Retaining Wall reinforcement problems

[Cjpwdesign]

Dear All

Got a comprehensive redesign job for a residential extension onsite at present. The Client had a lot of difficulties with the previous Architect's and Structural Engineers. Neither bothered in coming down to site to oversee the works etc.

Essentially this all came about for me as the Client wishes to build the boundary retaining wall from reinforced masonry to make it easier and faster to build on site, as the wall stem transitions into a standard cavity wall above the external ground level. The surrounding site is only farmers fields. The ground slope from rear to front of the extension from about 2m down to 0 over a 7m length.

The rc concrete base and starter reinforcement bars have already been cast as per the engineers sketch attached. The first glaring error is the L-bars are backwards. I did raise this concern with the Client however he doesn't want to spend the money ripping up what has already been done.

I have reworked the design to reduce the overall stem height to maximum 2m, which should allow me to use a reduced anchorage length for the starter bars (intuitively taking into account a reduced efficiency in the opening stem/base joint).
I also proposed an upper tiered reinforced masonry retaining wall to deal with the remaining height of earth. The upper tiered wall will be treated as a separate structure with a shear key in the base to resist sliding therefore not having it's base propped by the head of the retaining wall under.

Given the current starter bar config, a strut-tie model would use a C-T-T node at the L-bar turn into the base reinforcement, putting the Lbar longer leg into tension and the shorter leg in compression. Would anyone like to hazard how thid analysis would even work?

I am concerned with brittle fracture of the concrete (even with the reduce stem height) due to the opening up of the junction between stem and base.

The previous engineers calcs for the wall are a bit of a mess, even using Tedds software, as some of the assumptions were wrong and i get a tad apoplectic everytime I look at them. (I have redesigned the entire extension as so much was just left out, like 2m roof overhangs, wall movement joints, balcony beams under torsional loading etc. etc!)

All thoughts are greatly appreciated.
Many thanks, Chris

 

[BAretired]

The rc concrete base and starter reinforcement bars have already been cast as per the engineers sketch attached. The first glaring error is the L-bars are backwards. I did raise this concern with the Client however he doesn't want to spend the money ripping up what has already been done.

Sounds like a proper mess, but you may not have to rip up what has been built. Starter bars are H10-100 (850x400). I assume that means 10M bars @ 100 o/c. If by "backwards", you mean the horizontal leg is pointing to the toe instead of the heel, I would not think that is something to worry about, but others may disagree.

I have no idea what B1131 or B 785 Mesh Fibre means, and whether or not it is adequate for a 2m high retaining wall. I suggest you check the reinforcement which has been placed. If the starter bars are deemed to be too short, weld extensions. If they are not adequate, perhaps you can add new bars drilled and epoxied into the 360mm thick base.

If you continue with this project, and you accept what is shown on the drawing, you are accepting professional responsibility for the job. Make sure that you do not accept something which may get you into trouble later on.

Given the current starter bar config, a strut-tie model would use a C-T-T node at the L-bar turn into the base reinforcement, putting the Lbar longer leg into tension and the shorter leg in compression. Would anyone like to hazard how thid analysis would even work?

I don't agree with the red text, but I could be wrong. Maybe someone else will comment on that.

By the way, where is this project?

 

[Cjpwdesign]

Thanks for the reply BAretired. Project is in Mid Wales, UK. B1131 mesh is H12-100 centres longitudinal with H8-200 transverse. B785 mesh is H10-100 instead.
I have redesigned the wall for a reduced stem height with the starter bars extending into the stem as the main longitudinal reinforcement (no sense jumping up a bar size to H12's). Calculated the revised Anchorage length needed based on area of steel reqd vs area provided and the length is shorter than the amount if bar minus bendcradius etc. So am relatively happy with that.
All I meant by the highlighted part in red was a FBD of the stem and heel gives corner opening moments thereby inducing tensile stress to earth face of stem and top face if heel, meaning the LBar short leg winds up in a compressive zone. So my concern from a bar stress analysis point if view was how to deal with the transition from tensile stress to compressive stress with a radial bearing stress and bar slip...
Other than that I could spec a relief shelf to back of wall. Will check numbers in the morning

 

[Cjpwdesign]

Just to clarify the offence in question. Was down at site today and client informed me of reply from original engineers. So I fired off a sketch in response, as original engineers trying to justify bar orientation

 

[kcer]

I'm perhaps missing something, but each of the elements (I assume at this height) are designed as singly-reinforced cantilever sections. Detailing aside, the presence of the compression reinforcement in each of those elements is immaterial. Notably, if you've since altered this to a cavity reinforced wall, typical detailing provides only for a single layer of tension reinforcement. As such the tension lap orientation and length shown for the stem on the original calculation seems acceptable to me.

If compressive stresses in the reinforcement are a concern (I suspect not at the reduced height?) then each section could be analysed as doubly-reinforced to obtain.

The original height of the wall versus the wall thickness is curious, also.