Max Horizontal Displacement Cantilever King Post Garden Wall 1

[Dr.Y.Pack]

Hi All, first post on this forum. Please be gentle...

I have already checked the forum for similar topics etc...but I could not find exactly what I was looking for.

I am designing this king post wall with a retained height of 2.35m. This is going to be a garden wall inside a residential property and it will retain just garden so there are no roads, existing structures or services on the back of the wall. This is in UK.

Now I understand that there is not a specific max horizontal deflection limit to meet in the standards. I see also that in the CIRIA C760 says that for cantilever embedded walls a typical horizontal movement of 0.4% of the retained height is to be expected. This means in my case 9.4mm.

I understand also that the table 6.3 from CIRIA C760 is based on embedded walls with excavation that range in depth from 8m to 31m and I would like to hear from you what you think is a realistic horizontal limit. Am I overkilling the design if I use as a reference the CIRIA max horizontal deflection?

My top horizontal expected movement is about 17mm and before to change the design (reducing the spacing of the posts) I would like to hear your opinions regarding this.

Thank you for your help.

 

[BridgeSmith]

The loading condition and stability of the wall is not affected by the deflection, as it might be for other wall types, so it's typically not a design consideration, but it could be a consideration for serviceability.

Any limit for deflection for this type of wall would typically be governed by aesthetic criteria. If it being visually plumb is critical, it would be installed slightly out of plumb to compensate for the expected deflection, so that it is plumb in the final condition.



Rod Smith, P.E., The artist formerly known as HotRod10

 

[PEinc]

More info is needed.
1. Is the deflection solely a structural beam deflection or is it based on soil movement using a program such as L-Pile?
2. Did you design the cantilevered soldier beams with Ka or Ko?
3. Did you apply a safety factor to your passive earth pressure coefficient or did you use the full value of Ka or Ko?

http://www.PeirceEngineering.com

 

[Dr.Y.Pack]

Thank you for the answers so far.

For @PEinc:
1. To check the tip horizontal deflection only, I created a model in Tekla. The steel element is cantilevering freely above the excavation level for 2.35m. Below excavation level the steel post is restrained horizontally by a series of springs with the appropriate stiffness value @250mm c/c. The active and surcharge loads are applied only on the part of the steel post above the excavation level. The deflection is due to soil movement + steel beam deflection (even if the deflection is mostly dictated by the soil movement)
2. To check the deflection on this model I have used Ka characteristic
3. Apart from the deflection check, I did the stability check following the methods explained in this post:

https://www.eng-tips.com/viewthread.cfm?qid=432033.

 

[PEinc]

What about the safety factor? When and where is it applied?
Active earth and surcharge pressures can act on the soldier beam below the excavation subgrade or finished grade in front of the wall. Therefore, they could increase the deflection.
Are you designing and checking deflection for service loads or factored loads?

http://www.PeirceEngineering.com

 

[Dr.Y.Pack]

I am doing this deflection check only for service loads. No increasing factors are applied to the forces and no safety factors are applied on Ka. Below excavation level there is no load applied but only springs which should simulate the soil-structure stiffness interaction.

On the other hand, I have applied safety factors on the forces and the value of Ka and Kp when I checked the embedment length required as per Eurocode 7.
In EC7 I have used the design approach 1 (DA1) for STR and GEO limit states. The DA1 has 2 sets of combination values to be used.
DA1 combination 1 where factors are applied to actions only and DA1 combination 2 where factors are applied mainly to materials.
The STR and GEO checks are satisfied and I adopted the embedment length in the worst case scenario.

Now I was focusing only on the tip deflection check which I am treating as a SLS check. Why do you think I should apply factors for a serviceability check?

 

[PEinc]

I did not say that load factors should be applied to a serviceability check. I said that there was not enough information given in your original post for me to comment on your calculated amount of deflection. On my wall designs, I try to keep my deflections to below 1 inch (25mm). For cantilevered sheet pile and soldier beam walls using allowable stress design and Teng's Simplified Method, I design the beams for the full, theoretical Ka and Kp coefficients and calculate the embedment depth required for moment equilibrium. Them, I increase the calculated embedment depth by at least 20% to get my required beam length and embedment. If you put a safety factor on your Ka or Kp, you will have a less economical design (more bending and more embedment). There are enough references that show using full, theoretical Ka and Kp and then just increasing the embedded length by at least 20%. I almost never apply a safety factor to my Kp. Instead, I increase my calculated embedment depth by at least 20% which results in a safety factor of at least 1.44 on my passive resistance (which is a function of the embedment depth squared). 1.2[sup]2[/sup] = 1.44 = FS which is approx. 1.5.

I never had anyone complain about my calculated deflections. Apparently, your wall will not be supporting any critical structures or utilities. IMHO, your 17mm deflection sounds reasonable. Remember that the maximum beam bending moment is not usually located at the base of the wall. The required beam size should be greater when the beam is designed for the maximum bending moment which should be below the grade in front of the wall.

http://www.PeirceEngineering.com

 

[Dr.Y.Pack]

Thank you for the explanation. This makes sense. I was actually worried I missed some factors on the serviceability check...

I added also a 20% on the calculated embedment length and I have also considered a 10% of retained height unexpected excavation on the front of the wall as recommended just to be sure. This is why in terms of stability and strength I am pretty confident with my design.

I just did not want to overkill it following all these guidance which are probably meant for taller and more "serious" walls.