Designing Rock Anchors

[weab]

I have been doing work for a mining company for quite a number of years. The site engineers are steadily doing less and handing more over to us, an engineering consultant. I have been doing some inconsequential rock anchor designs with little loads. The client likes wall plates with 2 or 4 rock anchors, grouting the gap between the plate and rock face, and welding beams to the wall plates to provide lateral or vertical support for various structures.

What we use primarily are 20mm diameter anchors (rebar). I am not responsible for the grout or how deep they are drilled. However I would like to be comfortable in my designs.

I have been guaranteed a 5 metric ton working (service) load capacity per rock anchor. I have been given a 1 page sample calculation for design of the rock anchor based on a working load. Basically, I must determine the maximum load by taking the tension and shear, use "square root of the sum of the squares" of the loads, and compare that to 5 tons. Is it as simple as this?

Also, sometimes I am given a situation to design a concrete structure to be anchored. The rock anchor extends from the rock at least its development length. Concrete is designed based on ultimate loads, but 1.6 times the 5 tons does not equal the capacity of a 20 dia rebar. However, I say that I don't know the yield of the rebar but I'd conservatively assume 400 MPa (58 ksi) .

I want to design these safely. Is it really this easy? Is there more to it than what I've been given? If so, please provide for me a reference or something to point to so that I can tell the client that I need more information to design a complicated thing. Or tell me that I am over-thinking this.

Any help is appreciated.

 

[Qshake]

The SRSS for completely differing stresses is a bit unusual to me. I see and have used SRSS for bi-axial moments and shears and combinations of shears and resulting force couples (from moments) but not to combine shear and tension.

There is an interaction equation for that where the shear and tension are divided by their respected allowables and combined in sum to be compared to unity. Each term can be expressed to a power of n, where n is usually a value of 1.

This interaction equation is widely used for bolts subject to shear and tension and can be found in many steel texts.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[dik]

Yup Q... I typically use the summation of the shear and tension as the anchor force... the shear friction is developed by tension in the anchor and I consider them additive... without reduction factors.

Dik