Brynmawr Gateway Arch

Brynmawr Gateway Arch

Large white Brynmawr Gateway Arch bridge span being lifted into place by a crane between two support structures in across valleys in the Clydach Gorge
Large white Brynmawr Gateway Arch bridge span being lifted into place by a crane between two support structures in across valleys in the Clydach Gorge

Client: Welsh Government
Main contractor: Costain
Architect: Knight Architects
Engineering a 120m landmark arch bridge over a Welsh dual carriageway The Brynmawr Gateway Arch (Jack Williams Gateway Bridge) is a 120m span steel arch bridge carrying a concrete deck on steel girders over the A465 at the Heads of the Valleys, linking the town centre to the school across the Clydach Gorge.

Wentworth delivered full construction phase assessments, including lifting platforms, rock slope stability, temporary trestle foundations, deck concreting, arch lifts, hanger installation, and tower unloading, to enable the sensitive integration of this key transportation link into the picturesque surrounding landscape.
50m
High steel rib arch rising above the Clydach valley floor
118m
Span composite deck crossing the A465 dual carriageway
7
Temporary trestle positions requiring geotechnical foundation assessment

Our Role

Our Role
Wentworth delivered comprehensive temporary works and construction engineering consultancy, covering the entire construction sequence.

Our scope included:


  • geotechnical assessment of lifting and assembly platforms with rock slope stability analysis
  • foundation verification for temporary trestle towers
  • 3D finite element models of permanent and temporary works, simulating all construction stages
  • independent checking of temporary steelwork, including towers, bracing, bearings, transfer beams and jacking components
  • a rigorous technical review and certification for Costain, covering interfaces between the permanent works designer and steelwork contractor.

Capabilities

Capabilities

  • Bridges & Highways
  • Geotechnics
  • Structural Engineering
  • Construction Method Engineering
  • Temporary Works
  • 3D Modelling & Detailing
  • Specialist Lifting & Jacking

Geotechnical assessment

Geotechnical assessment
We needed to establish stable platforms in the steep Clydach valley conditions to support the temporary trestle towers carrying the deck and the arch. We assessed the rock slope stability while designing tower foundations capable of supporting loads through successive construction stages.

Wentworth’s geotechnical team analysed the slope stability and verified the design of tower foundations tailored to the specific ground conditions and construction loads at each location. We developed platforms capable of supporting the changing loads through the multiple stages of construction: deck assembly and erection, concreting operations, and subsequent arch installation.

This independent verification gave Costain confidence to erect the towers and progress construction, knowing the platforms would perform reliably under all loading scenarios.

Coordinating complex temporary works

Coordinating complex temporary works
The bridge design team involved multiple designers working on different elements: permanent works for the arch and deck, steelwork contractors for erection systems, and temporary works specialists for the support towers. Coordinating these separate designs across a multi-span construction sequence—where the same trestles had to support deck construction then carry the arch during installation—created critical interface challenges.

Wentworth acted as independent checker for all temporary steelwork, reviewing designs and supervising construction methodology development. We identified critical modifications to trestle arrangements (e.g. temporary tower bracing configuration and capacity), bearing details, and jacking sequences that eliminated clashes and controlled construction risks. This independent validation gave Costain confidence to proceed through the complex construction sequence knowing all interfaces were resolved.
Large yellow crane with steel supports lifting the Brynmawy Gateway Arch into place on the Jack Williams Gateway Bridge which is under construction at night, with blue illiminated temporary support towers
Large yellow crane with steel supports lifting the Brynmawy Gateway Arch into place on the Jack Williams Gateway Bridge which is under construction at night, with blue illiminated temporary support towers

3D modelling of structural interactions

3D modelling of structural interactions
The deck steelwork was first assembled on ground platforms and then lifted onto temporary trestle towers, followed by casting the concrete slab. Then the arch sections were lifted onto high-level trestle towers and welded in-situ, before jacking and suspending the completed deck from cables, and removing all towers. Each step produced different load paths and behaviour, requiring a precise understanding of the interaction between permanent girders, temporary towers, and the arch.

We developed 3D finite element models combining permanent deck and arch elements with all temporary substructures, simulating every construction stage and environmental effect at the exposed Clydach Valley location. This validated sequence gave the steelwork erectors and Costain confidence in loads, deflections, and structural performance throughout construction.

Outcomes

Outcomes
By combining our knowledge of temporary works, geotechnics and staged analysis with detailed structural modelling, we validated the structural response during every critical stage.

Key results:


  • Construction confidence: Rigorous technical review and certification for a complex four-segment arch lift to provide safety and structural design reassurance
  • Interface coordination: Our reviewer role ensured permanent works designers, temporary works designers and steelwork contractors were properly coordinated across all construction phases
  • Foundation certainty: Geotechnical assessments validated rock slopes, crane platforms, and seven trestle tower foundations for heavy lifting
  • Staged sequence validation: Separate analysis models for each major phase validated the transition from four-span temporary deck through arch installation to final cable-hung configuration
  • Safety improvements: Identified modifications enhanced stability and controlled risks throughout the multi-phase project.