Temporary Needle Beam with an Unanchored Rear Support
A contractor is using a single 5.00 m length of 48.3 mm scaffold tube as a temporary needle beam to land a 1.20 kN air-handling unit on a first-floor slab. The tube projects 1.80 m beyond the face of the supporting wall and is supported at two points: A at the wall face and B 2.40 m behind A. The rear end is merely resting on a timber sole plate. Tube self-weight is 0.16 kN/m. The site engineer is asked to sign the permit, but the rear support shows no holding-down detail. The unit will be landed at the tube tip; no lateral restraint is shown at B. The tube is 48.3 mm outside diameter with 4.0 mm wall, and the permit is for one day.
1. Determine the vertical reactions at A and B for the static tip load, including tube self-weight. State clearly the direction of each reaction.
Read the worked answer
Take A at x = 0, B at x = +2.40 m, the tip load at x = -1.80 m and the rear end at x = +3.20 m. Tube weight = 5.00 × 0.16 = 0.80 kN, acting at the midpoint x = +0.70 m. Sum moments about B: 2.40 R_A = 1.20 × 4.20 + 0.80 × 1.70 = 5.04 + 1.36 = 6.40 kN·m, so R_A = 2.67 kN upward. Sum of vertical forces: R_A + R_B = 1.20 + 0.80 = 2.00 kN, hence R_B = -0.67 kN. The negative sign means the rear support must pull downward with 0.67 kN. A timber sole plate cannot do this, so the arrangement as drawn is unstable and would tip about A.
2. Can the rear support be a simple sole plate? Calculate the anchor demand if a 2.00 kN tip load is applied with an impact factor of 1.60.
Read the worked answer
No. The static case already requires a downward reaction at B of 0.67 kN; a sole plate can only push upward. With impact, the tip load becomes 2.00 × 1.60 = 3.20 kN. Sum moments about B again: 2.40 R_A = 3.20 × 4.20 + 0.80 × 1.70 = 13.44 + 1.36 = 14.80 kN·m, so R_A = 6.17 kN upward. Vertical equilibrium gives R_B = 3.20 + 0.80 - 6.17 = -2.17 kN. A rated clamp or anchor must therefore resist at least 2.17 kN downward, before any additional factor for anchorage or dynamic effects. The anchor should be fixed to the slab or wall and checked for pull-out; a nail into a sole plate is not acceptable.
3. List the checks you would require before signing the permit, beyond the anchor calculation.
Read the worked answer
Check the tube in bending and shear at A, because the cantilever moment is produced by the tip load and self-weight; confirm the section properties for a 48.3 × 4.0 mm tube, not a nominal value. Check local bearing and crushing at A, and provide a spreader if needed. Confirm the anchor capacity, embedment and edge distance, and verify that the slab can take the downward pull. Provide lateral restraint at B and at the load point so the tube cannot roll or swing. Confirm the load is landed slowly, with a tag line, and that the area below is excluded. The permit should identify the actual anchor, the checker and the load limit; if any of these cannot be shown, do not sign.