Alternative Options for Shear Reinforcement at Walls of Tunnels 1

[Romano11]

Hi all,

I am working on a design of a tunnel in California. Based on my calculation, we need shear reinforcement at the base of the walls, where the walls are connected to the mat foundation (see attachment). However, for ease of construction (placing steel and pouring concrete), we prefer not to put stirrups in these highly condensed areas. I came up with several options to replace the traditional use of transverse stirrups. The options are described below (see attachment for pictures):

Option 1 (Traditional stirrups): 8#4@9" stirrups are used as shear reinforcement.

Option 2: The use of 2#9 diagonal bars to replace the traditional stirrup reinforcement. One ends of the bars are hooked to the mat foundation and the other ends are extended to develop full reinforcement capacity.

Option 3: The use of concrete blocks under the slabs on grade to increase the thickness of the critical shear sections (around the bases of the walls); thereby, eliminating the need of stirrups.

I am currently advocating option 2; however, I could not find any code or academic articles to support for it. Is anybody here knowledgeable about the use of diagonal bars for shear reinforcement? How do you think about option 2 and option 3? Any advantages or disadvantages in adopting these alternatives? Any ideas are welcomed.

Thanks,
Romano

 

[ishvaaag]

Certainly inclined rebar can be used for reinforcement against shear; the traditional concept was just a division by the sine of the inclination to get the force in the inclined bar; this is tantamount to one strut and tie scheme that must be checked. The most classical book in Spain about concrete (Montoya-Meseguer-Morán) had its section on inclined rebar for shear. Current code CTE has also specs for them as if inclined stirrups, and ACI 318 08 states

11.4.1.2 — For nonprestressed members, shear reinforcement shall be permitted to also consist of: (a) Stirrups making an angle of 45 degrees or more with longitudinal tension reinforcement;
(b) Longitudinal reinforcement with bent portion making an angle of 30 degrees or more with the longitudinal tension reinforcement;

so it is clear that they are permitted for RC sections.


The ACI 318 08 proportioning in 11.4.7.5 is tantamount to the classical approach.

So you need not any academic article, just the code is enough to justify the use of inclined rebar as reinforcement in shear.

As usual, good development length must be ensured at each end of bars; by the Spain's code we would add ld+d at both ends.

From your sketch, I don't like the idea of just concreting what seems a separate block of unreinforced concrete to supplement resistance against shear; my caution is that positive transfer of the shear is required, quite likely through shear-friction scheme (involving some amount of steel) or whatever the scheme you want to be in place.

I am every day less and less a fan of strut and tie works for what could be inadvertly overstressed situations, involving some disproportionate amount of manual work to identify the valid schemes. In these cases, a FEM model can help to quickly ascertain the maximum values of the tensile and compressivie stresses and their directions, and so be a more perfect way to identify the specific need of overall (tension plus shear) reinforcement in the area.

 

[ron9876]

Increase concrete strength or thickness. If not then thicker walls at the bottom would be my next choice.

 

[JedClampett]

I've done 1 and 3. As far as 2, I see a couple of drawbacks. It's not a code recognized approach. Common sense, maybe, but not in the code. I'd be concerned about spacing and getting concrete past the bars. And if your assumed failure plane is off a little bit, are the bars effective?
Are you worried about internal or external pressures? For external, you can check shear at a distance "d" from the support.
What's the problem with adding wall thickness? Concrete is cheap, it will help your moment design, the concrete will be easier to vibrate.

 

[Romano11]

@Ron and Jed, based on my calculations, I will need to increase wall thickness from 24" to 36" to totally overcome shear reinforcement. I am not sure I saw a wall that thick for underground tunnels (thicker wall means more excavation work and thereby more cost?)...

The height of the wall is 22', and if we use the traditional stirrup placement, we would need around 70" of shear reinforcement (8#4@9"o.c.). If we are allowed to use diagonal bars as Isvaaag pointed out, we would save some steel and make it much easier for construction contractor. But honestly, I haven't seen any diagonal shear reinforcement like that during my 3-yr career.

Isvaaag, do you have a link to the Spanish concrete code or the book that you mentioned?

I will run my calculation based on what Isvaaag pointed out in ACI 318 and will be back soon...

Thank you!

 

[JedClampett]

Excavation=Cheap
Concrete once you've paid for the wall forms=Cheap

On the other hand, you'll have walls with congested reinforcing, running the risk of rock pockets, plus that reinforcing isn't free.
I go out on a limb every once in a while, but a good rule is "If you haven't seen it, you better be really sure before you use it."

 

[ishvaaag]

http://www.fomento.gob.es/MFOM/LANG_CASTELLANO/ORGANOS_COLEGIADOS/CPH/instrucciones/EHE08INGLES/

This is the current concrete standar for concrete in Spain. Inclined bars contribution is acknowledged through its alpha inclination angle. See chapter 10, article 44.2.4.2.2

Note that if you don't feel covered by the setup in the article, that "forgets" to explicit the single inclined rebar case (for which no separation between bars can be stated) -something that was not the case in former codes- you are entirely covered by chapter 9, when dealing with strut and tie matters, of which the single inclined rebar is just a case, and properly one, since corresponding to the area of influence of the joint.

The strut and tie reasoning most surely can be extant in any modern concrete model in the world. However I would not forget to investigate the concomitant effects of moment and shearwith your proposed rebar setup to ascertain nothing is left out of consideration (i.e., you can't forget moment is also present, what will add to the compression in the concrete strut to check, whereas the longitudinal and steel (inclined) shear rebar may stay being accounted separately for equilibrium). This notional setup should be a conservative one.

I only have now at hand one 2000 edition pdf of the Jiménez Montoya book, my paper far older edition being at the moment inaccesible. Anyway the only "problem" with the 2000 edition is that it also counts on a "separation" between inclined rebars, as if for stirrups. If nothing explicitly prevents it in the standing code (CAL, AASHTO or whatever) or just with the strut and tie basis I might proceed along, if that is what you want.

However it is clear that to not force the concrete is shear is much convenient, yet, anyway, if we have some way to show code compliance enough, we need not be particularly timid.

 

[ishvaaag]

To be consistent, if any axial compression is extant, it must also count to contribute at least at its average value to the strut compression in the strut and tie scheme.

 

[Grahammachin]

Could you not do a hybrid of options 2 & 3 and add a 45degree chamfer between the wall and base. In this chamfer you could then run your diagonal reinforcement through the chamfer.

This would look similar to virtually any precast box culvert I have seen over here in the UK.

 

[Teguci]

Is this a seismic load condition subject to reverse loading? If so, I would consider confining steel a good idea. Also, 11.9.9 seems to require horizontal steel shear reinforcement.

The #9s will get your shear across the construction joint, but I would put the horizontals in for the wall itself.