Slab Shoring and Reshoring Load Distribution

it loads the whole stack of levels the shores connect, and the load is shared among them. THE DISTINCTION BETWEEN RESHORING AND BACKSHORING IS THE ONE THAT GETS LOST, and it changes the arithmetic completely. A RESHORE is installed after the slab above has been stripped and allowed to DEFLECT and carry its own weight, so it carries only loads added afterwards. A BACKSHORE is installed without permitting that deflection -- typically by stripping and reshoring in small areas -- so it continues to carry a share of the slab's own dead load as well. Treating a backshore as a reshore, or the reverse, misallocates the load through the whole stack, and both readings are computed here so the difference is a number rather than a word. GETTING IT WRONG IS NOT CONSERVATIVE IN A PREDICTABLE DIRECTION: it can under-count the load on a level or over-count it, and the design of the sequence depends on which it is. THE CONSEQUENCE IS A SLAB LOADED BEYOND ITS CAPACITY AT ITS AGE. A young slab has a fraction of its 28 day strength, and construction loads -- the wet concrete above, plus the forms, plus the crew and equipment -- are often the largest loads the slab will ever see in its life. Multi-storey construction failures during placement are a recognized category and the shoring sequence is usually at the centre of them. The governing level is the youngest supporting slab, because it is the one whose capacity today is furthest below its design capacity. AND THE STRENGTH TO USE IS FIELD-CURED CYLINDERS that experienced the same conditions as the slab, not laboratory-cured ones. A slab poured in cold weather is far behind lab cylinders that sat in a warm curing tank, and stripping to a laboratory strength on a cold weather pour is stripping to a strength the slab does not have. That is the trap that turns a correct calculation into a collapse. EQUAL STIFFNESS ACROSS THE CONNECTED SLABS IS THE SIMPLIFYING ASSUMPTION HERE and it is the standard first-order one, which makes the share a straight division by the number of connected levels. A real analysis follows the construction sequence step by step, tracks each slab's stiffness at its actual age, and accounts for shore stiffness and for the fact that the lower slabs are older and stiffer and therefore take more than an equal share. This is a screen against that, not a substitute for it. It does not design the shores, which is a separate calculation from tributary area, or evaluate slab punching, deflection, or cracking under construction load; it does not set stripping times; and it does not address post-tensioned slabs, whose stripping and reshoring rules differ, or the lateral bracing of the shoring stack. ACI 347 and ACI 347.2R, the shoring designer's sequence drawings, the field-cured cylinder breaks, and the engineer of record govern.

the load a new pour puts into the stack = slab dead + formwork + construction live, shared EQUALLY among the connected levels under the standard equal-stiffness assumption, so each supporting slab carries its own dead load plus that share; a backshore adds the slab's own dead load to what is redistributed.

The ACI 347.2R shoring and reshoring load distribution by name, taken at the standard first-order EQUAL STIFFNESS assumption. A RESHORE is placed after the slab above has deflected and carries only later loads; a BACKSHORE never permitted that deflection and still carries a share of the dead load. The slab's capacity must come from FIELD-CURED cylinders at its age today. A screen, not a sequence analysis. ACI 347 and ACI 347.2R, the shoring designer's sequence drawings, the field-cured cylinder breaks, and the engineer of record govern.

One sum and one division; no shoring design or sequence drawing is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: slab_dead_psf, construction_live_psf, form_dead_psf, connected_levels, backshored, governing_slab_capacity_psf, governing_slab_strength_psi, new_pour_load_psf, redistributed_load_psf, share_per_level_psf, governing_level_load_psf, capacity_margin_psf, utilization_pct, other_case_load_psf

• Equal stiffness across the levels a real analysis follows the sequence and each slab's stiffness at its actual agethe shoring designer's sequence drawings
• Capacity from field-cured cylinders laboratory-cured cylinders say nothing about a cold weather pourthe field-cured cylinder breaks
• Shores are not designed and punching, deflection and stripping times are separateACI 347 and the engineer of record