Calculating Bending in Anchor Rods 1

[amecENG]

I have come across two different methods that give very different results when calculating anchor rod bending from shear forces at the base of a column:

Some texts suggest to use a lever arm equal to the thickness of the grout when computing the bending moment in the rod.

However, recently I was shown a different method which calculates the moment arm as the sum of the baseplate thickness, grout, and half the embedment of five anchor rod diameters.

Can anyone offer their opinion on which method they use?

 

[Lion06]

Neither. ACI 318 App. D allows you to use a 0.8 factore on the shear force for the steel in anchor rods when a grout bed is used. This results in a 20% reduction.

If you look at bending in the anchors, you will take a hit more in the range of 50% to 80% reduction. You'll be upsizing your anchors a lot.

 

[slickdeals]

I have come across an equation in the HILTI catalog. It is located in Section 4.1. I think it is based on European code?

 

[WillisV]

AISC Design Guide 1 provides two allowable methods:

1. If you do not mind the plate slipping a bit into bearing and you do not use welded plate washers, then you design only two of the bolts for shear only with the aforementioned 0.8 reduction factor and ignore bending.

2. If you want to use all 4 (or more) bolts to resist shear you use welded plate washers and check combined bending/tension/shear in the rods using 1/2 the total thickness of (baseplate+0.5*washer) as the moment arm (assuming double curvature).

 

[amecENG]

Lion06 & WillisV

I guess you would consider ignoring the presense of grout as I have mentioned in my methods as being too conservative when designing the anchor rods?

I am just thinking that from my limited experience with inspecting baseplates the grouting is usually done very poorily. Often the grout space is too small to effectively place grout and when the grout space is large enough there is no way to inspect to ensure that the grout is in 100% contact with the rods.

Your thoughts please.

 

[rscassar]

Typically the grout pad is 1.5*dia of the anchors. At this span to depth, bending theory is not entirely applicable so I would only be using the depth of grout and assuming the rod is in double curvature.