Temporary Works Engineering: Shoring, Scaffolding & Cofferdams Explained

What Are Temporary Works?

In construction, temporary works are engineered structures, systems, or facilities that are needed during the construction process but do not form part of the permanent building. They are temporary by definition — installed for a specific construction phase and removed when no longer needed — but their failure can be catastrophic and permanent.

Construction industry statistics consistently show that temporary works failures account for a significant proportion of construction fatalities and serious injuries. In New York City, where construction occurs on dense urban sites with adjacent occupied buildings, underground utilities, and significant pedestrian traffic, the stakes are even higher. This is why NYC DOB regulates many categories of temporary works with mandatory PE design, filing, and Special Inspection requirements.

Types of Temporary Works

Shoring & Reshoring

Shoring is the temporary support of freshly poured concrete slabs or beams until the concrete achieves sufficient design strength. Shores (vertical props) carry the dead weight of wet concrete plus construction live loads (workers, equipment, material storage). Reshoring — the re-installation of shores at lower floors — is required when upper-floor concrete construction imposes loads that lower floors cannot yet carry independently. Shoring and reshoring require floor-by-floor load analysis.

Formwork

Formwork is the mold into which concrete is placed. It must resist the fluid pressure of freshly poured concrete (which behaves essentially as a hydrostatic pressure), plus impact and vibration loads from concrete placement equipment. Formwork failures are among the most common and deadly construction accidents. Engineered formwork designs specify sheathing, joists, stringers, shores, and lateral bracing — all sized for concrete placement rates, temperature, and concrete mix design.

Excavation Support

In NYC, virtually every below-grade excavation adjacent to an existing structure requires engineered excavation support. This includes sheet pile walls, soldier pile and lagging walls, secant pile walls, tie-back (anchor) systems, and braced excavations. The engineer must address both global stability (will the excavation wall tip over or slide?) and local element capacity, as well as the impact of dewatering on adjacent building foundations.

Underpinning

When a new building's excavation must go deeper than an existing adjacent foundation, underpinning transfers the existing foundation load to a deeper, stable stratum. Traditional pit underpinning, mini-pile underpinning, and jet grouting are common NYC methods. Underpinning sequences are critical — remove too much soil at once and the existing building moves.

Supported Scaffolding

Scaffolding that bears its loads directly to the ground through frames, tubes, or system scaffold requires engineering at significant heights. In NYC, supported scaffold over 40 feet high requires PE-stamped drawings filed with DOB. Engineers design the scaffold for self-weight, worker loads (25 psf), wind (per ASCE 7 using temporary structure wind speed factors), and ties to the building facade.

Sidewalk Bridges (Overhead Protection)

Sidewalk bridges protect pedestrians from falling objects during construction or facade repair. Standard sidewalk bridges use pre-approved DOB designs. However, when construction loads must be transferred through the bridge (materials staged on the bridge, heavy equipment rolling across it), or when the bridge is exceptionally long, PE-stamped custom drawings are required.

Cofferdams

Cofferdams are temporary enclosures built in water bodies to allow construction of bridge foundations, piers, or waterfront structures in a dry environment. They must resist hydrostatic pressure, earth pressure, and wave/current loading. Sheet pile cofferdams are most common; design involves soil-structure interaction analysis and watertight connection detailing.

Demolition Support

Controlled demolition of structural elements requires temporary shoring to prevent progressive collapse of the remaining structure. Bridge demolition, for example, requires a carefully engineered sequence of load relief and dismantlement that maintains the bridge's structural integrity at every stage of removal.

Why Engineering Design Is Critical

Temporary works are often treated as a contractor's responsibility and receive less engineering attention than permanent structures. This is a dangerous misconception. Temporary works loads can exceed permanent design loads — wet concrete is heavy (150 pcf), and construction live loads often far exceed the building's eventual occupancy loads. A residential building designed for 40 psf live load may see 100+ psf during concrete slab construction.

Additionally, temporary works are operated by personnel who may be focused on maintaining construction schedule rather than strict safety protocols. Engineered drawings, clearly communicated to the site team, establish the contractor's minimum installation standards and provide a clear record for inspection and enforcement.

NYC DOB Requirements

The NYC Building Code (Chapter 33) and OSHA 29 CFR 1926 jointly govern construction safety in New York City. For structural temporary works, DOB requires:

The Design Process

A typical temporary works engagement proceeds through these steps:

1. Scope Review: The engineer reviews the project's permanent structural drawings, contractor's proposed methods, and site conditions (soil test reports, adjacent building documents, utility locations).
2. Load Analysis: All applicable loads are determined — wet concrete weight, construction live loads (per ACI 347 and ASCE 37), wind, seismic (typically lower for temporary structures due to short exposure period), and any dynamic effects from equipment.
3. System Design: Member sizes, spacing, and connection details are determined through structural analysis. For excavation support, soil-structure interaction is modeled using pressure diagrams and deflection criteria to protect adjacent buildings.
4. Drawing Production: PE-stamped erection drawings, typical sections, and connection details are produced. These drawings become the installer's legal obligation to follow.
5. DOB Filing: Drawings are filed with DOB as part of the construction permit application. Special Inspection programs are established.
6. Construction Monitoring: The engineer reviews installation during site observations, responds to RFIs from the contractor, and may provide written monitoring reports, especially for movements of adjacent structures during excavation.

Responsibility & Liability

Liability for temporary works in NYC is distributed across parties per their defined scope of responsibility:

• Structural Engineer of Record (SER): Responsible for design correctness — the drawings must conform to applicable codes and adequately address the intended loads. The SER is not responsible for contractor deviations from the design that are not observed and reported.
• Contractor: Responsible for installing temporary works strictly per the PE drawings, maintaining them in safe condition throughout the construction period, and removing them in the specified sequence.
• Special Inspector: Responsible for verifying that installation conforms to approved drawings at specified inspection stages. Must promptly report non-conformances to the SER and DOB.
• Building Owner / Developer: Responsible for ensuring required permits and inspections are obtained before work begins and that all parties fulfill their contractual obligations.

Common Failures & How Engineers Prevent Them

The most frequent temporary works failures engineers encounter in forensic investigations include:

• Formwork collapse: Often caused by premature stripping before concrete gains adequate strength, overloading during concrete placement, or lateral instability from inadequate bracing. Prevention: minimum stripping time per concrete mix design, phased placement sequences.
• Scaffold overload: Materials stored on scaffolding exceed the design live load. Prevention: clear load signage, daily inspector checks, engineered deck loading drawings.
• Excavation over-dig: Contractor excavates below the permit limits, undermining adjacent foundations. Prevention: monitoring prism arrays with survey readings taken before, during, and after each excavation lift.
• Shore removal out of sequence: Reshoring removed floor by floor without load redistribution analysis. Prevention: sequenced removal plan specified in drawings, confirmed by engineer before each phase.