Coefficient of friction for steel base plate to concrete 3

[GD_P]

Hello,

What value of coefficient of friction do you consider, when base plate shear is transferred through friction?
If you refer Eurocode EN 1993-1-8, Cl. 6.2.2(6), it specified only 0.2 for Sand & cement mortar with steel.
Whereas for AISC Steel design guide 1 : Base Plate and Anchor Rod Design, Cl. 3.5.1 specifies 0.55 for steel to mortar & 0.7 for steel to concrete.

Thanks in advance.

GD_P

 

[WARose]

I've generally used 0.3 over the years.....but I can see a case for 0.2.

 

[BridgeSmith]

Not sure, but perhaps the discrepancy is the difference between a base plate placed on hardened concrete (0.2), and with the base plate set on wet mortar or concrete (0.55 or 0.7, respectively), so that there is full contact and bond between the steel and substrate.

 

[dhengr]

GD_P:
Undoubtedly, there is some friction existing btwn. the base pl. and the concrete, and you could argue a bit about the exact/best coef. of friction. But, is it good practice to count on this friction to transfer the base shear, when so many different things (slight movements during construction, some uplift, etc.) can radically alter this friction force or break the bond?

 

[BridgeSmith]

dhengr makes a good point; rarely can the higher friction values be counted on for a base plate. It may be higher initially, but if anything moves even slightly, it likely drops to around that 0.2 value, possibly lower if the contact area gets too small and there is crushing of the concrete.

 

[BridgeSmith]

The only condition where we consider steel-to-concrete friction is for seismic force dissipation, but we usually ignore it even for that.

 

[GD_P]

In short you use welded washers or shear stub every time.
Thanks for the suggestions.

GD_P

 

[GD_P]

One more point,
Have you ever used pretensioning of anchor rods for increasing the shear resistance due to friction.

For eg., there is staircase flight inclined member whose base plate has only 1 anchor rod.
Since the member is inclined, there is significant shear as compared to compressive force.
If I use the base plate with oversize hole & washers welded to base plate, the bending stresses are not within the allow limits.
Here cantilever length of anchor rod = grout thk + base plate thk + washer thk / 2,
How to remove this grout thk from cantilever span? if so bending stress will be within allow limits.
OR
Pretensioning (only 15% of the tension capacity of the M16 8.8 grade bolt) of anchor bolts by tightening using the calculated torque.


GD_P

 

[GD_P]

Oh, I forgot this.
Pretensioning is not possible for anchor rods, since leveling nuts are used which will not transfer the compressive force to concrete surface, but it will only compress the base plate.

GD_P

 

[Tmoose]

Is this an interior or exterior installation?

" Pretensioning is not possible for anchor rods, since leveling nuts are used"
First, I'd fix that. Leveling nuts are admittedly quite convenient, but have no place around structures or rotating machinery.
Second, if the staircase is "leveled" that way, it suggests the upper end of the stair case is secured to something, which means the load at the bottom of the staircase may be vertical with minimal shear.

"For eg., there is staircase flight inclined member whose base plate has only 1 anchor rod.
Since the member is inclined, there is significant shear as compared to compressive force.

 

[WARose]

Have you ever used pretensioning of anchor rods for increasing the shear resistance due to friction.

Most definitely. You do however have to account for some relaxation over time.

 

[BridgeSmith]

For our installations with leveling nuts (which is all of our overhead sign structures and high mast light towers), we keep the distance from the bottom of the leveling nut to the concrete less than the bolt diameter and ignore bending, per the AASHTO bridge design spec. We check tension and shear only on the anchor bolts.

We have a detailed tightening sequence that the contractors are required to follow to ensure the base plates remain flat and the bolts are fully tensioned.

 

[GD_P]

I wonder how do you ensure the bolts will be in tension with levelling nuts used.
My understanding is what I have written in the previous post.
How much relaxation will be sufficient?

GD_P

 

[GD_P]

@Tmoose
Since the flight member is supported on the secondary member near its landing and base plate (pin support) at the start,
Secondary member being flexible transfer the force to flight member which is transferred to anchor finally, resulting in significant shear.

GD_P

 

[BridgeSmith]

"I wonder how do you ensure the bolts will be in tension with levelling nuts used."

We use a turn-of-the-nut method, specifying some fraction of a turn beyond wrench tight. Although wrench tight is defined rather vaguely as "the full effort of a man using and 18" wrench", which I don't particularly like. Is the "man" PeeWee Herman or Dwayne Johnson?

 

[WARose]

[blue](GD P)[/blue]

How much relaxation will be sufficient?

It depends on how much it is tightened. (And other factors.) But (in general) I am use to seeing about 40-50% of pre-load loss over time.

 

[GD_P]

I am not able to understand how to achieve pretension in the anhors (i.e., compression in foundation).
Please refer attached file.
Now from the fig.
The base plates are positioned using the levelling nuts, hence their position is fix after levelling. So after this when we tight the main anchor nuts it will only develope compression in PART 'A' of the base plate and tension in the PART 'A' of the anchor. If so, let Dwayne Johnson be the man, it won't induce any pretension in the whole anchor bolts (apart from PART 'A'). In such situations how can one use the extra compression to resist shear by friction between the base plate & concrete.
Please correct me if I am wrong.


GD_P

 

[Ingenuity]

I am not able to understand how to achieve pretension in the anchors (i.e., compression in foundation).

With leveling nuts, you won't.

 

[BridgeSmith]

Yes, with leveling nuts and the main anchor nuts torqued, the forces in the anchor bolts below the leveling nuts are due to the loads applied to the base plate. The loading in the 'Part A' length of the bolt should only the preload, if tensioning is done correctly.

"In such situations how can one use the extra compression to resist shear by friction between the base plate & concrete."

You can't. Only the shear capacity of the bolts is all you get. However, as was stated previously, counting on friction between a base plate and concrete is a dubious proposition, but the shear capacity of the bolts can be reliably quantified.

 

[WARose]

I am not able to understand how to achieve pretension in the anhors (i.e., compression in foundation).
Please refer attached file.
Now from the fig.
The base plates are positioned using the levelling nuts, hence their position is fix after levelling. So after this when we tight the main anchor nuts it will only develope compression in PART 'A' of the base plate and tension in the PART 'A' of the anchor. If so, let Dwayne Johnson be the man, it won't induce any pretension in the whole anchor bolts (apart from PART 'A'). In such situations how can one use the extra compression to resist shear by friction between the base plate & concrete.
Please correct me if I am wrong.

I've handled this a few times and what I did was grout underneath while using a form to keep the grout off the corner rods/nuts (sometimes with caulking to seal the edges) and under the plate. Next (once the high-early, non-shrink grout set), you'd have them pull out the form, back off on the leveling nuts to where the plate would effectively be sitting on the new grout......then grout those corners.