Shear resistance with standard base plate 2

[FeSE]

When a base plate sees more shear than available friction, what actually happens? Certaintly the anchors go into bending. After some deformation would it not be reasonable that a concrete wedge forms as shown in the following sketch? Wouldn't that in turn add to the friction available?

Does anyone know of research or designs based on something like this?

On my current project, I have I have the following;
rxns;
.6D+.7E 97k uplift & 57k shear
D+L+.7E 337k down & 57k shear
W12x79 column.

So, I need to pretension the anchor rods in order to overcome the 97k uplift. AISC doesn't recommend that either... wouldn't that be pretty simple though?

 

[Lion06]

Wouldn't pretensioning them reduce the capacity of the concrete breakout cone? Well, I guess it wouldn't reduce the capacity of the cone, but it would reduce the load that could be place on the anchors from the column before the concrete would fail (from tension on the anchor rods).

 

[111213]

hi,

i have a report in german exactly about that problem.

there are 2 possible states of equilibrium for uplift:

1. if the anchors are not pretensed an long enough, the anchors bow in the direction of the resulting force-vector

2. if the anchors are strongly pretensed, they belong straight, and the friction takes the the force, because
there is a multdirectional stress state.

the german building codes allow to calculate this in that way.

 

[bjb]

I recommend a shear lug on the base plate to transfer shear. AISC Design Guide 1 has guidance for designing these.

If relying on anchor bolts, it has to be considered that the holes for the anchor bolts will be oversized. To count on all of the anchor bolts you'll need plate washers with standard holes that are field welded to the base plate.

If this is for a job in the US, I wouldn't rely on what a foreign code says.

 

[FeSE]

Correct me if I'm wrong, but pretensioning will not reduce the breakout cone or required development length.

As for codes, would you go to a doctor who would only treat you with procedures from a manual? No, we rely on doctors to use knowledge and reason.

 

[Lion06]

For a given embedment depth, I believe pretensioning will reduce the tension that can be imposed on the anchor rod and here is why.
For a given embedment depth, the concrete breakout has a specific value, which we'll call Z for this case. You can reach Z simply by pretensioning the anchor rod without ever having the column apply a load to it.
I don't think it is realistic to believe you can apply a tensile load to the anchor without having it affect the additional load that can be applied before the concrete breakout occurs. Does the concrete know whether that load is coming from uplift on the column of pretensioning of the anchor rod? I don't think so.
Based on that reasoning, I think that if you need to pretension the anchor rods to some force Y, then the most you can count on the anchor rods for (for uplift from the columns) is Z-Y. This is only for the limit state of concrete breakout. I know that high strength steel bolts do not require a reduction in capacity for pretensioning, I am not sure if that carries over into anchor rods or not.

 

[nutte]

StrEIT, you're describing a situation that stumped me and most of my colleagues at one time or another. The pretension force in the rod is an internal force. You can add external tension on the rod up to the pretension force without adding any more tension to the concrete cone. The pretension is resolved through concrete interlock at the bottom of the anchor rod and bearing of the nut (or whatever) against the concrete (or plate or whatever) at the top of the rod. Any tension applied to the end of the rod, above the nut, would reduce the clamping force between the nut and the concrete, but it doesn't change the force on the concrete cone below, until the applied tension meets and exceeds the pretension force.

 

[civeng80]

Why would a wedge form in the concrete? Why not just rely on the dowel action in the bolts or perhaps bending? If base plate is embedded in a concrete slab then tieing the base plate to the slab would avoid the problem.

 

[LowLax]

Are you using the ACI shear friction method? I watched an AISC seminar where the presenter recommended using the shear friction method instead of resorting to shear lugs.

 

[Lion06]

nutte, I agree that any tension applied to the rod above the nut would reduce the clamping force.
Wouldn't the uplift imposed by the column be applied by the nut (through bearing on the baseplate), not above the nut? That is why I think it should be additive with the pretension force.

 

[miecz]

It's not additive. Let's say you pretension 4 anchor bolts to 40 kips, each. This produces a compression force between the base plate and the concrete of 160 kips. Now apply a 70 kip uplift to the column. The result is a 90 kip compression force between the base plate and the column. The bolts still carry 40 kips of tension, each. If you apply 160 kips tension, the clamping force goes to zero and the bolts still have 40 kips tension. If you apply 200 kips tension to the column, each bolt has 50 kips tension.

 

[nutte]

Miecz has it.

 

[FeSE]

Nutte & Miecz; Thank you for your excellent explanations.

Now back to the real topic;
LowLax; You hit the nail on the head. Do you remember the name of the seminar? Is it available online?

 

[swearingen]

I second Nutte and Miecz.

As for pretensioning, the problem comes in when the concrete creeps. The concrete will slowly release some of the preload in the anchor rod over time. How much? Impossible to know - hence the practice is frowned upon.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[bjb]

FeSE, Why don't you want to use a shear lug?

 

[LowLax]

It's the "Designing Steel after College" seminar (or something like that) It's a "webinar" on the AISC website. I forget the presenters name, someone from Walter P. Moore.

 

[Lion06]

Alright, can someone explain this a little better to the EIT of the group?
When you pretension an anchor rod, the rod is put in tension by lengthening the rod between the bottom of the rod embedded in the concrete. The only way the rod elongates is by the nut bearing on the concrete and putting an artificial load on the concrete cone. With no load in the column, there is an elongation of the rod. As that column gets loaded in tension, the rod elongates further and the axial column load is now also in the rod being resisted by the concrete cone.
The only way I can see them being seperate is if there is a pipe sleeve or something around the rod.
Are you saying that if you hav 100 kips of concrete breakout strength that you can pretension up to 99 kips and still be able to apply 100 kips of axial tension on the column before the concrete breakout occurs?
Does anyone know of a reference that might explain this a little better?

 

[hokie66]

StructuralEIT,

nutte and miecz have done their best, and I can't do any better. I know it is a hard concept to get your head around, it was for me too. But they are correct, and this is the basic principle of prestressing. Perhaps having a new read of your prestressed concrete text will help.

 

[miecz]

EIT

I think the sketch included with the original post may have you confused. That sketch shows a nut on the underside of the base plate. When you say "the nut bearing on the concrete and putting an artificial load on the concrete cone", I guess you are thinking the nut under the base plate is turned down on the concrete and causing the pretension.

There is no nut on the underside of the base plate. The nut causing the pretension is on top of the base plate and is bearing down on the base plate, which in turn is bearing down on the concrete, or the grout son top of the concrete.

 

[nutte]

Are you saying that if you hav 100 kips of concrete breakout strength that you can pretension up to 99 kips and still be able to apply 100 kips of axial tension on the column before the concrete breakout occurs?

Yes.

The first 99 kips applied will reduce the clamping between the plate and concrete a little. They will still remain in contact, just pressed together slightly less.

Salmon and Johnson discusses this in their 4th edition steel book, Chapter 4, section 4.13, pages 157-160. They summarize with the following:

The important conclusion from this example is that no significant increase in bolt tension arises until the external load equals or exceeds the pretension force, in which case the pieces do not remain in contact and the applied force equals the bolt tension.