below the wall base with the intent to engage passive pressure and enhance sliding stability, lateral soil pressure on both sides of the keyway shall be considered in the sliding analysis.' This means the wall lateral force must be calculated to the bottom of the key rather than the bottom of the footing. This can significantly increase lateral force and require design modifications. This requirement does not affect overturning which is still calculated about the base of the footing."
www.PeirceEngineering.com
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
Does anyone really think that adding a small keyway to a retaining wall will make the problem worse?
Lets say you have a stable retaining wall without a keyway and decide to add a 2ft deep keyway for giggles, do you really think the addition will make your wall start to slide?
I personally have a hard time accepting that. If you find that easy to accept, please help me understand. I agree that the forces need to balance, but maybe our view of the forces is simplistic.
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
One- If active earth pressure should be extended below the bottom of the footing (when you have a keyway), does that active earth pressure start at zero at the base of the footing, or do you continue it from the lateral earth pressure value at the base of the footing. For a 10 ft tall wall with a 3 ft deep keyway that would make a huge difference.
Two-It should matter where the keyway is placed. Obviously if the keyway is below the toe the active earth pressure will not walk laterally to engage the keyway. Similarly if keyway is below the stem. The case ca be made for a keyway below the heel to be designed for active if it is flush with the stem. If the keyway is offset from the stem, the earth pressure line of application is broken, so we have to start the active at zero value. Logically, there has to be an offset distance that will eliminate this active earth pressure completely.
If you look to other countries, a relieving slab is incorporated into the stem to zero out the active earth pressure just below this slab. Same thing for the keyway.
Now we open the gates to problems with older designs. Retaining walls designed with IBC 2003 and 2006 with a keyway will now fail! How come- they are perfectly straight in the real world.
The backfill soils (active earth pressure) does not recognize property line, author's conclusions and changes in building code. It follows stresses and strains. Our designs should do the same. We should follow the soil loading.
I know few structural engineers who stopped using keyways because it is very conservative under the 2009 IBC. I have talked to others who aren't even aware because they just use sofwtare and check for o.k. output.
We should have in my opinion 4 cases: Keyway under the Toe, under Stem, between toe and stem and under L-shaped wall(flush with the stem).
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
www.PeirceEngineering.com
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
EIT
www.HowToEngineer.com
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
PEInc- Yes I though about the bearing pressure and you have a point. Technically, we can say the trapezoidal bearing pressure on the base slab has 0.3 to 0.4 lateral stress component. But when you have a keyway, these pressure are opposite and for the most part cancel each other. If you had a keyway and the passive side was exposed, then yes, you can apply the small active earth pressure due to the toe or heel bearing pressure.
Anyway, I will finish a small paper and submit to IBC in few weeks, I will attach a copy of it to this for at that time.
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
This now has me thinking about overturning...If checking about the toe you may just us the weight of the key but real for OT to occur there would be some sort of passive resistance from the assumed active soil pressure side.
EIT
www.HowToEngineer.com
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
www.PeirceEngineering.com
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
How far do you think this offset should be? A few feet for most small-scale retaining walls? I'm going to make a habit of putting the keyway at the edge of the retaining wall toe to eliminate this issue.
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
I am currently rewriting our design guide on cantilever retaining walls, and we will discuss the specific applications that require these loads to be applied (specifically saturated soils where they act as a fluid), and where they do not (everything else.)
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
It turns out on the extreme end of things, if your keyway depth is more than the heel offset, you are safe to avoid any active earth pressure on the back side of the keyway. See my diagram. Let us hope that the ICC agrees to this for the 2015 IBC version.
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
www.PeirceEngineering.com
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
It is reasonable to assume some reduction in the driving pressure as the keyway moves away from the end of the heel - I use the same concept as shown in the sketch by RFreund, exccept that I assume that opposing lateral pressures due to vertical surcharge from the slab above are balanced (approximation). The code only requires that the lateral pressures be "considered" since this is a complex issue with pressures varying with keyway position.
The design concept is based on a simple "free-body" diagram of the forces on each side of the wall - unfortunately, the exact values of all forces is not so "simple".
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
Here is a link to Section 5 of the Cal Trans Specs.
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
If you can dowel into a permanent slab, that helps the sliding. If its a site wall, then make it bigger. The added concrete for the footing may be cheaper then hand digging the key and install those bars from my experience. IF it's a large wall, you have no choice to make the sliding work but use a key.
Carry the active to the key depth, that's conservative.
RE: IBC 2009 1807.2.1 Retaining Wall Keyway
Now that we're Oh so much better at calculating than the old slide rule dudes, does that mean we can do away with the 1.5 safety factor against overturning and sliding? My opinion is the "old dudes" knew all these issues and agreed, among themselves, the 1.5 factor would cover them. Now, if you really, really want to stir up an issue, why take OTM moments around the toe of the footing and NOT include the effects of the upward soil pressure. If you don't include it then the 1.5 is absolutely needed.
More food for thought?
LonnieP