Specialty Structural Engineering: Roofs, Anchorage, Connections & Formwork

Roof Structural Engineering

In New York City's dense urban environment, rooftops are active work surfaces — often carrying HVAC units, cooling towers, elevator machinery, water storage tanks, and increasingly, rooftop terraces and green roofs. Each of these elements imposes loads on the existing roof structure that must be evaluated and accommodated by a structural engineer.

Mechanical Dunnage Frames

Dunnage frames are structural steel platforms (commonly W-sections or HSS tubes) that distribute mechanical equipment loads to the building's roof framing. A proper dunnage design documents the equipment weight, center-of-gravity location, and vibration loads; analyzes the underlying roof framing for the resulting forces; and sizes the dunnage beams for bending, shear, and deflection under all load combinations. Vibration isolation pads between equipment and dunnage frame are required to prevent transmission of operational vibration to the building structure.

Green Roofs & Ballasted Assemblies

Green roofs introduce significant additional dead loads — typically 15–25 psf for extensive (shallow) systems and 50–150 psf for intensive (deep substrate) systems — far exceeding the typical 10–15 psf for conventional membrane roofing. Before any green roof installation, a structural engineer must evaluate the existing roof framing capacity and determine whether reinforcing is required. The engineer must also consider the drainage design to prevent ponding loads during storm events, and account for saturated soil weight in the tributary area calculations.

Safety Railing & Guardrail Design

NYC Building Code §1015 and OSHA 29 CFR 1926.502 impose specific load requirements on guardrails and safety railings. Guardrail posts must withstand a 200-pound concentrated load applied in any direction at the top; the entire rail system must resist 50 plf in-fill loading per NYC BC. For open-sided rooftops, balconies, mezzanines, and elevated platforms, the rail post connections to the structural deck are a common failure point. The anchorage design must engage sufficient concrete area or masonry substrate to resist the imposed loads with appropriate safety factors.

Concrete Anchor Design (ACI 318 Chapter 17)

Post-installed concrete anchors — mechanical expansion anchors, undercut anchors, and adhesive (chemical) anchors — are ubiquitous in NYC construction. They are used to attach structural steel to existing concrete; to mount MEP systems and conveyor supports; to install facade elements, railings, and curtain wall anchors; and to connect structural additions to existing framing.

ACI 318-19 Chapter 17 (formerly Appendix D in older editions) provides the governing design procedure. Chapter 17 checks are required for all anchor failure modes:

• Steel strength: Anchor rod fracture in tension or shear
• Concrete breakout: Formation of a concrete cone under tensile load (or a prism for shear)
• Pullout: Anchor pulling through the concrete without cone formation (critical for mechanical anchors)
• Concrete side-face blowout: Near a free edge under eccentric tension
• Pryout: Short anchor rotating under shear load, breaking out a back-side concrete wedge
• Combined tension and shear: Interaction equation check when both loads act simultaneously

ICC-ES Evaluation Service Reports (ESRs) for proprietary anchors provide the test-derived design parameters (f1N, f1V, etc.) required to perform Chapter 17 calculations. The engineer must match the anchor to the ESR, confirm the installation conditions match the report limitations, and verify that the installed anchor is inspected per the Statement of Special Inspections.

Steel Connection Design (AISC)

Steel connections transfer forces between members — beams to columns, bracing to gussets, plates to existing framing. AISC 360 Specification Chapter J governs all steel-to-steel connections. The engineer checks each connection for: bolt shear and bearing (AISC J3); weld throat and length requirements (AISC J2 and AWS D1.1); plate bending and net section capacity (AISC J4); and block shear (AISC J4.3).

In seismic design categories C through F (less common in NYC but relevant for high-rise), AISC 341 Seismic Provisions require pre-qualified or tested moment connections at special moment frame locations. Pre-qualified connections from AISC Prequalified Connections for Special and Intermediate Steel Moment Frames (ANSI/AISC 358) include WUF-W (bolted web, welded flange), BFP, and RBS (Reduced Beam Section) connections.

Wood Connection Design (NDS)

Wood connections in NYC construction occur primarily in timber-frame light commercial buildings, historical renovation of 19th-century wood-frame structures, and mass timber (CLT/glulam) projects. The National Design Specification (NDS) for Wood Construction governs fastener design: nails (NDS Table 12N), lag screws (NDS Table 12.3.1), bolts (NDS Table 12A), and wood screws (NDS Table 12.2). Adjustment factors (Cd load duration, CM wet service, Ct temperature, Cg group action, Cδ geometry, Ceg end grain, Cdi diaphragm) must be applied to tabulated reference design values.

For mass timber connections, structural hardware suppliers provide ICC-ES evaluation reports with specific load tables for their proprietary concealed bracket systems — used to join panels at walls, floors, and ridges while maintaining the visual appeal of exposed timber.

Masonry Anchorage (TMS 402)

Masonry anchorage is required whenever structural or non-structural elements are attached to masonry walls — facade panel supports, shelf angles, sign mounts, MEP brackets, and safety anchors. TMS 402 Building Code Requirements for Masonry Structures governs anchor design, evaluating: bolt tension (governed by the lesser of bolt yield, embedment pull-out, and masonry cone breakout); bolt shear; and combined loading effects. Anchor locations relative to edges, mortar joints, and existing reinforcing critically affect capacity and must be coordinated with field conditions.

Formwork Engineering

Formwork — the temporary molds that give cast-in-place concrete its shape — must be designed to carry the pressure of fresh concrete (which behaves hydrostatically at placement) plus impact loads from the concrete pump, vibrator, and construction workers. The American Concrete Institute ACI 347.2R (Guide for Shoring/Reshoring of Concrete Multistory Buildings) and ACI 347R (Guide to Formwork for Concrete) provide engineering guidance.

In NYC, formwork for concrete placements exceeding 6 ft in formed height or over occupied spaces requires PE-stamped drawings. The typical formwork package includes: shoring layout, shore size and load rating at the required height, lateral bracing calculations, plywood form design, and allowable pour rate.

Sidewalk Shed Engineering

NYC's sidewalk sheds (overhead pedestrian protection structures — OHPP under NYC BC §3307) are required for all construction work on buildings taller than 25 feet. The shed must be designed to carry 300 psf live load on the roof deck and resist all applicable wind loads. DOB filings require a PE-stamped design with structural drawings showing the post foundations, canopy framing, decking, and tie-back to the building facade where applicable. Permit issuance typically takes 2–4 weeks under standard review.