Temporary Structures & Safety

Temporary Structures & Safety · Study · PE Civil: Structural · FE → PE Prep

Temporary structures fail more often than permanent ones, and formwork is the classic culprit: fresh concrete behaves like a dense fluid, and a wall form that is fine at the top can burst near the base where the pressure piles up. The PE rewards an engineer who can size that pressure from the placement rate and temperature, then trace how the weight of freshly placed floors travels down through shores and reshores into floors that have barely begun to cure. This is a NARRATIVE topic — the design pressure formula lives in ACI Committee 347 ·Guide to Formwork for Concrete· (ACI 347) and its load companion ACI 347R / SP-4, and the worker-safety rules in OSHA 29 CFR 1926 Subpart Q (Concrete and Masonry Construction), NOT in the NCEES PE Civil Handbook, which gives only construction context. Cite ACI 347 and OSHA 1926.700 here, never a handbook page.

Concrete as a fluid — and why pressure is capped

Before it sets, plastic concrete pushes on the form with a full hydrostatic pressure

p = w h

, where

w \approx 150\ \text{pcf}

for normal-weight concrete. If you placed an entire wall instantly, the base would see the full fluid head. In reality concrete stiffens as it is placed, so once the lower lifts begin to set they carry their own weight and stop transmitting pressure upward. The design lateral pressure therefore rises with depth only until the concrete at the bottom has gained enough stiffness, then levels off at a maximum

p_{max}

. ACI 347 captures this with rate-of-placement and temperature terms: place faster or colder and the set is delayed, so the fluid head — and the design pressure — climbs.

p_{\text{hydrostatic}} = w\,h, \qquad w \approx 150\ \text{pcf}

The ACI 347 wall-form pressure formula

For walls placed at a rate

R \le 7\ \text{ft/hr}

(the common slow-placement case), ACI 347 gives the maximum lateral pressure as a base term plus a rate/temperature term, multiplied by a unit-weight coefficient

C_w

and a chemistry coefficient

C_c

Columns and the design pressure diagram

For columns ACI 347 uses

p_{max} = C_w C_c\big(150 + 9000R/T\big)

Shoring: carrying a slab before it can carry itself

A freshly cast slab cannot support its own weight, so vertical shores transfer that dead load — plus a construction live load — down to a level that can. OSHA and ACI require shores to be designed for the fresh-concrete dead load, the weight of forms and equipment, and a construction live load (commonly

50\ \text{psf}

minimum, more for motorized buggies or stockpiled material). The shores must bear on a surface that has gained adequate strength or be carried all the way to the ground (falsework). Load combinations follow the formwork design load, not the in-service ASCE 7 combinations — never mix the two.

w_{\text{shore}} = w_{\text{slab,fresh}} + w_{\text{forms}} + w_{LL,\,\text{constr}}

Reshoring and the multistory load path

On a multistory building you strip the forms early and replace the shores with ·reshores· — but the new slab now leans on the slabs below it through that shore/reshore stack, and a freshly placed slab can momentarily load a slab below it to more than its own weight. The classic Grundy-Kabaila analysis distributes the construction load among the connected levels: with

N

interconnected levels of equal stiffness, the most heavily loaded supporting slab can carry on the order of

1.5

2.0

times a single slab dead load while concrete is being placed above. The number of shore/reshore levels is chosen so no slab is loaded beyond its age-adjusted capacity — fewer levels concentrate load, more levels spread it.

Q_{\text{slab}} = \beta\,D_{\text{slab}}, \qquad \beta \approx 1.5\text{–}2.0\ \text{for typical shore/reshore stacks}

Falsework

Falsework is the temporary support for an entire elevated structure — bridge superstructure, large transfer girder, cast-in-place arch — until it is self-supporting. It is a designed structure in its own right: towers, sills, and bracing checked for vertical load, the lateral load from wind and placement, and stability against overturning and buckling, with foundation bearing checked against settlement that would distort the permanent work. Codes treat falsework seriously (Caltrans and AASHTO falsework manuals, ACI 347.2R for bridge falsework) because its collapse during a concrete pour is catastrophic. Design loads include the full wet-concrete weight, forms, a placement live load, and a horizontal load taken as a fraction of the vertical (commonly

2\%

) for stability.

H_{\text{lateral}} \ge 0.02\,\Sigma P_{\text{vertical}}\quad(\text{minimum stability load, falsework})

Exam strategy

First decide wall vs. column — the rate term and the cap differ (

150h

vs.

3000\,C_w

). Plug

R

\text{ft/hr}

and

T

Key equations

Hydrostatic (full fluid) pressure

p = w\,h

Upper bound on form pressure if concrete never set; $w\approx150\ \text{pcf}$ normal weight, $h$ = depth of plastic concrete (ft). Result in psf.

ACI 347 wall pressure ($R\le 7$ ft/hr)

Worked examples

Wall-form pressure and the resultant on the ties

Problem. A $12\ \text{ft}$ tall, normal-weight concrete wall is placed at a rate $R = 4\ \text{ft/hr}$ at a concrete temperature $T = 70\,^\circ\text{F}$

Common pitfalls

•Citing a handbook page for the ACI 347 pressure formula. The NCEES PE Civil Handbook has NO formwork-pressure equation; the source is ACI 347 (worker safety is OSHA 1926 Subpart Q). Attributing it to §2.3 or §4.3 is a sourcing error.
•Reporting the raw formula value without applying the bounds. ACI 347 floors wall pressure at $600C_w$ psf and ceilings it at the fluid head $C_w(150)h$

References

ACI 347-14 (R2021) — Guide to Formwork for Concrete (lateral pressure $C_w$/$C_c$ formula, design loads)
ACI SP-4 / ACI 347R — Formwork for Concrete (resultant pressure diagrams, shoring/reshoring practice)
OSHA 29 CFR 1926 Subpart Q — Concrete and Masonry Construction (1926.700 formwork and shoring requirements)
Grundy & Kabaila, 'Construction Loads on Slabs with Shored Formwork in Multistory Buildings,' ACI Journal — shore/reshore load-distribution method

A. Special inspections

Special Inspections & Submittals

C. Formwork, falsework, and scaffolding

Formwork Pressure, Shoring & Reshoring