Formwork Engineering in NYC: ACI 347 Design Loads, PE Requirements & Failure Prevention

What Is Formwork and What Does It Include?

Formwork is the complete temporary construction system that holds fresh concrete in place until it achieves enough strength to be self-supporting. It is a multi-component system:

• Sheathing (form face or decking): The surface in contact with the fresh concrete. Typically plywood (structural sheathing, HDO or MDO), steel deck forms for slabs, or prefabricated steel panel forms for walls and columns. The sheathing carries the direct fluid pressure of the fresh concrete and transfers it as a line load to the supporting framing.
• Studs and walers (for wall and column forms): Vertical studs span between walers, carrying the concrete lateral pressure as a uniform distributed load. Wales (horizontal members) span between ties or vertical supports and deliver the lateral pressure as concentrated loads to the tie system or vertical shores.
• Form ties: Devices (snap ties, she-bolts, coil ties) that resist the lateral pressure pushing the two faces of a wall form apart. They also leave cone holes or recesses in the finished concrete that must be patched.
• Shoring and reshoring: The vertical load-carrying system — Acrow props, adjustable steel shore posts, tube-and-clamp scaffolding, engineered aluminum frames (system shoring) — that transfers the weight of the slab form, fresh concrete, construction live loads, and equipment down to the ground or previously hardened floors.
• Bracing: Diagonal bracing of the shoring system for stability against lateral loads — wind, construction equipment impact, eccentricity from uneven concrete placement, and vibration from form vibrators.

Vertical Load Design: Dead + Live + Construction Loads

The vertical load on formwork shoring is the sum of:

For a typical 10-inch CIP concrete slab in NYC: 125 psf (concrete) + 10 psf (formwork) + 75 psf (construction LL) + 25 psf (pump) = 235 psf design load on shoring. This figure surprises many contractors who budget only the slab dead load.

Lateral Pressure of Fresh Concrete: ACI 347 Formulas

For slab forming, only vertical loads matter. For wall and column forming, the lateral (hydrostatic) pressure of fresh concrete governs the wall form and tie design. Fresh concrete behaves as a fluid initially — it exerts full hydrostatic pressure against form faces, equal to 150h psf where h is the height of fluid concrete above the point of consideration.

As the concrete stiffens and begins to set (due to cement hydration), the effective fluid head decreases below the actual pour height. ACI 347.2R provides two design equations:

For columns and for walls poured at rate R ≤ 7 ft/hr, pour height ≤ 14 ft:

For walls poured at R > 7 ft/hr, or pour height > 14 ft, or pour height > 7 ft for columns:

The calculated maximum lateral pressure is capped at Cw × 150h (full hydrostatic) as the upper bound. Minimum Pmax is 600 psf.

Pump pours are faster than hand-placing concrete — a common NYC concrete pump can place at 20–40 cy/hr, equating to pour rates of 3–12 ft/hr depending on the pour area. Engineers must obtain the contractor's intended pour rate before finalizing form designs.

NYC DOB PE Stamp Requirements

NYC Building Code §3301.6 specifies when PE-stamped formwork and shoring drawings are required:

• Clear height of pour or shoring exceeds 6 feet (one story): Virtually all multi-story NYC construction.
• Formwork supports loads over occupied or public spaces: Any pour above an occupied floor, public sidewalk, or occupied streetbed. This covers almost every renovation project in an occupied NYC building.
• Reshoring extends through three or more floors: Common in fast-track NYC construction where multiple floors are under construction simultaneously.
• Any other situation where DOB determines engineering is required.

PE-stamped formwork drawings must be filed with the DOB permit application and be on-site during construction. They must show: shoring layout plan at each level, shore type and size, spacing, base plate size, cross-bracing location and type, and the reshoring load flow analysis.

Shoring and Reshoring Load Flow Analysis

In multi-story construction, the fresh concrete pour on an upper floor is carried by shoring down through lower floors — each of which may still be gaining strength — to the ground or a floor slab with confirmed adequate capacity. A shoring and reshoring analysis is required to confirm that no partially-cured slab is overloaded during construction.

The analysis must track the construction sequence:

1. Determine the age and concrete strength at each floor level at the time each new floor is poured
2. Calculate the load delivered to each shored floor from all above-level pours and construction loads
3. Verify that the concrete at each level has reached sufficient strength (typically 75% of f'c) before it receives construction loads
4. Identify the critical construction stage — the sequence step at which any slab is closest to overstress

Pour Sequence and Construction Loads

The pour sequence — the order in which concrete is placed across a floor plate — creates asymmetric loads on the shoring that can cause lateral instability. Engineers must consider:

• Concrete placement typically proceeds from one end to the other or from the perimeter inward — the weight grows progressively on one side of the shoring field
• The pump line (typically 4-inch or 5-inch steel pipe carrying concrete at 600–900 psi) acts as a concentrated moving load on the form deck
• Vibrator operation creates dynamic impact loads; powered screed equipment (laser screeds, vibratory screeds) transmits additional dynamic loads
• Standard pour sequence restriction: do not pour more than 50% of the floor plate weight on one side of any selected column line without confirming stability of the shoring

Special Inspections for Formwork

NYC BC Chapter 17 §1705.3 requires special inspection of concrete forming and placing. The engineer of record must include formwork inspection requirements in the Statement of Special Inspections (SOSI). Key inspection tasks:

• Pre-pour formwork inspection: The special inspector performs a pre-pour inspection — confirming shores are plumb, positioned per the layout plan, fully engaged (base plates bearing, screw jacks extended within limit), cross-bracing is in place, form sheathing is sound without gaps or damaged panels, and embed items (anchor bolts, conduit sleeves) are secure
• Concrete placement inspection: Placement rate is confirmed to not exceed the rate assumed in the formwork design; pump pressure is monitored; vibration duration and depth per ACI 309 (vibrator overlap, maximum vibrator spacing)
• Cylinder sampling: Concrete test cylinders cured under field conditions are required to confirm concrete has reached the contractual minimum strength before shores are stripped. NYC DOB typically requires 28-day cylinders for all structural concrete; engineers may specify 7-day cylinders to evaluate early stripping viability
• Reshoring verification: Inspector confirms reshores are set at each required level before the level above is poured

Formwork Failure: Causes and Prevention

According to OSHA's construction fatality statistics, formwork collapses are among the most lethal construction incidents. The dominant causes in documented NYC-area incidents:

• No engineering at all: Contractors who self-design formwork without a PE on informal rules of thumb are the single largest cause of catastrophic failures. Construction crews have inherent incentives to minimize shoring (cost and schedule) without an independent engineering judgment to counterbalance.
• Rapid pump pours exceeding design assumptions: A form designed for a 3 ft/hr pour rate that is actually poured at 9 ft/hr develops approximately twice the design lateral pressure — enough to blow out wall form ties.
• Shore base settlement or punch-through: Shores set on unsupported wood mud sills over soft or wet ground, or on slab with a void below (from soil settlement), settle suddenly under load. Base plate sizing must be appropriate for the substrate.
• Premature shoring removal: Removing shores before the concrete cylinder breaks confirm adequate strength — often driven by owner schedule pressure — transfers loads to under-strength slabs.
• No lateral bracing: Slender shore props without adequate cross-bracing buckle under destabilizing lateral effects of the pour sequence, equipment movement, or wind.
• Damaged or undersized shores: Using bent, kinked, or corroded Acrow props that have significantly reduced capacity. Every shore prop should be inspected for damage before use on-site.

Stripping Criteria: When Can Shores Be Removed?

Shoring cannot be stripped until the concrete has achieved sufficient strength to carry the loads that will be placed on it after stripping. ACI 347 provides minimum stripping criteria, but structural engineers typically impose more conservative project-specific requirements:

In cold weather (below 50°F ambient), concrete gains strength significantly more slowly. NYC winter construction requires heated enclosures, insulated blankets, or heated concrete mixes to meet stripping criteria — and the structural engineer must account for the reduced strength gain rate in the stripping schedule.