Christian Revival Church (CRC) on Witkoppen road in Douglasdale Johannesburg features structural steel in the main auditorium roof, the spiral ramp and the feature ring structure supporting the main staircase.
The structural framing
The lower and upper gallery seating accommodates 4500 congregants and is enclosed with a curved structural steel roof consisting of triangular radiused roof trusses spaced 10m apart over an area of 80m x 68m. The seven unique structural steel triangular space frame trusses (also referred to as ‘Toblerone trusses’) are manufactured from circular hollow sections with varying spans up to 50 m. These radiused trusses support the purlins, roof sheeting, insulation, air-conditioning ducts, a service walkway, and audio-visual systems.
The auditorium roof structure was analysed using the Prokon frame analysis program with loading modelled in 20 different load combinations. The Toblerone trusses which span across the entire main auditorium building, are supported by reinforced concrete columns with corbels at each end of the trusses as well as by an intermediate reinforced concrete ring beam. The trusses are constructed from a variety of circular hollow sections with two top chords and one bottom chord connected with diagonal members along the span. The top chords, which under most load combinations are in compression, are braced together with horizontal circular members. The trusses are roughly 1.9 m deep and 2.4 m wide in plan and are curved to provide a curved roof profile and suitable slope for rainwater disposal.
Each of the seven space trusses is unique as these have intermediate support from a ring beam which is curved in plan and which forms the rear perimeter of the auditorium. The intermediate support point of each of the trusses consequently occurs at a different point along its length resulting in changes to the arrangement of each truss.
The size of the steel trusses makes thermal expansion and contraction a significant consideration which was dealt with by providing movement joints and flexible connections thereby allowing the trusses to move freely when expansion and contraction occurs. These movement joints, which consist of steel plates with PTFE sheets between, have been provided at the steel to concrete connections on the corbels for each bottom chord and at the ring beams above for the top chords on each end of each truss.
Due to the length of the steel trusses, each truss was constructed into 3 or 4 units to facilitate transport to site. Each unit was welded together in the workshop and provided with connection plates at each of its ends so that these could be bolted together during erection on site. The steel connections consisted of steel end plates and stiffener plates welded to the ends of the two top chords, one bottom chord and two diagonal chords. The plates were provided with bolt holes of varying number and diameter bolts determined by the maximum load being transmitted through each connection.
The spiral ramp is a significant architectural and structural feature in the entrance hall of the church. The ramp starts in the lower parking basement and provides safe and comfortable vertical access to all floors for all users including those in wheelchairs.
One central reinforced concrete column in the center of the spiral ramp supports a circular reinforced concrete beam and slab system at the roof of the spiral ramp. This slab and beam system is braced by perimeter columns and beams which also support the glass facade. 55mm Diameter steel hangars with adjustable couplers are hung from the slab and beam system and support the inner steel stringer channels. The outer steel stringer channels are connected to the inner stringers with I-beams and are braced at various points to the columns surrounding the ramp. A reinforced concrete slab forms the floor of the ramp and is supported between the two stringers and intermediate I-beams for the length of the spiral ramp.
The steel ring structure was added to overcome an aesthetic challenge encountered during the initial design of the concrete staircase. In order to achieve the required aesthetic outcome, the design of the staircase was modified by removing the reinforced concrete columns which initially extended from the first landing to the second landing and replacing these with a circular hollow section steel ring structure.
The steel ring structure is supported at its base on either side of a reinforced concrete column beneath the landing of the first flight. This is the base of the circular structure and from here it circumvents the first flight’s staircase landing and meets the base of the second staircase at the apex of the circle. The steel structure is fixed to the base of the second flight staircase landing where it provides support to the staircase. The steel structure is connected at either side of the landing of the first flight for stability.
The steel ring solved an aesthetic challenge and provides a unique structural and architectural feature in the building and introduces a fun element on the side where the children’s church is located.
From the Project Manager’s Perspective
The first issue we had from an erection point of view was that the tower crane was in the wrong position and hence caused issues when erecting the final 3 bays.
- Transporting of the sections to site was also a logistical challenge to the amount of trips required.
- The setting out and drilling of the M30 anchors on the concrete ring was also a challenge which was overcome by the use of templates and coring the holes.
- Working out and erecting a support scaffold for the 3rd and 4th truss bay was a challenge and we overcame that by allowing for scaffold jacks and timber batons on the top of the scaffold so we could fine tune the support structure.
- The installation of the last section of each truss was difficult due to the tight 50mm (design) tolerance from the back of the truss to the RC upstand this was often down to 15mm.
- Doing tandem lifts on the last 3 trusses with a tower crane and a mobile crane and having to walking though the truss that was partially suspended to connect the splice connection. This often took a few hours as the trusses where picked up with a 4 sling rigging system and caused deflection of the truss sections when suspended. The use of mechanical lifting and pulling equipment assisted greatly in ensuring every splice had no gaps.
Setting out of the levels of the eyelets on the hanging rods was a challenge due to access and when installing a section below a section that as set we found that rotation was happening and changing the levels.
- Installation of the spiral stringer sections was difficult due to access and don’t being able to use a mobile crane for the first ring due to head clearance from the soffit above.
We overcame this by having cast in sleeves positioned in the slab above to allow for lever hoists to do the lifting and manipulating of the ramp sections.
- The access scaffolding had to be adjusted and lowered for every section to be installed.
- Once all sections where installed we noticed significant creep was happening on the ramp which in turn caused the handing robs to be out of plumb. This was overcome by shortening and lengthening the ramp sections and cutting off and rewelding the fin plates that connected the ramp sections to the eyelets on the hangers.
- Finishings to create an aesthetically pleasing structure took a significant amount of finesse with the grinder and body putty work.
Construction Director’s Perspective
The CRC Church developments roof structure looks simple at first glance until one realises that it is made up of curved triangular “Toblerone” main support structures with varying spans brought about by an intermediate, curved in plan, concrete support beam which heads the back wall of the main auditorium.
The seven number Toblerone trusses span an opening of 68 meters and are made from circular tubular sections of varying diameters and wall thicknesses to match the varying spans and in order to make them all to the same curved shape they were fabricated incrementally in a layout jig approximately 30m in length in the workshop.
These trusses were further complicated by walkways which are in the lower ‘V’ of the truss mounted on welded on brackets on the bottom chord at approximately 920mm.
The Toblerone trusses were fabricated in transportable lengths taking into account their shape and curvature which are bolted together using stiffened pipe flanges of varying thicknesses.
Erection of the trusses was fairly straightforward using the site tower crane and mobile cranes where the tower crane was unable to reach with the only real problem being the close tolerance between the perimeter ring beams which required some interesting manoeuvring of the suspended sections to coax them onto their holding down bolts.
A second more interesting structure from a constructability point of view is the spiral ramp at the car park entrance to the church. This ramp starts at basement level and services three levels through the structure.
The spiral structure is suspended from the concrete roof by suspension bars on its inner stringer with the outer stringer bolted to perimeter concrete and steel columns in unsymmetrical locations.
The ramp structure was fabricated in a specially designed jig framework in our workshops which allowed for the various unsymmetrical support locations to be accommodated.
Due to the inner stringer of the ramp being supported on suspension bars the structure had to be erected from the top down which presented some really difficult challenges in terms of headroom to get the top sections in and the entire structure wanting to rotate as more sections and weight were added.
Cast in sleeves in the roof slab with tirfor cables passing through were utilised to install the initial sections and the entire structure had to be installed first and then aligned from top to bottom to get the rotation out such that all the suspension bars were vertical in the final setup.
A third interesting feature on the Project is the “Feature Ring” supporting the mid landing of the concrete access stair at the one end of the entrance lobby. Although not particularly challenging from a fabrication and installation point of view it does provide a unique structural solution for supporting the stairways second landing which is aesthetically pleasing and provides an interesting talking point for the congregants.
Project motivation editorials are provided by the project nominator. If any technical details, company names or product names are incorrect, please notify the SAISC so that the error can be corrected.
|Completion date of steelwork||November 2018|
|Completion date of full project||February 2019|
|Tons of structural steel used||200 tons|
|Structural profiles used||Hollow sections, hot rolled sections, CFLC|
|SA content – if this is an export project||100% SA Content|
|Cladding profile/ type used||0.53mm ISQ550 Novotexi 440 profile sheeting|
|Cladding area/ coverage and tonnage||8520 m² (47 tons)|
|PROJECT TEAM ROLE||COMPANY|
|Client/ Developer||Christian Revival Church|
|Architect||DBM Architects JHB (Pty)Ltd.|
|Structural Engineer||Fortem Consulting Engineers (Pty)Ltd.|
|Civil Engineer||Tekciv Consulting Engineers
|Quantity Surveyor||Mellet Quantity Surveyors|
|Project Manager||Deo Gloria Developments (Pty) Ltd|
|Electrical Engineer||NALA Consulting Engineers(Pty)Ltd|
|Mechanical Engineer||PV3 Engineers|
|Main Contractor||Mike Buyskes Construction (Pty)Ltd|
If you were a part of this project, and your company details are incorrect or missing – please notify the SAISC so that the error can be corrected.