Metal Culvert 3

[dik]

I have a 7' dia corrugated metal culvert cast into a 20" concrete wall. Is there a method (other than FEM or an involved energy method) of calculating the pull-out capacity of this. It has not been fastened to the concrete wall and the only withdrawal resistance is caused by the corrugations.

Dik

 

[Lion06]

I might try doing something like using a maximum achievable bond stress between the concrete and the culvert to treat it like a very large diameter rebar. That seems reasonable to me.

 

[dik]

Thanks EIT... I'd looked at that type of approach as well as a 'shear friction' approach as concrete only and was concerned about the culvert retaining it's shape and not collapsing...

 

[Lion06]

That is a valid concern. I wonder if anyone has any literature or references to check something like that.
Anyone??

 

[ChipB]

dik,
I don't think you need to worry about it retaining its shape before collapsing. If I were worried about it, I'd calculate the pullout capacity on the shear cone of the concrete. Then calculate the tension capacity of the culvert at F_u and compare to the concrete strength. I believe the concrete would break or the culvert would rupture before the metal corrugations buckle in unison.
Chip

 

[Lion06]

ChipB-
I could see just a few corrugations failing, adding more load to the others causing them to fail (not necessarily in unison, but many failing as a result of the first few failing).
dik-
I might use a conservative bond stress to account for this.

 

[ChipB]

Well, we might have to disagree on this one.

Each corrugation is braced by the opposing corrugation. As it is trying to pull out, the concrete between the corrugations goes into compression, putting pressure on the face of the corrugations resisting the pullout. The backside of these corrugations are acting like columns bracing the top of the corrugations. It's basically a fixed top and bottom as the stiffness coefficient would be the same across all the corrugations. It would take a significant amount of compressive force to buckle the backside of these corrugations. Keep in mind, while these forces are attempting to buckle these corrugations, there is a tensile force attempting to pull out the culvert. So you are going to have f_a - f_t. So, no, I don't believe the corrugations are going to fail in this loading condition. Put the culvert into compression however, and yes, they would likely buckle before the concrete fails.

One failure mode under pullout may be the concrete shearing across the tops of the corrugations.

 

[dik]

The problem is that the corrugated culvert is fabricated from spiral sheet and the tension could tend to 'unwrap' the spiral... reminds me of the Pillsbury container...

Dik

 

[miecz]

I doon't see buckling of the pipe as a failure mode. The corrugations have continuous lateral support. If the pipe were to fail, it would be yeiding in compression. To determine the compression force, you could do a vector analysis with the angle of the corrugations, assuming zero friction between the pipe and the concrete.

My gut feeling, unless the pipe is very thin or the corrugations very shallow, is that the concrete would shear near the tops of the corrugations, with some slight flattening of the corrugations, before the pipe would yield.

Do you have an axial force in the pipe resulting from a bend downstream? Is it not possible to put a thrust block at the bend and eliminate the force at the intersecting wall? It seems that a corrugated pipe with an axial force would see some significant axial deflection.

 

[dik]

Already constructed... the axial load is generated by waterhammer from energy dissipator...

Dik

 

[cvg]

unless I am mistaken, headwalls are not designed to rely on any lateral support from the pipe to which they may or may not be connected to. Although I have seen plans which showed anchors through the cmp designed to provide a posative connection to the headwall. Your energy dissipater structure should stand on it's own without help from the pipe.

 

[dik]

CVG...

Thanks, the consultant is concerned about a 'waterhammer' force generated from the flow impacting on the energy dissipater...

Dik

 

[cvg]

granted, I understand the concern - but I still would design the dissipater to handle the impact and not rely on the pipe to do anything structurally to hold back the dissipater structure. this is usually done accomplished by a cutoff wall as well as by the dead weight of the dissipater. If this is already constructed and now you have the concern, perhaps you can consider some small amount of restraint supplied by the connection between the headwall and the pipe. Otherwise, I would mitigate it by anchoring the structure somehow into the ground.