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# Re: Stripping Treads

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• To: seaint(--nospam--at)seaint.org
• Subject: Re: Stripping Treads
• From: Roger Davis <sds_rdavis(--nospam--at)yahoo.com>
• Date: Fri, 29 Sep 2006 14:38:54 -0700 (PDT)

Another way of dealing with that problem is to put a coupler on the exposed bolt and add another short threaded rod. Then cut the base plate hole larger and use plate fillers and washers to get you to a point that a nut works properly. I would trust that solution more than I would trust a welded extension.

Roger Davis
Architect
SDS Architects, Inc.

Rich Lewis <seaint04(--nospam--at)lewisengineering.com> wrote:
I?m looking for the design equation for stripping threads.  I have anchor bolts in tension and the bolt doesn?t extend through the nut.  Now I have to sharpen a pencil and see if the design tensile force is less than what would strip what is left of the threads on the bolt.  If not then the contractor will need to weld an extension onto the bolts.

I?m having a difficult time finding design literature on this.  AISC doesn?t cover bolts.  Logic tells me it is the area of shear of the pitch diameter of the threads.  Since the pitch diameter occurs midway between the peaks and valleys, it would be ½ the perimeter area of the threaded section put in shear by the tension force on the nut/bolt.  I found a reference for a bolt guide in one of Harold S old posts to a 1998 NASA RP-1228 ?Fastener Design Manual?.  In this report they recommend using 1/3 the perimeter area of the pitch diameter to account for loose fit of the nut and bolt.  It makes sense to me.  The equation is P=(PI*DIA*Fs*L)/3 where DIA is the pitch diameter.  It is written for yield strength and ultimate tensile strength.  I was going to input 0.6*Fy and 0.5*Fu to come up with allowable loads.

I?ve been searching other books and haven?t found any reference to analysis or design recommendations for stripping forces on a thread.  Surely there had to be some recommendations prior to the 1998 NASA report.  Can anyone give me other references to analysis and design of bolt thread stripping?

Thanks.

Rich

Roger Davis
Architect
SDS Architects, Inc.

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