Shear Transfer at Column Base Plates 3

[jike]

Is there a need to grout the oversized holes in column base plates to transfer horizontal shear without subjecting the anchor bolts to bending?

This question is related to how the shear is being transfered from the top of the base plate to the bottom. Is it thru bending of the anchor bolts or thru shear friction of the plate against the grout where the anchor bolts provide a tensile force to hold the plate in contact with the grout.

This, of course, assumes that there is no other device to transfer shear like shear lugs, etc.

 

[DaveAtkins]

I assume the anchor bolts act in tension and create shear friction between the plate, grout, and pier. I use the article entitled "Design of Headed Anchor Bolts", by Shipp and Haninger, from the Second Quarter 1983 AISC Engineering Journal. This article also addresses the related question of anchor bolt development in reinforced piers.

DaveAtkins

 

[VoyageofDiscovery]

Hi jike,

I've also seen these with no grout under the base plate. Nuts on the threaded rods support the vertical load from the base plate. But I plead ignorance here w.r.t. your good question. I assume bending and tension must be accounted for in the design of these.

For oversized holes perhaps friction between the washers and plate transfers the load.

Anyone else?

VOD

 

[steve1]

AISC 9th edition, Commentary Section J10,p 5-172, answers the question that you raise.

According to the AISC, shear is transferred by friction from the column base to the foundation.

 

[J1D]

Jike,

Reinforcement detail and anchor bolts in piers (or pile caps, or pedestals) are not well studied and standardized, in my opinion. The lack of research is remedied in practice somehow by conservative design. The following is what I know (may not be accurate) and what we do usually.

Anchor bolt, it’s right, all the capacity tables (from codes to company’s tables) are based on un-reinforced concrete. Although, in most cases, the pullout cone will intercept rebars (including anchors at center of a pier, if the embedment is deep enough, since the tip of the cone is at the anchor plate/head). I know Hilti did tests in reinforced concrete and acknowledges the benefit, but never gave a guide/table for bolts in a reinforced concrete base. Maybe the actual reinforcing is too case-by-case.

When subjected to shear, the anchor bolts need some edge distance to prevent lateral bursting or shear spalling off. (but, again, that distance is based on un-reinforced concrete.). In addition, we check the shear capacity of the bolts (metal material shear capacity) and bearing capacity of the concrete behind the bolts (the way of this check is quite diverse, usual way is to consider a 1/4 depth of bolt embedment or 5 diameter depth, whichever smaller, but someone uses more conservative calculation on this, I had a post on this the other day). But with axial force from the column, lots of shear transfer of the anchor bolts should be by “shear friction”. The ties however are rarely checked against the shear force like what we do in a concrete beam. Maybe because the section is a big mass, and because the ties are pretty close, less than d/2 for sure.

You are right, when in tension, the anchor bolts pull-out capacity is guaranteed by the number of bolts chosen by engineer and standardized embedments, which are calculated based on the rule that pryout is impossible prior to breakout of anchor bolts. However, not like in a beam design to consider the rebar development and splice. But if we do so, we can consider in a 45 degree angle (from anchor plate/head) the rebars in piers are intercepted, then to check whether the development length from that point is enough to develop the force in rebars you need. When a whole pier or pilecap is in tension, we should consider the pull-out capacity with tensile capacity of the whole section, but this is a rare case and rarely critical. In pilecaps, you can see frequently the anchor bolts directly overlap with the dowels from piles, in these cases, the vertical reinforcement in pilecap/pier is less important, sometimes just plays a role as the skeleton for ties.

 

[Joecandraw]

This is a great question. I believe the question is do you need to grout the oversized holes in the base plate. For reasons of hor. shear tranfer so as not to subject the
anchor bolts to bending. The holes do not need to be grouted
but oversized holes need min. 3/16 plate washers. The reason we do not grout them is because the columns are not perfect and this allows for there adjustment in the erection process. The anchor bolt will bend a small amount
which is there design. The reason you will sometimes see nuts above and below a column base plate is because the lower nuts are used to level the column. Later they will be grouted solid. Leaving the leveling nut in place.

 

[MrStohler]

Generally in the absence of a shear lug a shear friction method will attempt to develo at the base plate-grout interface. If there is no grout there is no shear friction ant the anchors must act in combined shear and bending.

Please refer to ACI 349 Appendix B - Steel Embedments for a thorough discussion of how anchors develop shear resistance.

Also refer to AISC Design Guide #7 and the AISC Journal- 2nd Qtr. 1983

 

[andyr]

Yes, grouting the oversized holes is a necessity purely from a protection against corrosion aspect point of view. No grout - bolts exposed to groundwater.
Low shears can be transmitted through the bolts, but in cases of high shear, consider as noted above, welding a stub section to the underside of the base plate and forming a pocket into the concrete base. this can then be grouted up as a solid and forms a shear key into the concrete, just make sure its deep enough and there is sufficient rebar around the pocket to act as containment steel.
I've done this on a job with a horizontal "kick" of about 1100kN (ULS) and the building is still there.

 

[pba]

J1D has covered my undersanding of this situation.

My policy is not to rely on bolts in bending for shear force transfer due to the lack of clear guidance. Where low horizontal forces are present and the baseplate will always be in compression then base friction is sufficient. Where tension is present or the horizontal force is large base friction is absent of insufficient and some method of shear key is required for the whole horizontal force. This can most easily be achieved by recessing the baseplate into the top of the foundation.

This approach is inline with the recommendations given by Steel Construction Institute here in the UK.

As a point of interest, how would you grout an oversized bolt hole? The space would be tiny and top access is surely prevented by the nut and washer.

 

[pmkPE]

Have you ever noticed light standards and signs near roads and highways that do not have grout under their base plates? They have leveling nuts underneath to support the assembly. The reason for not having grout is to "encourage" active shear through the bolts thereby allowing them to shear quickly during impact from a moving vehicle thereby reducing the resistance to the vehicle. (Lower resistance equals less damage to the vehicle and its occupants).

Should grout be in place then the tensile strength of the bolts comes into play and offer greater resistance to any impact. On the issue of grout inside of the drilled -through holes in the base plate I do not believe that the volume of grout is significant and worth calculating for any degree of load transfer. The majority of the forces will be taken as direct tension with the addition of grout under the base plate. Shear still exists but the addition of grout makes the attachment much stronger in its resistance to applied forces as demonstrated by the light standard example given above.