AS3600/AS5100.5 CL8.4

[SayGoodDay]

Hi

I have a question about the longitudinal design shear strength.

The application in AS5100.5 stated, this clause also applies to the transfer of shear across any specific interface such as between precast and in situ concrete or across construction joints.

So my question is, now, I am going to anchor some bars into rock bed to control the shear, what is the value of d in the equation at CL8.4.3 then?

Thanks very much.

SayGoodDay

 

[csd72]

SayGoodDay,

This clause only applies if the material onn either side of the interface is required to act as a single member.

I very much doubt that you need the rock and the concrete to act as one, what is your exact situation?

 

[SayGoodDay]

Hi csd72

My case is, I want the anchor (say N32) take shear force (only) between the rock and concrete footing. I don't need them to act composite, but shear only to resist the lateral force.

My other case is, some bars come out from the column into the headstock to resist the braking load. The bars used only for shear also.

I have trouble to understand these two case by applied the CL8.4.3. Not sure what is d in the equation.

Thanks.

 

[apsix]

I also can't see how to use the equation in your situation, although the additional sentence in AS5100.5 seems to imply that you can.
I've attached an extract from BS8110 which is straight forward to use. Being conservative, I would use the coefficents from AS3600.

 

[SayGoodDay]

Thanks apsix.

 

[csd72]

SayGoodDay,

What you describe is NOT longitudinal shear, it is technically just frictional resistance.

I would suggest you look up a geotechnical text for a typical friction value of sandstone and use that.

 

[SayGoodDay]

Hi csd72

I prefer only use the bar to resist the lateral force. I know there may be some friction, but because the sandstone is very smooth, so I prefer to ignore it.

 

[OzEng80]

SayGoodDay,

If you intend to ignore friction, just use the shear strength of the dowels (bar shear) and forget about trying to apply longitudinal shear formulas. As CSD has stated these formulas are for ensuring composite action - which is not what you are trying to achieve.

 

[apsix]

If it is a low strength sandstone failure maybe due to crushing/pullout and not limited by the dowel strength.

csd72, OZEng80;
I think that we all realise that the code clause deals with longitudinal shear within a beam but AS5100.5 adds the line;
"This clause also applies to the transfer of shear across any specific interface such as between precast & insitu concrete or across construction joints"

Do you understand this to still refer to longitudinal shear in beams only also?

 

[OzEng80]

Interesting. Pretty specific statement.

Sounds like you could use it providing you can adapt the d/s terms (d =effective depth of composite beam, s= spacing of reinforcement crossing the shear plane) to suit.

Not sure why you would bother if the friction is not intended to be utilized as the formula does not account for low concrete strength either (no f' term with the As).

You would be best served to take a conservative estimate of the rocks compressive strength and use the chemical anchoring manufacturer's (Ramset/Hilti) shear values for your specified embedment and epoxy.

As a side rant, I recently questioned a ‘longitudinal shear’ seminar presenter about whether steel crossing the shear plane (the As in the formula) was required to be developed (in tension). He said that a tensile development was required and the shear force across the plane should be thought of as a tension (hence the f’ct) with the steel acting as tension ties (applying a normal force to aid friction). He then proceeded to show about 50 slides of ‘composite’ construction with steel that was not even close to being developed in the topping. Challenged again regarding the requirement of a tensile lap he gave the infuriating reply ‘that is just the way that it is done’. I have since reasoned that possibly a larger area of steel may have been provided with a reduced stress, but am not convinced. I believe that value of fsy in the formula should be used carefully….

Your thoughts would be appreciated?

 

[apsix]

OzEng80
I guess different mechanisms are described by different theories. Three that I can think of;
1. Relies on tension in bars crossing the shear plane to enhance friction. (Described in your seminar and in my link to BS8110)
2. Relies on a dowel (or shear stud) in shear.
3. Relies on adhesion of concrete; eg insitu topping on precast planks.

I'm certain that in reality there will be interaction between the mechanisms.

 

[OzEng80]

Aspix
The mechanisms you are suggesting make sense, and are accounted for in the formula except for ‘dowel shear action’. Perhaps this is a secondary effect that occurs after initial tensile yielding of the reo, and is a serviceability concern?