The “Jesus Dome” roof structure at the Durban Christian Centre was a well-known feature roof very visible on the sweep of the N2 coming into Durban. The auditorium burnt to the ground in a devastating fire in 2016 leaving the local congregation, some 3500 strong, without a place of worship. The rebuilt was commissioned in 2017 and the Team was gathered.
The Client wished to retain the memory of the original dome into the future, unfortunately, a shape which belies the modern understanding for acoustic performance. To produce a world-class auditorium the Architects, EPA, threw the memory of the dome up into the air and a tri-bifurcated arch was conceived. Simple, smooth extruded box shapes rising over a sheeted, duo-pitched and acoustically attenuated free-spanning roof structure. On plan confined onto a hexagonal concrete ring beam squeezed to focus on the stage.
There seemed little purpose, other than feature, for the arches if they were not to be used to hang the roof below. So two possible schemes were followed in parallel. One with no arches – the roof structure truss grillage made deep enough to free span, and a second with the arches used to hang the primary apex girder so economizing on the truss grillage. This gave cost comparisons to the Client in respect of the spend to achieve the arches over a more conventional structure.
Taking it to the Ground
Steel arches need pouncing points – for our structure, these were some 15m off the ground. Off shutter concrete prisms seemed to fulfill the need in every way. The primary arch buttresses needed to be slimmed to allow access on a restricted footprint so this arch was tied – the primary girder running under the primary arch forms the arch tie, interconnected in a bold move through large welded steel elbows set into the 2.5m wide reinforced concrete buttresses.
The secondary arch buttresses were free to sweep to ground along the steel arch thrust lines, or thereabouts.
For the balance of the roof structure, a chunky concrete ring beam sits atop 7m high concrete columns and tied into the double story structures at front and back of house.
Making the Geometry Work
The arches are, as a trio, wedged from a single sphere, with the origin deep below the floor slab. Each arch is the rind of a perfect slim slice through the origin of the sphere and as such, in radial section, is very slightly trapezoidal – this variation was so small that the shape could easily be rationalized to rectangular allowing the designers and fabricators to select UC sections radiussed about the major axis. The Hulabond cladding box adopted the final trapezoidal form. Round tubes were used for the lacing in combination with web plates.
With the concept realized and agreed with the Architect, the structural design team got to work developing a parametric algorithm in the visual programming environment of Dynamo, which works seamlessly with Revit. This was then used to solve the geometry of all of the arches by iterating through the parameters of the sphere, and the arch truss centerlines imported into the global OASYS analyses and REVIT models and ultimately into TEKLA.
Getting the Design Done
The internal truss grillage is a simple 6×6 grid, allowing the designers some latitude to offer up future-proofing for spot loads at the intersections, as well as providing a sensible patchwork for the arrangement of acoustic lining, services reticulation and suspended acoustic blades above the 3500 seated auditorium array.
Simple IPE purlins set up a bottom flange shelf frame for the acoustic lining and a platform for the Klip-Tite sheeting system. A clamped bracket was devised for all the in-roof services so that post erection drilling or screwing could be prohibited globally.
A series of global analysis models were set up, augmented by a suite of simpler two-dimensional model checks. The primary software used was the OASYS GSA Building Suite program allowing the import and export of geometry into and out of both Revit and Tekla.
Global buckling considerations paid attention to the arch arrangement, and overall stability, as well as serviceability for pre-cambers, and interim construction step models incorporating temporary support towers to protect the geometry during erection.
All the arches end with exactly the same “cheese wedge”, terminating on articulating pot bearings designed to resist pull-off (when the wind blows).
Getting to the Fabrication
Negotiating with a chosen fabricator, Impact Engineering was hotly debated at the outset. For such an iconic structure it is our view that this was a primary factor in the success of the project. Agreeing on fabrication methodology and understanding fabrication economies with the fabricators during the design development is a process most designers would wish for. We applaud our Clients and fellow Professionals for engaging in this way.
NJV decided from the outset to partner with a selected detailing house, Strutech 3D Modelling. The final negotiation TEKLA model became the fabrication model in a fluid process which eliminated in our estimate 4 to 6 weeks of critical path time.
Impact Engineering fabricated the arches as a whole, one in each of their fabrication sheds. Once complete they were carved up for blasting and painting. The arch crossover units were then preassembled for fit in the sheds before transport to site.
Innovating for Erection
Steffanuti Stocks have been exemplary in their attention to setting out of the large HD bolt sets ( note from Author: in 30 years of practicing as a structural engineer both here and abroad the use of survey and delivery of accuracy by both Steffanuti Stock and Impact Engineering was at a level of excellence I have not witnessed before).
Program necessitated that the arch pieces be pre-clad with the Hulabond panels almost entirely prior to lifting, this required coordination and care between Façade Solutions and Impact with only a small window of time after the fact to access the arches utilizing cherry pickers for snags and joint sealing.
An erection plan well considered reduced the temporary tower suite to two, one under each hanger intersection. To say that site access was tight would be an under-exaggeration, and every truss piece lift brought the site action to a halt and a gathering audience. Each piece was maneuvered into position, lifted and slipped into place – no double lifting, no cladding damaged and no site work required. After each lift pre-set survey points were checked and structural behavior deflections matched with expectations of the analytical OASYS models.
The eight tubular hangers lay green at the fabrication works until the primary girder was de-propped and then each was site measured and purpose made to fit. Once the arches were de-propped and the roof was hanging the trusses could be completed below the arch formation and the cladding installed.
The arches will be accessed for cleaning by rope access cleat methods devised under consultation, the cleats having been installed prior to the arch cladding.
|Completion date of steelwork||March 2019|
|Completion date of full project||October 2019|
|Tons of structural steel used||280|
|Structural profiles used||Hot rolled sections, nominal tubular|
|Completion date of cladding||March 2019|
|Cladding profile/ type used||Klip-Tite 700 0,53mm CIS|
|Cladding area/ coverage and tonnage||3300m2|
|Nominator||NJV Consulting (Pty)Ltd|
|Client/ Developer||Durban Christian Centre|
|Architect||Elphick Proome Architects Incorporated|
|Structural Engineer||NJV Consulting (Pty)Ltd|
|Quantity Surveyor (Lead)||RLB Pentad Quantity Surveyors|
|Project Manager||M3 Africa Consulting|
|Main Contractor||Steffanuti Stocks Building KZN|
|Steelwork Contractor||Impact Engineering (Pty)Ltd|
|Steel Erector||CMGC Projects|
|Cladding Manufacturer||Global Roofing Solutions|
|Cladding Contractor||Impact Engineering (Pty)Ltd|
|Fabrication detailing||Strutech 3D Modelling|
|Bearing Supplier||Nova Engineering Works (Pty)Ltd|
|Access Consultant||Façade Solutions|
|Arch Cladding Sub Contractor||Scope Of Work|