Basic Retaining Wall Question

[STrctPono]

In regards to the design of the basement wall (shown in Magenta) and the cantilever wall above it, would the presence of the gravity retaining wall on the backside have additional negative effects or induce additional lateral earth pressures on the basement wall (Shown in Case 1) vs. as if the gravity retaining wall were not even there at all (Case 2)? In essence, which is the more critical scenario in designing the basement wall?

 

[bratty]

Case 2

 

[PEinc]

The existing stone wall will surcharge the lower portion of the proposed basement wall. Check the design of the new wall with and without the surcharge.

Is the new basement wall to be designed as a continuous wall 18'-6" high or is it two, individual, lower height walls? In any case, a 9'-7 5/5" cantilevered wall will be significantly more critical than a 3'-7 5/5" cantilevered wall. If the wall is full height continuous, the length of the upper cantilevered section will affect the moment in the lower section.

http://www.PeirceEngineering.com

 

[STrctPono]

Thanks for the input.

PEinc, if talking specifically about the lower basement wall (not the cantilever one) is it possible that the surcharge of the nearby rock wall could be more critical than the load on the basement wall as depicted in Case 2? Theoretically, could the pressures at the toe of the rock wall impart more load on the basement wall than if I designed the basement wall with the additional 6ft in retained soil height. Problem is that I don't know much information about the rock wall geometry so I'm trying to rationalize an upper bound limit state approach...

 

[PEinc]

You would need to run the numbers. The lower portion of the new wall has less earth pressure in Case 1 but has surcharge pressure. The lower wall in Case 2 has higher earth pressure but no surcharge load.

How is the upper wall connected to the upper slab and/or lower wall? The upper wall will add bending moment to the upper slab which may add moment to the lower wall.

http://www.PeirceEngineering.com

 

[STrctPono]

Thanks again PEinc,

Lower basement wall is being detailed as pinned at the base and fixed at the top with the slab and wall above. I am making a lot of conservative assumptions on this job. so even though I am detailing it that way, I am still designing the interior face reinforcing as if the wall is pinned-pinned.

I guess I can make some assumptions regarding the batter of the rock wall and embedment depth, run some numbers for the bearing pressure and superimpose that on the basement wall. How about the friction that develops at the base of the rock wall due to sliding? This will create a lateral force on the basement wall that needs to be accounted for?

 

[PEinc]

I am more worried about the vertical bars on the outside face of the structure, near the upper floor slab. With a fixed reaction at the upper slab, you will have moment reversal compared to mid-height of the lower wall section. The taller the upper wall section, the greater the moment of for the outside bars.

http://www.PeirceEngineering.com

 

[STrctPono]

Hmmm. What's to be worried about? It's a 12" CMU block which leaves me enough room to insert an additional bar on the back face where negative flexure exists. See attached sketch for Case 2.

 

[PEinc]

Hmmm. Run the numbers. The cantilevered moment is based on the length squared. 9.63[sup]2[/sup] / 3.63[sup]2[/sup] = 7.04. In addition, the magnitude of the triangular earth pressure at the upper slab for the taller cantilever will be 2.65 times (= 9.63 / 3.63) the earth pressure for the lower cantilever will be 2.65 times the earth pressure at the upper slab for the lower cantilever. All together, the cantilevered moment for the taller 9.63' cantilever will be about 18.7 times that of the moment for the lower 3.63' cantilever. Try squeezing that extra resteel into a 12" block.

http://www.PeirceEngineering.com

 

[STrctPono]

(40x9.67^3)/6 = 6030lb-ft/ft.

For Masonry Design: Kf = 73.0 and Rho = 0.00373.

Mb = (73*12*9.25^2)/23 = 6250 lb-ft/ft OK
As = 0.00373*8*9.25 = 0.276in^2 per Cell or #5@8" O.C.

Thanks.

 

[hotmailbox]

No seismic?