Repairing platform broken beam 2010
Section links:
17mar10: allowable za range, tiedown tensions for
mar10
8mar10: moving the dome,ch,az, tiedowns into
position.
8mar10:
truss repair schematic with 8mar10 updates (.pdf)
15jan10: pictures of beam before break.
Dome weight
unbalanced
loads on
azimuth arm
timeline:
Intro:
A beam connecting the upper and lower cords of the
platform was found broken on 03feb10. Looking from corner 12 to
the tiedown (td) back stay connection point it was the left
back stay cable and it was the rightmost beam of the pair of beams. The
break was right above the connection of the back stay cable to the
flange (which connected to this and two other beams forming a triangle).
17mar10: allowable za range, moments, tdkips
after beam fixed.
The patch on the broken member was completed on
17mar10. After this, ammann and whitney would like us to limit the
telescope motion so that:
- We have at least 20 kips of td tension on each corner
- We limit the unbalanced load to 2000 to 3000 kip feet.
- A constraint for our pointing model is that the carriage house is
at stow (8.835 degrees).
On 17mar10 az
swings were done to find the azimuth arm balance position.
- ch at stow
- A 1az fit was made to each tiedown tension vs az and then the
za for 0 was found.
- the az arm is balanced with ch at sto and the dome at
7.968 degrees za.
- Using the accepted weights,offsets, radii, we had computed this
to be 7.5 degrees.
The plots
below show the az,za,temp,td range we can use and still meet the
above constraints.
The
za range that the dome and ch can move(.ps)
(.pdf):
- Top: The dome can go 6 to 10 degrees za with the ch at stow and
remain within the 3000 kip-feet limit.
- Bottom: With the dome at za=8.4, the carriage house can
travel from 0 to 20 degrees za and stay within the 3000 kip-feet limit.
The dome,ch collision limits are: [8.33,8.27] for pre and final limits
so the dome at 8.4 should be ok as long as there isn't too much
overshoot in the positions.
The temperature range we can
keep 20kips in each td (.ps) (.pdf):
- Each page shows the td tension vs azimuth for za=6, 8 and
10
degrees za.
- pages 0-4 plots this for temps: 65,70,80,85,90 degF
- At 85 degrees F we drop to 15 kips tension in a tiedown which is
too low.
- The kips model overestimates the daytime tensions (because the
temp sensor does not reflect the cable tension). So the limit may be
hit at 80 degF.
Summary:
- At night we can:
- run the dome from 6 to 10 degrees za with the carriage house at
stow (8.835).
- run the ch from 0 to 20 degrees with the dome at 8.4 degrees za.
- During the day:
- To keep the 20 kips in each tiedown with the above motion
you may need to pull the telescope lower (out of focus) to increase the
tension. this can be done with:
- tiepos 75
- this means the telescope is out of focus.
- The operator should probably do this each morning.
- When nighttime observing starts again (below 80 degF):
- tietrk .. to turn back on tiedown tracking.
- How to handle these limits:
- The agcProg program that controls the az,ch,dome has been
modified to keep the dome between 6 and 10 degrees when tracking
sources.
- The cima gui uses tracking mode so this should be ok for
remote users of the dome.
- The changes will not stop the vme preset or rate
command from moving outside these limits so don't use them.
- The OCU, and upstairs LCU,PCU have not been modified so they
will send the dome wherever you request it (so be sure and stay within
the 6-10 limits for the dome when the ch is at stow).
- The 6-10 degree dome limit expects the ch at stow. When
tracking with the dome, the ch is always sent to stow. Don't go moving
the ch manually.
- Using the Ch.
- If you want to track with the carriage house:
- Do the trkinit ch to initialize tracking. This will
send the dome to 8.835 degrees.
- Resend the dome (with preset) to 8.4 degrees.
- then go ahead and send your tracking commands.
- If you reinitialize the ch tracking (trkinit ch) you'll
have to resend the dome back to 8.4.
- To move the ch to a position say 0 and leave it there:
- pnt mode pos 8.4 1 (move the dome to 8.4)
- pnt mode pos 0 4 (move the ch to 0)
processing:
x101/100317/unbal_moments_17mar10.pro
8mar10: moving the dome,ch,az, tiedowns into
position.
On 8mar10 we moved the tiedowns, az, dome,ch into
position to relieve tension on the broken beam side in
preparation for the welding. The starting conditions were:
- 13:00 overcast, 69deg F
-
|
az
|
dome
|
ch
|
td
pulldown inches
|
td
tensions (kips
|
|
|
|
|
td12
|
td4
|
td8
|
td12
|
td4
|
td8
|
beforeMove
|
268
|
15.52
|
0
|
5
|
5
|
5
|
31
|
55
|
0
|
After Move
|
296.4
|
8
|
12
|
12.4
|
12.4
|
18.7
|
42
|
43
|
43
|
processing: x101/100302/chkmove.pro
- amman and whitney procedure
to move the dome,az,ch, tiedowns (.pdf)
- This shows the expected tensions in the beams during the motion.
- frame 5 shows the final configuration with 40 kips. The broken
beamside has 125 kips (there temps say 90F but it was 69F).
- frame 6 shows the final configuration with 60 td kips. We
probably can't get that high unless the temperature goes below 69F
Images of the broken beam
15jan10: picture of beam where break occurred.
On 15jan10 tom balonek took some pictures of the
platform.
- There are two spots of coloration above the bolts where the break
occurred.
- It doesn't appear that the beam has pulled apart yet.
Timeline
- 15jan10 : pictures of platform show that beam is not yet broken
- 03feb10: (wed) broken beam found
- 08feb10:(mon) threaded rods installed (this is a guess).
- 08mar10: moved azimuth to 296.4, dome to 8, ch to 12 in
preparation for repair.
- 19mar10: (fri) start installation of plate across break. Finished
but weld not sufficient
- 23mar10:(tue) welding redone, welding inspector ok's welds.
- 24mar10: (wed) finished removing equipment, observing starts za
6-10 deg.
home_~phil
