Shear force values of concrete in shear pin design

[VanNuysDave]

As stability analysis software becomes more sophisticated, we can now realistically incorporate stabilization mitigation methods into the slopes - specifcally - shear pins.

Typicaly we find that shear pins are either driven steel piles or CIP bored piles with either a cage or a beam and CIP's are 2-6 feet diameter. The range of shear forces we see that are used for the steel and concrete section are 11,000 to 75,000 lbs per foot of pile out of plane spacing.

My question is - what is an appropriate shear force to be used for a preliminary shear pin design assuming a CIP aproach with a reinforcing cage ? There are so many combinations that can be used by the structural engineer after we are finished with our stability analyses - so are we being too liberal or too conservative for the preliminary analyses ?

 

[ishvaaag]

Shear is a sectional solicitation and needs be derived for a section. Say a pin embedment trough soil and nailed to bedrock below. Shear may be some assumed force to be taken by the pin at some level below ground, on a tributary volume basis, be it arrayed as a single line or an array in plan. Apart from shear, moment is also likely to be present both for the underground cantilever situation to a maximum near the interface with the soil, within the rock.

The estimates of resistance of anchors to concrete, of every kind of anchor in general, be it preinstalled or post-installed of the variegated patented and not patented forms is an area of continuous investigation and code estimate correction. Anchors to concrete may have some similarities to rock bolt applications yet it is always in so variegated sea of information keep tight to the use; so vendors in the field use to know what they can warrant and a text on say rod anchors for tunnel lining would be of better application in such use than for nailing meshes trough loose slate to degraded rock belos. That is, try to find the exact application you want portraited in the information for otherwise one is lost in the variegated info.

I would try making a search for the exact application you want in google, it will discover nice articles and thesis, and as well a number of vendors ready to show the reach of their products.

 

[VanNuysDave]

Understood - maybe didn't make myself clear - this application is for slope instability (landslide) where the factor of safety is less than the Building Code minimum (1.5) and the shear pins are installed based on the results of site investigation of the soil/bedrock profile and the subsequent slope stability analyses. The mitigated stability relys on the shear capacity of a CIP pile added by by interupting a discrete landslide surface or a theoretical arc of the minimum acceptable factor of safety. The diameter of the CIP shear pin is preliminarily determined by the shear force needed to resist the driving force of the slope. It is the determination of the un-factored applicable shear force of the steel and concrete shear pin that we are having trouble identifying and I am being provided with a large range as discussed in my previous message and wondering for this type of structure (reinforced CIP pile) if this range is realistic.

 

[ishvaaag]

Well, yes ... the numbers don't look bad. I take ACI 318-08 code, examine the shear capacity of one 2 ft and then 6 ft diameter section and I find some consistent statement of for the loads per linear meter you state, a solid stub of say 4 ft be able to meet in arcing action at the point of the interruption of the slope such force. Say, if you only have a service level reaction of the stated values on 4 ft projection from the rock coming from arcing action towards some stub 4 m tall, the capacity of a modest concrete of 3.5 ksi pile is enough by itself to meet such reaction.

For your numbers, each 2 ft pile would be able (more or less) to take shearing action, service level, normal to the pile, just with the concrete, 4x11=44 kip and each 6 ft pile, 4x75=300 kips total service level reaction for the tributary arc of underground volume of earth tributary to the pile.

In all this I have reduced by 1.6 the limit strength of the code, to get the corresponding allowable.

Shear reinforcement at the interface will make these numbers more sure, since spiral or stirrups add to shear strength, and the numbers above are a bit optimistic. A more proper analysis would require the study of the pile under lateral loads, and a proper reinforcement may be found meeting as well what happens upwards of this bottom stub. The capacity in shear rarely would be compromised by that study (otherwise our main arcing action would be being quite underestimated or misplaced) but it may drive the design of longitudinal rebar in the pile, out of sheer projection from the bedrock.

 

[dgillette]

Dave - Unless the failure surface is a specific thin weak layer within very strong material (e.g., a clay parting or slickensides within sedimentary rock), you must check bending also. Think of a vertical beam that is fixed against rotation at both ends but with the upper end moving laterally with respect to the lower end. In soil slopes, you cannot expect direct shear action through the shear pin, and the sheared thickness is likely to be measured in feet, something limit-equilibrium stability analysis can't tell you. Even if the sheared thickness was close to zero, the soil isn't stiff and strong enough to prevent bending moments and there will be shearing around the pin. This can be seen in the literature on laterally loaded piles and on the seismic mod for Sardis Dam, for example.

Regards,
Dave