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Lateral strenght of Post Embedded in Concrete Curb

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ajk1

ajk1

Structural
Apr 22, 2011
1,791
How can I determine the horizontal force resistance of the embedment of posts in a concrete curb?

The posts are subject to periodic inadvertent impact from vehicles, and the concrete curb has broken out in a shallow wedge-configuration (I had nothing to do with the original design; it was put in by a contractor based on his own initiative; this contractor does not like engineers).

The posts are 3" diameter galvanized steel pipes embedded 4.5" into an 8" wide curb. This leaves 2.5" of the concrete curb on each side of the post at the post centreline. The posts have no concrete fill in them, but have an aluminum removable cap at the top. The top of the concrete curb is about 6" above the adjacent asphalt paving. I don't currently know how far down below the asphalt that the curb extends, but I may be able to find out from the installer. If the post were a Hilti fastener (or other manufacturer's fastener), I could use their technical manual or software to calculate the shear resistance, but a 3" diameter post is too large to do this.

I could calculate the reaction couple force against each side of the post, for an assumed reactive lever arm, but I don't know where to go from there.

Any ideas?
 
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One of the problems with that detail is the possible expansion of the steel post in case of temperature change. This could easily fracture the concrete, so I would avoid the detail.

BA
 
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The key term you used was "periodic inadvertent impact from vehicles" - a major unknown factor. A large truck 78,000# or just a VW moving slowly? The bigger impacts come from large vehicles that normally have a higher bumper (lever arm height).

We had a problem in our parking area and found the cheapest long term solution was a 12" diameter bored hole 4' deep, filled with concrete and a 4" steel pipe filled with concrete.

A 3" pipe embedded only 4.5" into a curb will never last and remain plumb very long. Even concrete wheel stops in parking lots are anchored more securely.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.
 
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AASHTO Design Specifications will have the information for how to calculate the resisting force. May want to look into AASHTO Highway Signs Luminaires and Traffic Signals.

Thanks,

FEM4Structures
 
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BARetired - I completely agree with you that this is bad detail, but architects like it for its neat appearance. I dislike it because water accumulates around the post and corrodes the post. Also, the post is grouted into the curb, so if the grout shrinks a little and opens an annular space, then water can get into the space and freeze and split the curb apart. In fact, this may be part of the problem, although I think some of the failured occured in summer. Also the oxidation of the galvanizing produces an expansive product.

I had not thought about the post steel expansion that you raise. Since the steel post and the concrete both have essentially the same coefficient of expansion, would this be a problem?

slickdeals (Structural) - thanks for the attachment. I will study it. It seems to deal with my issue.

concretemasonry (Structural) - the vehicles do not include trucks.

FEM4Structures (Structural) - do the AASHO Standards deal with embedment in curbs?
 
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AASHTO deals with height of bollards and they should be designed to resit the vehicular impact at critical angle.

Thanks,

FEM4Structures
 
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ajk1,

You raise a good point about water finding its way into the recess and splitting the curb apart when it freezes.

In any case, the detail you describe could not stand any significant impact from a vehicle with only 2.5" concrete cover over the pipe. It would need to be much more robust than that.

BA
 
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FEM4Structures (Structural) - thanks. You would not happen to know what height AASHTO says for bollards? Is if 3 feet?

BAretired (Structural)- I am going to try to work out what lateral force it could withstand.

slickdeals (Structural)- looks like a neat spreadhseet. I don't imagin that it is available for use?
 
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ajk1, it is a spreadsheet output (pdf) that was submitted by the railing designer as part of the shop drawing process. Most of the equations are spelled out and you should be easily able to follow it.
 
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slickdeals (Structural)- yes ok thanks.
 
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Hi Slickdeals;

Are you able to help me with the following questions with regard to the shop drawing copy of spreadsheet that you sent? I have tried to read it in conjunction with ACI 318-02 but so far have still not figured it all out.

Page 1:

1. I understand where the hef=4.67 comes from (it is 2/3 of 7" slab depth), but where does the 6.25" on the same line, come from?

2. Where does hef=4.5" come from?

Page 2:

3. He seems to be working with a 45 degree breakout cone of concrete(i.e. 90 degrees at the vertex). But ACI says it is a 35 degree cone (i.e. [90-35] x 2 =110 degrees at the vertex). He seems to be using a more conservative model than ACI, but why?

4. Where does the formula for Af = root (bef / d0) come from?

5. Where do the limit Af< 2 come from?

Thanks

 
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Since Slickdeals is not repsonding, does anyone else know the answers to the follwoing questions that pertain to the attachment that Slickdeals sent.

Are you able to help me with the following questions with regard to the shop drawing copy of spreadsheet that you sent? I have tried to read it in conjunction with ACI 318-02 but so far have still not figured it all out.

Page 1:

1. I understand where the hef=4.67 comes from (it is 2/3 of 7" slab depth), but where does the 6.25" on the same line, come from?

2. Where does hef=4.5" come from?

Page 2:

3. He seems to be working with a 45 degree breakout cone of concrete(i.e. 90 degrees at the vertex). But ACI says it is a 35 degree cone (i.e. [90-35] x 2 =110 degrees at the vertex). He seems to be using a more conservative model than ACI, but why?

4. Where does the formula for Af = root (bef / d0) come from?

5. Where do the limit Af< 2 come from?
 
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1. Why is 2/3 slab relevant? Not sure about the other.

2. hef I believe is selected by the user.

3. Again not sure

4. This seems to be checking the bearing area. You may increase the allowable bearing pressure if the surrounding concrete bearing area is greater than the contact bearing area.

5. The limit for the increase for the above mentioned increase is 2. (See ACI-08 Sect. 10.14 I think)



EIT
 
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RFreund - ok, your answer to number 5 is helpful. I should have realozed that. Thanks.
 
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Hi Rfreund - I had a look at what you suggested, about the limit for the bearing area multiplier being 2, and I don't think that can be what this number is because it is in square inches. Also, by this formula, the smaller the post diamter is, the greater the bearing strength! I think there is something wrong here. I think I should forget about this example and try to work things out from the Code Appendix equations, but use my own thought on how to account for the large eccentricity of application of load that I don't think is accounted for in the Code equations.

Unless anyone else has a procedure to put forth. Hard to believe that this issue isn't frequently encountered.
 
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2 1/2" is not enough cover. You could kick the post and break-out the concrete.
Any calc you do will be wishful thinking at best.
As mentioned, corrosion jacking will likely cause it to fail as well.
I would let the Architect put his seal on it.
 
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