Double flanges on base plate for stack 1

[charliealphabravo]

I am designing the anchor rods for a 20" diameter stack (80' tall). There are two 32" flanges (base plates) with stiffeners. I understand that the stack is designed for complete exposure but in this application only the top 30' will be exposed to wind load.

Does anyone have a reference for how the anchor rod connection is achieved in this case where there are two plates/flanges? I'm not sure yet if I will need to engauge both plates but if I do I'm wondering how the nuts and washers are configured in order to share the load between both base plate flanges. My guess is that in the full exposure case the plates were to accomodate pretensioned rods of some type.

Thanks in advance.

 

[Althalus]

Please clarify something for me. I believe you are saying that two flanges + stiffeners essentially create what is known as a "chair". Is that correct?

The top flange is there to transfer load from the nuts to the stiffeners (on the tension side).
The bottom flange is there to transfer load from the grout to the stiffeners (on the compression side).

The bolt must be embedded into the concrete at the appropriate depth for pullout and breakout for your anchor bolt arrangement regardless of the baseplate. The anchor bolt projection must be sufficient to take into account the full height of the chair. Then place the nuts on top of the top flange. Do NOT place nuts in the middle of the chair (i.e. on top of the bottom flange).

The top flange should be sufficiently thick that the load from the nut will distribute to the two stiffeners that straddle the bolt shaft.
The bottom flange should be sufficiently sized to transfer load from the grout to the stiffeners.
The stiffeners should be properly sized to be able to transfer the load from top or bottom flange to the stack wall through shear.

The little bit of rotation on the stiffeners is also distributed by the flanges so the stiffeners don't penetrate the stack wall.

 

[charliealphabravo]

You have described my problem and answered my question exactly. Thank you very much.

 

[dhengr]

Charliealphabravo:
That looks like a fairly common base detail for a self supporting stack. The washers and nuts go on atop the upper flange plate. This does allow some pre-stretching of the anchor bolts over the 6 or 8" height of the stiffeners btwn. the top flange pl. and the lower base pl. The anchor bolts are most often assembled as a round cage with .25" plate doughnuts t&b as spacer/locators. This general base arrangement is needed to get the concentrated anchoring loads into the fairly thin shell of the stack at reasonable stress levels. You may not need all of the A.B’s. since only the upper 30' of stack sees wind load. Is the stack supported in any way at the 50' elev.?

I think several of the Civil and Structural Handbooks have some limited info. of this subject. See if you can get your hands on “Tubular Steel Structures, Theory and Design,” by M.S. Troitsky, from The Lincoln Arc Welding Foundation. It’s a good book and inexpensive too.

 

[charliealphabravo]

Thanks again.
These will be retrofitted anchors so the rod spacing (about 4" o.c.) will be looked at very closely. I may be able to omit some rods or pick up support at the roof level if the embedment analysis isn't working out.