Reusing Concrete Pedestals for New Light Poles

[CANeng11]

We have been asked to look into the possibility of reusing existing concrete pedestals for the installation of new poles. I have attached an example picture of the pedestals. When designing the anchors for the new poles, I am having difficulty making it work using cracked concrete state, but I can make it work with uncracked concrete. I'm looking for clarity on when it is appropriate to assume the uncracked concrete state and whether I can apply that in this case.

 

[r13]

Is the load increased? Do you need to replace the anchors? Did you check to see how the existing was designed?

 

[phamENG]

Generally, you use cracked when analysis says that it will crack (tensile stresses in the concrete exceed the modulus of rupture). It does not refer to typically microcracking found in all concrete. Locations would be the underside of a beam with positive flexure, the top of a beam with negative flexure, or in a lot of seismic applications where cracks occur in more places.

I wouldn't expect to need to use cracked values here.

 

[CANeng11]

retired13, the original anchor bolts were already cut off in line with the top of the concrete. The loading should be fairly similar to the original loading.

 

[AskTooMuch]

Taken from Hilti Q&A


Concrete cracks when tensile stresses in the concrete imposed by loads or restraint conditions exceed its tensile strength. Concrete is typically assumed to crack under normal service load conditions. Crack width and distribution are generally controlled through the use of reinforcement. With consideration for the protection of the reinforcing steel, crack widths are assumed to be less than approximately 0.012 in (0.3 mm). Under seismic loading, flexural crack widths corresponding to the onset of reinforcing yield are assumed to be approximately 1-1/2 x static crack width = 0.02” (0.5 mm). Both ACI 318 and the International Building Code assume cracked concrete as the baseline condition for the design of cast- in-place and post-installed anchors since the existence of cracks in the anchor vicinity can result in a reduced ultimate load capacity and increased displacement at ultimate load compared to uncracked concrete conditions.


Design for uncracked concrete conditions is permitted only for cases where it can be shown that cracking of the concrete at service load levels will not occur over the anchor service life. For cases involving design for seismic actions, post-installed anchors must be prequalified for use in cracked concrete as well as for seismic loading.

 

[engineering_patrol]

Concrete tensile strength is usually ignored when design against ULS, so I highly doubt that is would be possible to use uncracked section.

 

[r13]

I think you might be able to use longer threaded rod rather than shallow anchors, if the footing and pedestal both are in good condition. It looks like the cracks on the pavement are caused by settlement.

 

[phamENG]

engineering_patrol - ignoring flexural strength of concrete is not the same as investigating crack sizes. For ultimate strength and limit states, no, we don't look at concrete contributing to tensile strength. But we absolutely consider its effect on the stiffness of members, frames, and systems. Knowing when it cracks is important to understanding the response of a structure to various load levels. It's also an important durability consideration. Limitation of crack width is especially important in corrosive climates (close to the beach, parking garages where de-icing salts are used, etc.) or underground structures exposed to potentially corrosive soils.

In this example, what applied load and resulting stress do you see creating a crack in the top of the pier that would compromise the anchor? I don't see one.

 

[r13]

I think it is prudent to use cracked base for post installed anchors, unless the load is static in nature, and applied on massive solid concrete, for which chipping or splitting is not a problem. For this project, since the already cut bolts are difficult to get out, there are two approaches for re-bolting - 1) butt-weld same size/material anchors to the existing bolt with well considered preparation and welding procedures, and 2) drill and install new anchors around the existing bolts. The second approach will reduce the edge distance, and risk in cracking between the new and existing holes, so it is justified/essential to use cracked base in design.

 

[phamENG]

3) rotate base plate 45 degrees.

 

[TLHS]

Given that this is a primarily moment load, have you considered taking into account the additional capacity you get from the compression side of the base plate confining the breakout cone (or acting as a strut, depending on how you want to mentally model it).

There's been a bunch of research, but I can't find a simple summary of it right at the moment and I'm running out the door.