Temporary Structures & Safety

6% of exam

Formwork and falsework lateral pressures, shoring and reshoring, scaffolding and bracing, anchorage, special inspections and submittals, construction impact on adjacent facilities, and OSHA construction safety.

4 concepts

Exam areas

A. Special inspections

Special Inspections & Submittals

The IBC Chapter 17 statement of special inspections, what must be inspected continuously vs. periodically for concrete, steel, masonry, and soils, and the shop-drawing review chain.

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C. Formwork, falsework, and scaffolding

Formwork Pressure, Shoring & Reshoring

The ACI 347 lateral pressure of fresh concrete on wall and column forms, how shores and reshores distribute construction loads through a multistory frame, and falsework basics.

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 up to 14 ft tall placed at less than

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

. Here

R

is the placement rate in

\text{ft/hr}

T

is the concrete temperature in

^\circ\text{F}

C_w = 1.0

for normal-weight concrete, and

C_c = 1.0

for ASTM Type I/II/III cement with no retarder. The result is bounded: it must not be taken less than

600\,C_w\ \text{psf}

nor more than the full liquid head

wh

(

w

= unit weight of the concrete). A wall taller than 14 ft, or any wall placed at 7 to 15 ft/hr, uses

C_w C_c(150 + 43{,}400/T + 2800R/T)

with the same bounds; columns use the slow-wall equation.

p_{max} = C_w\,C_c\left(150 + \dfrac{9000\,R}{T}\right), \qquad 600\,C_w \le p_{max} \le w\,h

Columns and the design pressure diagram

For columns ACI 347 uses the same

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

at any placement rate, bounded like a wall: at least

600\,C_w

psf and never more than the full liquid head

wh

. Once you have

p_{max}

, the design pressure diagram is simple: it grows as the equivalent fluid

150\,C_w

pcf from the top down to the depth where it equals

p_{max}

, then stays constant at

p_{max}

to the base. Integrating that trapezoid gives the total load the form ties, wales, and studs must resist — the number that actually sizes the formwork members.

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 and check the placement rate and wall height, which pick the equation; every case then gets the same

600\,C_w

floor and the full-liquid-head ceiling

wh

. Plug

R

\text{ft/hr}

and

T

^\circ\text{F}

into

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

, then immediately apply the

600\,C_w

floor and the

wh

ceiling — a common trap is reporting a raw formula value that the cap would have governed. For the resultant, build the trapezoid: equivalent fluid

150\,C_w

to depth

p_{max}/(150\,C_w)

, then constant. For shoring/reshoring, work with construction loads (

D_{\text{fresh}}+

construction

LL

), never ASCE 7 service combinations, and remember a supporting slab can see more than one slab dead load. Cite ACI 347 and OSHA 1926.700; do not attribute any of this to handbook §2.3 or §4.3.

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 (height ≤ 14 ft,

R < 7

ft/hr)

p_{max} = C_w C_c\!\left(150 + \dfrac{9000\,R}{T}\right)

$R$ = placement rate (ft/hr), $T$ = concrete temperature ( $^\circ$ F), $C_w$ unit-weight coeff, $C_c$ chemistry coeff. psf. Walls up to 14 ft tall placed at less than 7 ft/hr, and columns at any rate.

Wall pressure bounds

600\,C_w \le p_{max} \le w\,h

Floor of $600C_w$ psf and ceiling at the full liquid head $wh$ ( $w$ = concrete unit weight, pcf; $h$ = depth of plastic concrete below the top of placement, ft), for walls and columns alike. Apply AFTER the formula.

ACI 347 column pressure

p_{max} = C_w C_c\!\left(150 + \dfrac{9000\,R}{T}\right), \qquad 600\,C_w \le p_{max} \le w\,h

Columns (no plan dimension over 6.5 ft), any placement rate: the same equation as a slow wall, with the same bounds, never less than $600\,C_w$ psf and never more than the full liquid head $wh$ (ACI 347R-14 §4.2.2.1). The old 3,000 psf column cap was dropped in ACI 347-04.

Depth to maximum pressure

h_{p} = \dfrac{p_{max}}{150\,C_w}

Depth (ft) at which the equivalent-fluid line reaches $p_{max}$ ; pressure is constant below this.

Resultant lateral load per unit width

P = \tfrac{1}{2}p_{max}\,h_p + p_{max}\,(H - h_p)

Area of the design pressure trapezoid (triangle to $h_p$ , then constant to wall height $H$ ). lb per ft of wall.

Shoring design load

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

Dead load of fresh slab + forms + construction live load (min $\sim50$ psf). Drives shore and falsework sizing.

Reshore load amplification

Q_{\text{slab}} = \beta\,D_{\text{slab}}, \quad \beta \approx 1.5\text{–}2.0

Grundy-Kabaila distribution: a supporting slab can carry $\beta$ times one slab dead load during placement above.

Falsework minimum lateral (stability) load

H \ge 0.02\,\Sigma P

Horizontal design load $\ge 2\%$ of total supported vertical load, for falsework stability/bracing.

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 $T = 70\,^\circ\text{F}$ , using Type II cement with no retarder. Find the ACI 347 maximum design pressure and the total lateral load per foot of wall length.

Solution. Coefficients:

C_w = 1.0

(normal weight),

C_c = 1.0

(Type II, no retarder).

p_{max} = C_w C_c\!\left(150 + \dfrac{9000R}{T}\right) = 1.0(1.0)\!\left(150 + \dfrac{9000(4)}{70}\right) = 150 + 514.3 = 664\ \text{psf}

. Check bounds: floor

600C_w = 600\ \text{psf}

and ceiling

wh = 150(12) = 1800\ \text{psf}

;

600 \le 664 \le 1800

, so

p_{max} = 664\ \text{psf}

governs. Depth to

p_{max}

h_p = 664/(150) = 4.43\ \text{ft}

. Resultant per ft of wall:

P = \tfrac{1}{2}(664)(4.43) + 664(12 - 4.43) = 1471 + 5028 = 6.50\times10^{3}\ \text{lb/ft}

. **Answer:

p_{max} = 664\ \text{psf}

;

P = 6.50\ \text{kip per ft of wall}

.** Sanity:

p_{max}

corresponds to only

4.4\ \text{ft}

of fluid head — far less than the full

12\ \text{ft}

— confirming the set of the lower concrete relieves the form, which is the whole point of the ACI rate/temperature term.

p_{max} = 150 + \dfrac{9000(4)}{70} = 664\ \text{psf}

Column-form pressure at fast, cold placement

Problem. A column is placed at $R = 6\ \text{ft/hr}$ with concrete at $T = 60\,^\circ\text{F}$ , normal weight, Type I cement, no retarder. The column is $10\ \text{ft}$ tall. Find the design lateral pressure and confirm which limit, if any, controls.

Solution. Coefficients:

C_w = 1.0

C_c = 1.0

p_{max} = 1.0(1.0)\!\left(150 + \dfrac{9000(6)}{60}\right) = 150 + 900 = 1050\ \text{psf}

. Bounds: floor

600\,C_w = 600

psf; ceiling

wh = 150(10) = 1500

psf. Since

600 \le 1050 \le 1500

, the formula value governs. **Answer:

p_{max} = 1.05\times10^{3}\ \text{psf}

(1050 psf), neither cap controlling.** Sanity: dropping the temperature from

70

60\,^\circ\text{F}

and raising the rate to

6\ \text{ft/hr}

pushed the rate term to

900\ \text{psf}

versus

514\ \text{psf}

in Example 1 — colder and faster means higher pressure, as expected.

p_{max} = 150 + \dfrac{9000(6)}{60} = 1050\ \text{psf} \le wh = 1500\ \text{psf}

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 the pressure at $600C_w$ psf and caps it at the full liquid head $wh$ , for walls and columns alike. The cap often governs cold or fast pours.
•Using the retired column cap or the wrong wall equation. The flat $3000C_w$ column cap has been gone since ACI 347-04: columns use the slow-wall equation at any rate, bounded by $600C_w$ and $wh$ . A wall taller than 14 ft, or placed at 7 to 15 ft/hr, uses $C_w C_c(150 + 43{,}400/T + 2800R/T)$ .
•Plugging the rate $R$ in the wrong units. The 9000 constant requires $R$ in $\text{ft/hr}$ and $T$ in $^\circ$ F; using m/hr or $^\circ$ C silently corrupts the rate term.
•Using ASCE 7-16 service/strength load combinations for shoring and falsework. Formwork uses construction loads (fresh dead load + construction live load $\ge 50$ psf); never mix in-service ASCE 7 combinations with construction-load design.
•Assuming a reshored slab carries only its own weight. The Grundy-Kabaila distribution can load a supporting slab to $1.5$ – $2.0$ times one slab dead load during placement above — the reason the number of shore/reshore levels matters.
•Treating the design pressure as constant hydrostatic over the full height. Above $h_p = p_{max}/(150C_w)$ the diagram is triangular; only below it is it constant — the resultant is a trapezoid, not a single triangle or rectangle.

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

D. Impact of construction on adjacent facilities

Excavation Support & Impact on Adjacent Facilities

Braced and tieback excavation support, the Terzaghi-Peck apparent-earth-pressure diagram and strut loads, dewatering effects, and protecting adjacent structures from settlement.

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E. Safety

OSHA Construction Safety: Fall Protection & Steel Erection

The OSHA fall-protection trigger heights and acceptable systems, the steel-erection rules, and the trench/excavation protective-system thresholds and sloping ratios.

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