Erecting and Stripping Telescopic Steel Props: A Site Sequence

Most prop failures trace back to installation rather than to the prop. The unit left the factory tested and sound, then stood on a wet sub-base, extended further than the drawing allowed, and left out of plumb because nobody had time to check. A short, repeatable sequence catches all of that.

Setting Out Before the First Prop Goes In

Start from the design grid, not from the stack of props nearest the door. Mark the prop positions on the sub-base or on the deck below, using the spacing the falsework design specifies, and check that the grid lines up with the primary and secondary beams that will sit on top. If the beams will not land where the marks are, resolve it now. Moving props after the formwork is laid means unloading and resetting an already loaded system, which is how accidents happen.

Check the height before anything is carried. Measure the soffit level and the bearing level, subtract the thickness of the formwork, secondary beam and forkhead, and confirm that the prop size on site covers the remaining range with room to spare. A prop sized exactly to the calculated length has no adjustment left when the slab level moves, and slab levels always move.

Base Preparation and Bearing

Every prop load ends up in the ground, or in the slab below. That surface has to be able to take it. Clean off mud and loose material, level the bearing point, and use sole boards of adequate size and thickness where the base is anything other than sound concrete. Where an adjustable screw jack is used at the bottom of the prop, make sure the jack sits squarely on the sole plate and that the thread is not run out to its last few turns.

On upper floors the question changes to what is underneath. Props on two or three consecutive levels should be stacked in line so the load path runs straight down through the structure. Staggered props point-load a slab that was never designed for it, and the cracking shows up weeks later, far from anywhere anyone connects to formwork.

Erecting, Plumbing and Locking

Rough-set the length with the inner tube and pin, then fine-adjust with the collar nut. The pin must go fully home and be retained; the nut must cover the thread engagement the class requires and be tightened properly. Tighten by hand plus a light tap from the correct spanner, not with a length of scaffold tube used as a lever. Overtightening damages the thread and, worse, hides the fact that the prop is being asked to carry load through a thread that is barely engaged.

Plumb matters more than it looks. A prop leaning out of vertical carries an eccentric load, and eccentricity turns a compression member into a bending problem. Check in two directions, not one. Keep the specified minimum overlap between the inner and outer tube, and never make up a shortfall in height by packing the base with offcuts or by welding an extension onto the tube.

Forkheads, U-Heads and Secondary Beams

The top of the assembly deserves as much attention as the bottom. Forkheads and U-heads have to seat the secondary beam centrally, and the beam has to bear properly along its length rather than on one corner. A beam sitting crooked in a forkhead puts a side load into the top of the prop, which is the least favourable place for it.

Check that every head is adjusted to the same level before the plywood goes down. Crews often adjust heads after sheeting, lifting one corner of the formwork at a time, and the result is a deck that is flat in patches. Fix the heads first, sheet second, and the finished soffit comes out level with no rework.

During the Pour

Concrete is the load, and it arrives quickly and unevenly. Pump or skip placement should follow the sequence the designer assumed, spreading out rather than heaping in one corner, and there should be someone watching the props while it happens. That person is looking for movement, for a pin walking out, for a base settling, and for any prop that starts to look less than vertical.

Nobody belongs under the formwork while it is being filled. If something moves, stop placing, clear the area and reassess rather than shoring up the moving unit with whatever is lying nearby. Over-vibration on a soft soffit produces the horizontal forces a prop resists least.

Stripping and Safe Release

Formwork comes off when the concrete has reached the strength the specification requires, not when the programme wants the props back. Where the specification is silent, guidance such as ACI 347R gives indicative values, but the decision belongs to the engineer responsible for the temporary works. Striking early is the most common cause of avoidable deflection in new slabs.

Release in the sequence the design gives, generally working from the middle of the span outwards rather than from the edges inwards, and lower each prop gradually. Slackening a loaded prop in one go dumps its share of the load onto its neighbours, which then shed to their neighbours. Props are designed to be reused; treat them that way when they come out, clean the threads, stack them by size, and keep damaged units out of the next pour.

References

BS EN 1065, Adjustable telescopic steel props: Product specifications, design and assessment by calculation and tests, CEN / British Standards Institution.

BS EN 12812:2008, Falsework: Performance requirements and general design, CEN / British Standards Institution.

BS 5975:2019, Code of practice for temporary works procedures and the permissible stress design of falsework, British Standards Institution.

ACI 347R-14, Guide to Formwork for Concrete, American Concrete Institute.