Tie-rod capacity on a wall-mounted jib before a second hoist point is added
A maintenance team is modifying a wall-mounted jib inside a fabrication bay. The horizontal beam is pinned to the wall at A. A tie rod runs from a wall bracket at B, 0.90 m vertically above A, down to the beam at C, 1.50 m from A. A single hoist load of 6.0 kN hangs at 2.20 m from A. The existing tie rod is an M16 grade 4.6 threaded rod with a tensile stress area of 157 mm squared and a yield strength of 240 MPa. Beam self-weight is neglected at this stage. Production now wants an identical 6.0 kN hoist point added at 3.00 m from A, arguing that the jib looks heavily built and that the rod is only lightly stressed today. There is no engineer on site until next week and the bay is needed for a shutdown job in two days.
1. Determine the tie-rod force and stress for the present single-load arrangement and judge how much margin the M16 grade 4.6 rod actually has. Include the reactions at A.
Read the worked answer
Geometry of the tie rod. BC = square root of (0.90 squared + 1.50 squared) = 1.749 m. The vertical component factor is 0.90/1.749 = 0.5145; the horizontal component factor is 1.50/1.749 = 0.8575.
Moment about A (equilibrium of the beam):
T x 0.5145 x 1.50 = 6.0 x 2.20 0.7718 x T = 13.2 T = 17.1 kN
Tensile stress: 17,100 N / 157 mm squared = 108.9 MPa, which is 109/240 = 45 percent of yield. On the face of it the rod has roughly a factor of 2.2 against yielding.
Reactions at A:
- Vertical: 6.0 - (17.1 x 0.5145) = 6.0 - 8.80 = -2.80 kN, i.e. 2.80 kN downward. The wall pin has to hold the beam down.
- Horizontal: 17.1 x 0.8575 = 14.7 kN pulling the beam toward the wall.
The important judgement is that the rod is not the only element to check. A 14.7 kN sustained pull-out on the wall bracket anchors and on the pin at A is often the real weak link on old jibs, and it is invisible in a simple rod stress check. The 45 percent figure is a stress ratio, not a proof of adequacy: it says nothing about the threads, the clevis, the welds or the anchor bolts.
2. Evaluate the proposed second 6.0 kN load at 3.00 m and quantify the options if the tie rod has to be upgraded.
Read the worked answer
Recompute with both loads. Both loads create clockwise moments about A:
Moment = (6.0 x 2.20) + (6.0 x 3.00) = 13.2 + 18.0 = 31.2 kN.m
0.7718 x T = 31.2, so T = 40.4 kN.
Stress in the existing M16 rod: 40,400 / 157 = 257 MPa. This exceeds the 240 MPa yield strength of grade 4.6 at the working load, before any safety factor. The rod would stretch and the jib would deflect permanently; fatigue at the threads would follow. The proposal is not acceptable as it stands - the visual impression that the jib is heavily built is not evidence.
Options, quantified:
- M20 rod, stress area 245 mm squared: 40,400 / 245 = 165 MPa, 69 percent of yield. Workable but thin on margin.
- M24 rod, stress area 353 mm squared: 40,400 / 353 = 115 MPa, 48 percent of yield. Preferred if the bracket can take the larger clevis.
- Change the geometry: moving the wall bracket up so the rod sits at 45 degrees gives a vertical component factor of 0.7071, so T = 31.2 / (1.50 x 0.7071) = 29.4 kN, about 27 percent less force for the same load. Usually cheaper than a bigger rod if the wall allows it.
Also recheck the wall bracket: horizontal pull now approaches 2 x 14.7 = about 34.6 kN, which the original anchors were never sized for.
3. State the procedural conditions that must be met before the modified jib carries a load, and the records that must change.
Read the worked answer
A tie-rod change is a change to load-bearing equipment, so the sequence matters more than the arithmetic:
- No field modification without a signed calculation and a revised drawing. The workshop can measure and propose, but the acceptance decision belongs to a competent person. A verbal go-ahead from a supervisor who has seen the arithmetic is not a substitute, because the liability sits with the employer, not the individual.
- Check the whole load path, not only the rod: wall bracket anchors, pin at A, clevis and thread engagement, and the beam where the hoist points attach.
- Proof load test before service, typically 1.25 times the rated load for the new configuration, with the result recorded. Hold the test load over the full reach and both hoist points.
- Re-mark the safe working load plate to the new rating. If the jib is now rated for one hoist point only, say so on the plate and in the permit.
- Update the records: revised drawing and calculation sheet, load-test certificate, equipment register entry, inspection and maintenance schedule, and a briefing note to the operators. The old rating plate left in place is the classic cause of an overload after a modification.