Under-Capacity Four-Point Lift of a Prestressed Box Girder
At an erection site, a 30.0 m prestressed concrete box girder weighs 540 kN and has four cast-in lifting inserts, each rated 150 kN for a vertical lift. The contractor plans a four-point lift with slings at 45° to the horizontal, using a single crane hook and a spreader beam that is not equalizing. During a trial lift, one insert pulls out about 10 mm and a diagonal crack appears at the web-flange junction near that insert. The contractor wants to continue because the girder is still hanging and the crack is 'hairline.' The lift plan is not stamped by an engineer, and the inserts were last proof-tested three years ago. The girder has 35 mm of camber, and the bearing area at the abutment is blocked and ready.
1. Calculate the available vertical lift capacity of the four inserts at 45° and compare it with the girder weight, including a reasonable dynamic allowance.
Read the worked answer
Each insert rated 150 kN is not 150 kN vertical when the sling is at 45° to the horizontal. Vertical component = 150 × sin 45° = 106.1 kN per insert. Four inserts give 424.3 kN vertical, which is less than the 540 kN girder weight even before dynamic effects. With a 1.10 dynamic factor, the required static-equivalent capacity is 594 kN; with 1.25 it is 675 kN. The lift is therefore undercapacity by calculation alone. Because the spreader is not equalizing, load may not be shared equally; two inserts could carry much more than 106 kN each. The trial-lift insert movement and crack confirm that the calculated overload is real, not theoretical.
2. What should be done immediately when the insert moves and the crack appears?
Read the worked answer
Lower the girder immediately onto stable blocking or the transport supports; do not continue the lift or move the girder while it is hanging. Keep personnel out from under the load. Mark and photograph the crack and insert movement. Inspect the insert, surrounding concrete, and web-flange junction for spalling, splitting, and strand disturbance. Notify the erection engineer and the precast manufacturer. The crack is not 'hairline' in the sense of being acceptable without evaluation: it formed during an overloaded lift at a highly stressed region. The girder should be considered damaged until an engineer evaluates the crack depth, location, and effect on prestress and shear. Do not re-lift with the same plan.
3. What must be corrected before a second lift?
Read the worked answer
Require a stamped lift plan prepared or reviewed by a professional engineer. Use lifting points and inserts with adequate capacity for the actual sling angle and dynamic factor; at 45°, a 150 kN insert is only good for about 106 kN vertical, so a heavier insert or a different rigging geometry is needed. Use an equalizing spreader or a lift beam so each insert shares load, and verify sling angles. Proof-test or inspect the inserts and document their capacity. Repair or strengthen the cracked region per the engineer's design, and consider a strongback or additional pickup points. Recheck camber and support conditions. Only after the engineer releases the repair and the lift plan should a trial lift be repeated, with the girder just clear of supports.