Return to index: [Subject] [Thread] [Date] [Author]

Re: Erecting Steel Braced Frames with Bolted Connections

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Harold Sprague wrote:
> Garry,
> As subscribers to this list know, I used to be an iron worker (and wasn't
> very good at it), I became a steel detailer (was also inept), then I became
> a structural  engineer (where the weather is better).
> The problem is the fit up with large braced frames using bearing bolts.  I
> too have designed telecommunication towers, and they generally use bearing
> bolts, and there is no fit up problem because the pieces are small and a
> bull pin (or drift pin) cures the majority of fit up opportunities.
> Single story, and 2 story buildings also don't have much of a problem with
> fit up also because the smaller member sizes lend themselves to persuasion
> by a bull pin, a hammer, and an average pissed off iron worker.
> When frames get larger, the bull pins and rage of the average iron worker
> are less effective due to the large sizes of the frame members.
> How do the fabricated pieces get so far off? The answer is tolerances,
> temperature, and luck.  What is the temperature in the shop when the
> diagonals are fabricated?  What is the temperature in the shop when the
> horizontals are fabricated?  What is the temperature in the shop when the
> columns are fabricated?  Ask the same sequence of questions upon erection.
> Obviously, temperature and solar thermal gain can not be controlled.  And
> the tolerances will not offset the temperature problem.  You can, hopefully,
> see the problems with fit up.
> If your shop and project are in San Diego, you preassemble, and the days are
> all cloudy, you have a fighting chance of having a well fit frame.
> What is the engineer to do?  If you use a gusset plate with connector plates
> on each side - use oversized holes in the gusset plate, standard 1/16"
> oversized holes in the connector plates, slip critical connections, and
> forget about bearing bolts.
> If it is essential that you have large frames with bearing bolts, then you
> must pre-assemble.  Even with pre-assembly and controlled shop temperatures,
> you will need some luck to erect the frames with no fit up problems.  I have
> been involved in only 2 projects where I was forced to do bearing bolts and
> joint preassembly was required.  There was no reaming required, but bull
> pins were still required to align the parts, and they erected well.  We were
> lucky.
> With modern shops (plasma hole burners, core drills, and CNC equipment)
> pre-assembly isn't as big a deal as it once was.  But it is not free.
> Milling is not required any longer because of modern cold saws.
> I would also look to an AISC certified shop with a Cbr or a Cbd rating.
> If the base plate elevation is off, watch the iron worker work magic with
> wedges and a hammer.
> For another example, look at the St. Louis Arch.  It did not line up when
> the final center assembly was placed.  The solution was to run cold water on
> half of the arch until the center assembly fit.
> Regards,
> Harold Sprague
> The Neenan Company
> -----Original Message-----
> From: Garry Frederick [mailto:GFREDERICK(--nospam--at)]
> Sent: Thursday, June 24, 1999 3:41 PM
> To: 'seaint(--nospam--at)'
> Subject: Erecting Steel Braced Frames with Bolted Connections
> > What facilitates erecting steel braced frames with bearing bolted
> > connections when using standard holes whose diameters are 1/16-inch
> > greater than their matching bolts? Recently a 22-foot by 22-foot by
> > 100-foot high tower whose X-braces crossed at the mid-span of beams at
> > alternating floors went up poorly because the holes in the braces did not
> > match the holes in the connecting plates on the columns and beams. Even
> > after reaming some of the mismatched holes to accommodate the next larger
> > diameter of bolts, more than 120 mismatched holes remained. What finally
> > enabled the tower to be erected within tolerances per section 7.11 Frame
> > Tolerances of the 1986 AISC Code of Standard Practice was by welding
> > instead of bolting the braces to the beams.
> >
> > What may have caused the bolt holes to be so mismatched? The base plates
> > for the columns had been positioned within the erection tolerance of plus
> > or minus 1/8-inch of the design elevation per section 7.6 Bearing Devices
> > of the AISC Code of Standard Practice. The wide-flanged beams, columns and
> > braces had been milled and fabricated within the tolerances per ASTM A6.
> >
> > Does the accumulation of mill, fabrication and erection tolerances
> > preclude using bolted braces with standard round holes unless extra
> > measures are taken beyond standard construction? Do base plates need to be
> > exactly at the design elevation with no plus or minus 1/8-inch tolerance?
> > Do bolted braced frames need to be assembled in the shop by the fabricator
> > to insure ease of erection by the erector? Does the contractor always need
> > the option of welding the braces?
My goodness Harold, I haven't heard that story about the Arch since I
watched the closure piece inserted on B&W TV as a kid in St. Louis! 
Brings back memories - 1964 Cardinals and all that.

Laurence B. Oeth, P.E.
Suffering Cardinal Fan on the West Coast