concrete to steel plate friction coefficient 2

[Lucian09]

Hi guys.
Flat (i.e. smooth) steel plate, or tube, embedded into concrete footing. Say 3" wide x 1/4" thk and a 24" vertical embedment. Does anyone know of a method to calculate the maximum pull out strength (force) of the steel plate out of the concrete? They say to use a 0.7 friction coefficient, but applied to what force?
Thanks for any info.

 

[STrctPono]

You're looking for cohesion values, not friction coefficient. As you have realized, without a normal force, there is nothing to multiply your friction coefficient against.

AASHTO 5.8.4.3 gives cohesion values of 25 psi for normal weight concrete cast against rolled steel. For direct steel plate pullout of concrete... Your surface area of steel in contact with the concrete is = (2 x 3 x 24) + (2 x 0.25 x 24) = 156 in2. 25 psi x 156 in2 = 3,900 lbs.

Seems reasonable to me. Apply strength reduction factor as required.

 

[r13]

While the above is correct, but for a permanent embedment, I would weld a small bar at the end to provide positive pull out resistance.

 

[Lucian09]

Thanks STrctPono, cohesion makes sense (and thanks retired13 - though not sure what you mean by "positive"; isn't the cohesion strength positive too? Btw, I do have a rod at the end but I wanted to isolate the problem in order to calculate the total embedment pull strength).

I still wonder about the true cohesion strength though, and whether micro expansion-contraction would lessen it over time. I also find it hard to intuitively visualize a 3900 lbs SUV hanging from such embedment (but so I find it hard to visualize like 8 such SUV's hanging from a 1 in2 steel bar). What kind of reduction factor would you apply? Thanks.

 

[r13]

You will be advised throughout your career that friction/adhesion is difficult to quantify, it's not convincingly reliable, and durable. If there is simple way to develop the required strength without doubt, I would skip the adhesion but treat it as an added benefit.

 

[JStephen]

In the olden days, plain anchor bolt embedment in our work was based on a specified bond strength (as I recall, 1.5*sqrt(fc') or around 82 psi for 3,000 psi concrete.).
I would assume that number came from old ACI provisions, perhaps for undeformed rebar or undeformed rebar hooks; it was applied to hooked bolts.
But this was abandoned and the ACI-318 Apppendix D design used instead, which is based on a positive pull-out resistance as noted above. IE, a nut or washer plate on the bottom of the bolt, sized for bearing at the bolt/washer face rather than friction on the bolt shaft.
Watch for corrosion at the interface.

 

[WinelandV]

"Positive" in this sense just means to create a mechanism to transfer the load, not just the cohesion.

Example: "I welded the end of the beam to the bearing seat to create a positive connection, instead of relying on friction"

Your second paragraph gets at why you want a "positive" connection - there's a lot that can go wrong when counting on cohesion.

 

[WARose]

Agree with what retired13 is saying. Adhesion (concrete to steel) has been out of ACI codes quite sometime. (In part, for some of the reasons he stated.)

You need to consider another mechanism for failure.

 

[EngineeringEric]

Concrete shrinks. cohesion or bond goes to zero. weld a plate, or shear studs, or something that will always engage the system. This value you can count on, even if you cover the steel with grease before you place concrete

 

[Lucian09]

Thank you all. A well-defined mechanical resistance is always preferable but this is an as-built situation and, as I said, I needed to determine any possible strength from cohesion as well. Your numbers above vary greatly, from 25 psi to 82 psi.

Meanwhile, I have a hard time convincing myself that a safe cohesion strength, over time, can only be negligible (0 psi). Images of demolition sites come to mind, with galvanized steel tubes of sign posts or railing posts still monolithic with their concrete footing. As my case actually consists of stainless steel (3"x3" sq tube, 24" embedm.) and a well controlled pour, I'm inclined to think that by the time the concrete around the steel would have loosened its bond, the whole footing would be on the brink - in hundreds of years. I think I'm going to work with a 10 psi long-term cohesion strength value. I mean, AASHTO must have done their testing with that 25 psi figure.