Formwork Shore Post Load and Spacing (ACI 347)
The vertical load path of an elevated pour.
Example
You enter
- Slab / pour thickness (in) 8
- Concrete unit weight (pcf) 150
- Formwork dead load (psf) 10
- Construction live load (psf) 50
- Shore spacing one way (ft) 4
- Shore spacing other way (ft) 4
- Rated shore capacity (lb) 6000
- Combined-load floor (psf) 100
You get
- Slab dead load 100.0 psf
- Design pressure 160.0 psf
- Load per shore 2560 lb
- Utilization 0.43
Details, formula, and sources
Design pressure = max(slab_in/12 x unit weight + form load + construction live load, 100 psf floor), and the load on any one shore is that pressure times its tributary area. An 8 in slab on a 4 ft grid carries over 2,500 lb per post. Motorized buggies raise the live load to 75 psf and the floor to 125 psf. The rated capacity is the manufacturer's allowable for the extended height and bracing; reshoring and the slab below are separate analyses. A design aid, not a stamped shoring plan.
slab_load = slab_in / 12 x unit_weight; design_psf = max(slab_load + form_load + live_load, 100); shore_load = design_psf x spacing_x x spacing_y; utilization = shore_load / shore_capacity.
ACI 347 (Guide to Formwork for Concrete), by name; the tributary-area load path is first-principles statics.
The construction-load minimums are stated in ACI 347; the tributary-area load per shore is public statics.
Estimate. AHJ and licensed professional govern.
Field names used by the API: slab_in, unit_weight, form_load, live_load, spacing_x, spacing_y, shore_capacity, min_design_psf, slab_load, design_psf, shore_load, utilization
- Design pressure max(slab dead + form + construction live, 100 psf floor) per ACI 347ACI 347
- Load per shore design pressure x tributary area (the two shore spacings), the vertical companion to formwork-pressurefirst principles
- Rated capacity the manufacturer's allowable for the extended height and bracing governs; reshoring is a separate analysisACI 347.2R