Retaining wall with Key Footing per AASHTO LRFD 2

[precast78]

I am creating a spreadsheet to design a cantilevered retaining wall per AASHTO LRFD. I feel like I am missing something. When I add key at the bottom of the footing, I will increase my triangular load of my active pressure. But then my passive pressure on my key isn't that high because I use .5 resistance factor per Table 10.5.5.2.2-1. It seems to me making the key deeper doesnt help because you are increasing your active pressure as well. What am I missing?

 

[RFreund]

There is some debate, see here:

http://www.eng-tips.com/viewthread.cfm?qid=332224

EIT

http://www.HowToEngineer.com

 

[precast78]

Thanks RFreund! I am still not sure what to do even after I read all of the replies .

 

[RFreund]

Unfortunately I'm not as well versed in AASHTO, do they have similar language as to the IBC about 'considering' active earth pressure to extend to the bottom of the keyway?

What you are saying appears to be correct according the letter of the code. I'm just not so sure it is warranted. In any case your passive pressure would be increasing about 10x 'faster' than your active so you should still get some help.

It's an interesting situation. If you take away the footing and imagine it's just an embedded wall, than it seems obvious to include active pressure on the keyway. However, when you consider the footing with the keyway below the footing and you start drawing pressure diagrams, then it seems like the active pressure is interrupted and there is will be a bearing surcharge on each side and it gets a little funky. Then you start to see that the position of the key affects these pressures, which seems odd that the horizontal position of the key could actually affect its resistance capabilities. Finally when you start to apply reduction factors to the passive pressure you would think that the soil above the footing is less reliable than soil below the footing. For factors such as erosion, frost, someone who digs holes in front of walls, poor compaction if backfilled etc, not to mention that the soil below the footing and in front of the keyway has an additional surcharge load due to overturning which will push against the key wall.

So, yeah I guess I'm not sure. Conservative to consider the active pressure though.

EIT

http://www.HowToEngineer.com

 

[Surya77]

I assume you have issue with sliding

this is my take

make sure u are calculate upward earth pressure correctly at the base. It will be different at heel and key face. similarly it will be different from key back to toe.

From toe to key the Fric Resistance is product of upward normal force internal friction

BUT from key to heel it is product of normal force to coeff of sliding fric

Check this and see what happens

 

[precast78]

Surya, yes I am planning to do that. Basically in front of the toe is Soil to Soil friction, behind the key is Concrete to Soil friction. I found 2 examples now from old textbooks that do not use active pressure on the key.