Offset Dead Load Surcharge

[jagti]

I am looking for a simple mathematical solution for magnitude of offset surcharge above CMU block wall. The wall above is also modular block wall about 1.2m(4.0ft) high. The lower wall is about 6.0ft high. The offset distance is about 6.0ft.between walls. Do i have to included Dead load surcharge from wall above? If i am designing the wall as gravity wall and if yes by how much magnitude. Is there any equation to calculate this type of Load Geometric configuration.

Thanks for suggestion in advance

 

[Boondog]

Yes you have to include the surcharge of the upper wall. If the distance between the tiers is less than 2H of the lower tier, you should include the surcharge. Basically you would take the applied bearing pressure from the upper tier and apply it as dead load on the lower for internal and external stability. From what you describe, once you have performed a global stability analysis of the entire system, you will likely have to include reiforcing in the lower tier.The NHI design manual provides information on the design of tiered segmental walls.

 

[jagti]

Thanks Boondog,

But what I am trying to design is a Multidepth gravity wall system. So within the space limitations I can achieve 2 things eliminate need for geogrid and hence additional excavation. Do just enough excavation for placing the blocks and reduce the width of block as I stack up due to lesser earth pressure from top. I am not sure what type of analysis I should do in order to take the load from the wall above and multidepth gravity wall model since I am designing a gravity wall I will only to external analysis. I tried to model this situation in MSEW software (for geogrid walls) applying a strip surcharge of upper wall from behind. I am wondering if there is any mathematical equation for stress distribution at depth to calculate the soil/ offset dead load surcharge

 

[Boondog]

Delta sigma h is applied as a uniform load to the retained soil. So, your applied load is the surcharge load x the height of the lower wall x Ka. This is added to the retained load (1/2 gamma h squared Ka). Also, MSEW is designed specifically for reinforced walls not gravity walls. It takes a little manipulation to make it work. If you email Dov, he can send you something to guide you in modeling gravity walls in his software. What type of block are you planning to use for the gravity wall? Your foundation width will be quite large with the extra lateral loads to resist sliding. Some block systems are better suited to this than others.

 

[blgeo]

jagti

The NHI Segmental Retaining Wall manual (2nd Ed) does address this case (Refer to section 2.5.4 Tiered Walls), as was previously noted by Boondog. The diagram shown in the NHI manual shows straps however paragraph 2.5.4 addresses how to modify the method for gravity walls.

I have also seen that certain wall manufacturers use an approximatation technique that will effectively neglect the surcharge of the upper wall if the setback is greater than the lower wall height and the upper wall height is less than the lower wall height. Seemingly the wall geometry you have described would satisfy these requirements.