Anchor Bolt Design 6

[fasboater]

I apologize if this has already been addressed.

I am trying to figure out the process for designing anchor bolts with the base plates NOT in contact with the concrete.

For example if you have a two column sign that is subject to wind and dead load there should be shear, tension, and compression potentially applied to the anchor bolts. I typically design a 1 ½” grout pocket between the base plate and the foundation that is fully grouted after sign is leveled. If the grout is not placed how can I check the anchor bolts? Do I treat the anchor bolts as columns with the distance between the leveling nut and the foundation as the un-braced length?

Thanks

 

[DarthSoilsGuy]

good question,
i'll be watching this post. i haven't designed structural steel but have done tons of steel insp. i would wonder often about whether thread fatigue in/at the leveling nut would make a major problem if grout wasn't placed. Any comments out there on this too?

 

[csd72]

Treat each of the bolts as point supports to get axial loads. Shear calculated as usual.

In addition to these you will have cantilever bending in the bolts from the shear loads.

Over 1.5" the section stresses are the important criteria and buckling doesnt come into it.

Limit both to the recommended fatigue criteria stated in the relevant code.

csd

 

[whyun]

AISC Solutions Center discussed this topic a year ago. This is the link:

http://www.aisc.org/MSCTemplate.cfm...tomSource/Faq/SteelInterchange.cfm&FaqID=2662

 

[UcfSE]

You have to watch how you calculate loads in the rods. Some of the diameters and distances between the foundation and base plate will cause the bolt to behave more like a deep beam. A 1-inch bolt spanning 3 inches isn't the typical shallow beam we normally see.

Hilti, in their 2006 product technical guide, has some information on this in section 4.1.

 

[transmissiontowers]

We do this all the time. If the gap between the bottom of the base plate and the top of concrete is less than 2 bolt diameters, we ignore the bolt bending. Here is a section of a soon-to-be-published ASCE document on Electric Substation Structure Design (ASCE 113):

7.4.2.2 Base Plate Supported By Anchor Bolts With Leveling Nuts
Anchor bolts used with leveling nuts should be designed with consideration of
tension, compression, shear, and bending. The base plate supported by anchor bolts
with leveling nuts differs from the base plate bearing on concrete since a portion of the
shear is not resisted by friction between the base plate and concrete (e.g. coefficient of
friction (?) is not a factor). In addition, if the clearance between the base plate and
concrete exceeds two times the bolt diameter, then a bending stress/buckling analysis
of the bolts is required (ASCE 48).

There is also an ASCE document ISBN 0-7844-0262-0 "Wind Loads and Anchor Bolt Design for Petrochemical Facilities" that covers anchor bolt design.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[Lion06]

I would think that you would have to account for the bolt bending.
Transmissionstowers-
I didn't see anything in there that said you can ignore bolt bending for less than 2db clear, just that you MUST include it (and a buckling analysis) for greater than 2db clear.
Also, the OP said 1 1/2" clear. If you consider a 3/4" AB, then you are right at the 2db.

 

[transmissiontowers]

You can always be conservative and include the bending in the bolt, but if the baseplate is on leveling nuts, the gap between the bottom of the bottom nut and T.O.C. is about 1 bolt diam since the nut height is 1 bolt diameter. We consider such a short moment arm to lend the problem to be just shear.

When we wrote the section on bolt bending, we decided to put in the positive side that you must include bending if the gap is >2D as opposed to the negative that you can neglect it if it is less than 2D. Keep in mind that the structures I design get a max load once every 50 years. The original post was about a sign which would be wind controlled.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[Lion06]

won't the moment arm of the bending in the bolt be from the TOP of the baseplate (where the plate washers are bearing on it) to the top of footing? The levelling nuts under the baseplate aren't doing anything for you in terms of transferring shear.

 

[transmissiontowers]

If the bolt is in compression, the friction force will be on the bottom nut but we always assume that with a sufficiently thick baseplate with a top and bottom nut, the nuts clamp the plate and the top of the anchor bolt is not free to rotate like a cantilever but behaves like a fixed moment connection so there is a point of contraflecture between the T.O.C. and the bottom nut.

_____________________________________
I have been called "A storehouse of worthless information" many times.