Base Plate Anchor Bolt Loading

[MFJewell]

I have an access platform I am adding along a boiler. The design is cantilevered and supported by 6x6x0.25 HSS tube. I did a quick calculation based on max loading to see what tension would be on the anchor bolts (see calc below and model of baseplate). I was looking at using quick bolts and reviewed the hilti specs against the calculated load. My calc shows a tensile load of 5655 pounds along the centerline of the anchor bolts. This load would be shared across two or three anchor bolts. Hilti has a pullout load of 3345 lb in cracked concrete for 5/8" bolts. When I use the spacing factor in Hilti's spec, I get that it would take 7125 pounds (0.71*3345*3) to pull out all three bolts. That of course assumes all bolts share the load equally. Any tips or advice on this?

 

[JGard1985]

MFJewell

I agree with your approach, its common for concrete anchors like other bolted connections to share load (so long as your interaction affects are taken into account).

However considering you have a cantilever column baseplate resisting an applied moment, I suggest you review column baseplate design. (Blodgett "Design of Welded Structures" Section 3.3-8) You have to look at the fact that your compressive force is a triangular pressure distributions. This affects the force-couple moment arm and tension loads in the bolts. Because you are so close, I'd recommend this exercise to verify you have a sufficient moment arm.

Jeff
Pipe Stress Analysis
Finite Element Analysis

http://www.xceed-eng.com

 

[dik]

and if you make your baseplate a tad thicker, you can eliminate the stiffeners... Thicker plate is far less costly than adding stiffeners.

Dik

 

[MFJewell]

Thanks for the replies. Unfortunately I don't have Blodgett's book, but I did use a base plate design guide through AccessEngineering (via ASME). I believe I am ok with the plate dimensions. Additionally, I changed the geometry of the baseplate to lower tension on the bolts and plan to use 1/2" plate, so some of the stiffeners were removed.

 

[dik]

Can you remove them all? Generally cheaper, unless you have huge moments and members, to not use them even if they are an inch or inch and a half thick unless you have a kazillion of them.

Dik

 

[MFJewell]

I probably could remove them all, but I'm not concerned with the cost. I would rather have the extra stiffness just in case one of the maintenance guys tries to use this platform well in excess of my already conservative design.

 

[SlideRuleEra]

My calcs shows a tensile load of 5655 pounds along the centerline of the anchor bolts.
...it would take 7125 pounds (0.71*3345*3) to pull out all three bolts.
...just in case one of the maintenance guys tries to use this platform well in excess of my already conservative design.

Failure of one of the three bolts allows failure at design load.

At design load with all three bolts fully engaged: Safety Factor = 7125 lb. / 5655 lb. = 1.26

For safety purposes, say a scaffold is equivalent to an elevated platform. Scaffold Safety Factor = 4 is a widely accepted value.

1.26 SF < 4 SF

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[MFJewell]

For safety purposes, say a scaffold is equivalent to an elevated platform. Scaffold Safety Factor = 4 is a widely accepted value.

1.26 SF < 4 SF

Agreed. However, actual design load is 350 pounds plus dead weight of platform (so I already have an FS of 2 in my design load). Additionally, as I previously stated, I revised the geometry of the base plate to get tension bolt load lower. I am almost at 4 (it is like 3.8x).