Transferring shear from column base plates 1

[AYMAN5000]

Reference to AISC design guide 1 second edition,

There are three principal ways of transferring shear from column base plates into concrete:
1. Friction between the base plate and the grout or concrete surface.
2. Bearing of the column and base plate, and/or shear lug, against a concrete surface.
3. Shear in the anchor rods.

My question, if I have a big value of shear force at the base which cannot be resisted by shear lug separately, could I resist part of the shear force by friction and the remaining one to be resisted by shear lug?
Example: Ultimate shear force (Vu) = 8847 kN, ultimate compression force (Pu) = 7796 kN
available shear strength (phi*Vn) = 0.75*0.55*7796 = 3215 kN
Required shear to be resisted by shear lug = 8847-3215=5632 kN.

 

[DaveAtkins]

I think that is legitimate. Friction will resist the shear force until the base plate begins to slide a tiny bit, then the shear lug will take the rest of the shear.

DaveAtkins

 

[KootK]

My question, if I have a big value of shear force at the base which cannot be resisted by shear lug separately, could I resist part of the shear force by friction and the remaining one to be resisted by shear lug?

I vote no for the most part. One might make an argument for combining mechanisms as a way to get the shear out of the base plate and into the concrete initially. Unfortunately, that is rarely the governing mode of failure.

It is usually not getting the shear into the concrete that is the problem but, rather, keeping it there without initiating a shear breakout failure etc. And, unfortunately, the friction and lug bearing mechanisms will substantially share the same shear breakout mechanism. In this sense, it is double dipping to count the contributions of both friction and lug bearing.

 

[AYMAN5000]

As received from AISC SOLUTIONS

Could i say that shear force from friction described in section 3.5.1 which is caused by column compression load can be added to shear strength provided by shear lug

I believe this is possible. I missed this in my previous response, but AISC Design Guide 1 states, “If the designer wishes to use shear-friction strength as well, the provisions of ACI 349-06 can be followed. Additional comments related to the use of shear lugs are provided below:”

I believe the authors are indicating that one could consider shear-friction in addition to the shear lug.
Sincerely,
Carlo Lini, S.E., P.E.
Director of the AISC Steel Solutions Center
American Institute of Steel Construction
866.ASK.AISC

 

[Tomfh]

I vote no for the most part. One might make an argument for combining mechanisms as a way to get the shear out of the base plate and into the concrete initially. Unfortunately, that is rarely the governing mode of failure.

Yes it all depends on what sort of shear failure we are talking about.

A breakout failure close to an edge would be double dipping.

Shearing off the rods, remote from an edge, it wouldn’t be double dipping.

 

[AYMAN5000]

ACI349-13 STATES THE FOLLOWING
D.6.1.4 Friction between the baseplate and concrete shall
be permitted to be considered to contribute to the nominal
steel shear strength of the anchor in shear. The nominal shear
strength resulting from friction between the baseplate and
concrete—that is, without any contribution from anchors—
shall be taken as 0.40CF.

RD.6.1.4 The friction force that develops between the
base plate and concrete due to the compressive resultant
from moment or axial load, or both, contributes to the shear
strength of the connection. For as-rolled base plates installed
against hardened concrete, the coefficient of friction is
approximately 0.40

............................................................

The only difference (when making the combination between shear friction and shear lug ) that the friction coefficient should be 0.4 instead of 0.7