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RE: Erecting Steel Braced Frames with Bolted Connections

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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)HNTB.com]
Sent: Thursday, June 24, 1999 3:41 PM
To: 'seaint(--nospam--at)seaint.org'
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?