Bridgewater: The latest impressive addition to Century City

By Holger Bischoff, technical director at WSP Group Africa

This development adds onto the Rabie Property Group’s portfolio of 5-star projects located in this Cape Town precinct.

In mid-2018, Rabie Property Group gave the green light for the proposed multi-use development then known as The Hudson. The professional team, led by Vivid Architects, commenced with the conceptual planning and design in earnest, and the first spade hit the ground in November 2019.

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The project was initially programmed for completion in April 2021, however, the unforeseen appearance of the Covid pandemic threw a significant spanner in the works and as a result, the final completion date was pushed out to August 2021.

The development, now under its final name of Bridgewater, was successfully completed and is currently undergoing fit-out of the hotel component, as well as several of the office tenants and residential purchasers already having taken occupation of their new premises.

Project overview

Bridgewater is a mixed-use development comprising of:

• 80 key hotel. This is operated by Century City Conference Centre and Hotels.
• Residential: twelve luxury penthouse units occupying the top two floors of the building, plus 92 one and two bed apartments, of which 18 are designated as serviced apartments for use by the hotel.
• Three separate office and retail buildings totalling some 8 300m² of lettable area.
• A super basement over two levels, mainly for parking but also including large plant rooms for mechanical, electrical, wet services and fire plant equipment.

The individual buildings comprising the development form a U-shape, the centre of which is an open air recreation area which is beautifully landscaped to offer office workers, hotel guests and residents the opportunity to unwind in a relaxed environment.

The location of Bridgewater, being on the boundary of the former Ratanga Junction Adventure Park, is ideal in that it is in close proximity to the Canal Walk shopping centre, and right next door to the Century City Convention Centre. It also benefits from an extensive infrastructure development which has been undertaken concurrently with the building’s construction, including a new water body which bounds almost directly onto the residential side of the complex.

The HVAC design concept

WSP Group Africa was appointed by Rabie Property Group for the design of the air conditioning and ventilation as well as the Building Management System (BMS) for the new development.

Having successfully completed various projects for this client in the past, WSP was familiar with their vision and expectations for this new project. Nevertheless there were new challenges to be faced in terms of a construction sector which was experiencing a significant increase in construction costs, and thus to design and deliver a satisfactory project within a budget and timeframe which would meet the financial feasibility criteria set out for the development of the project.

A central chilled water system was designed to provide cooling for the hotel, offices and retail component only. This system was considered to be the most cost effective for this application, including annual energy consumption costs, with building loads considered to be fairly predictable and consistent throughout the course of a year. The chilled water piping reticulation was designed to enable individual energy metering, via the BMS, of each of these three components, as well as the hotel.

The residential component (apartments) is not generally air conditioned, apart from the twelve penthouse apartments on the two top levels, which have DX split mid-wall units installed, as well as being mechanically supplied with filtered fresh air. Bathrooms and WC’s in the apartments are mechanically ventilated via common ducted extraction systems.

WSP was requested by the client to make provision for the possible future addition of split air conditioners in the apartments, depending on the requirements of individual purchasers. This was not a simple task, as this includes the provision of mechanical fresh air supply in accordance with the current SANS 10400 regulations.

Figure 1 & 2

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Chilled water plant

The central chilled water generation plant has a total diversified cooling capacity of 1460kWr, with two water cooled Climaveneta screw compressor chillers of equal capacity, each providing some 65% of the design total building load.

This plant is located in a designated plant room in the lower basement level, immediately adjacent to the electrical LV and generator plant room, which significantly reduced the cost of expensive cables feeding the chiller plant.

The chilled water system is configured as a de-coupled system, with two primary chilled water and two secondary chilled water pumps with variable speed drives. The secondary (building) loop is a variable flow system, with all air handling units and fan coil units fitted with two way modulating type control valves.

{os-gal-223} Images by RACA Journal | Benjamin Brits

There are two BAC cooling towers, of the low profile open type, installed on the roof of office A building. Due to the close proximity of these towers to the apartments, the original design specified sound attenuation on both the inlet and discharge of these towers. As a result of subsequent value engineering, and with input from the appointed acoustic consultant, it was decided to omit the discharge sound attenuators. This has since proven to be the correct decision, as the overall ambient noise levels have been found to be well within acceptable limits.

MEP services in corridor ceiling. Image credit: WSP

The large bore chilled water and condenser water piping is routed within the upper basement level to a number of riser shafts, from where it is reticulated vertically through the buildings.

Interestingly, it should be stated that at the initial design stage, a study was conducted jointly between WSP and the electrical consultant to determine the comparative energy consumption costs between open type cooling towers and closed-circuit coolers. This study revealed, that in this case, the annual operating costs for closed circuit coolers were some 47% higher than those for open cooling towers. This, coupled with the much higher capital cost for closed circuit coolers, decided in favour of the latter for use on this project.

The Hotel

The hotel comprises of 80 keys of approximately 20m² in size over five floors and is fitted out to a four-star standard. On level 2 there is a restaurant / lounge with an open view kitchen and reception area, whilst the staff areas, administration office and BOF / stores are located on level 1.

Guest rooms are fully air conditioned by two pipe chilled water fan coil units with electric resistance heating. These units are Intramech, and utilise two way modulating chilled water control valves for capacity control. These fan coil units operate on a stand-alone control basis (not linked to the Building Management System).

A single fan at roof level supplies filtered fresh air (un-conditioned) to each guest room as well as to the corridors, lobbies and linen rooms. Extraction from bathrooms is achieved by means of a single axial type extract fan located at roof level, with individual riser ducts, common to two adjacent rooms on each floor. Fresh air fans and extract fans are monitored by the BMS and operate on a 24/7 basis.

Chilled water piping to the rooms is reticulated vertically from the basement plant room in a single riser shaft, and horizontally on each floor in the corridor ceiling voids.

Most mechanical and electrical / electronic services reticulation had to share the corridor ceiling voids for horizontal reticulation, which required careful co-ordination and planning between these services within the limited space available. Such co-ordination and clash detection was all carried out in Revit 3D model space, by the various consultants and architect.

The restaurant / lounge / reception area for the hotel, located on Level 2, is served from a single chilled water air handling unit, and was designed as a variable air volume (VAV) system. The interior finishes in these spaces are of a high standard, and hence WSP selected linear VAV ceiling diffusers which were finished in matching colours (mainly black) as specified by the interior designer.

The kitchen serving the restaurant is open to view from the restaurant patrons, which again put emphasis on high class finishes and neat layouts. To this extent, the stainless steel extraction canopy had to be powder coated matt black to fit in with the overall colour theme of this space.

Offices and retail buildings

The development has three distinct buildings designated for offices and retail:

• Office block A comprising of 4 floors, total lettable space approximately 5 200m².
• Office block C comprising of 4 floors, approximately 375m² retail space on ground level, and 2 320m² of lettable office space on the remaining three floors.

It was the client’s decision that the MEP services for these should be designed as a ‘grey box’ finish. In terms of the HVAC systems, it was decided to opt for smaller, low to medium static pressure hydronic fan coil units, each capable of covering a floor area of approximately 30m² in terms of delivered cooling capacity. These units were pre-purchased by the client, and as part of the main contract works only the chilled water reticulation and fresh air supply ducts were installed, based on an estimated location layout of the FCU’s. Chilled water pipes terminate in isolating valves, and fresh air branch ducts in butterfly balancing dampers at each unit ‘location’.

Figure 3 & 4

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The design intention is that, once a particular tenancy is let and actual internal furniture and partition layouts become available, the hydronic units are installed in their final positions and connected to the chilled water tails including a two way control valve. Supply air is via an internally insulated plenum box with three or four spigots from which flexible ducts can connect onto constant air volume ceiling diffusers.

Condensate drains are installed to the intended positions of the fan coil units, connected to riser stacks cast into the concrete columns.

Offices B comprises of six offices, each with a ground floor plus a mezzanine level, all open to one common space. The client’s design intent for these offices was to create a bespoke, fully fitted out space, into which tenants could move in with minimal fit-out changes to the MEP services.

The HVAC design for each of these offices comprises of two hydronic fan coil units serving the ground floor, with units and ducting concealed within a corner ceiling bulkhead. The mezzanine floor is served by a single fan coil unit, exposed within the open roof space, and exposed spiral wound ducting with sidewall grilles.

Residential (apartments)

There are a total of 104 apartments as part of the Bridgewater development. Eighteen of these (on levels 1 and 2) are serviced apartments for exclusive use of the hotel. The remaining apartments are intended for private ownership and comprise of one or two bedroom units plus twelve luxury penthouse units on the top two floors.

The apartments (with the exception of the penthouse units) are generally not air conditioned. Two central fan powered systems provide extraction ventilation from all bathrooms, whilst two central fan powered systems supply filtered (unconditioned) fresh air to the apartment corridors and lift lobbies, as well as to the penthouse units and the 18 serviced hotel units.

The penthouse units have air conditioning in the lounge / dining room as well as in the master bedroom. This is by means of Samsung AR range digital inverter mid-wall split (DX) units, with the condensing units installed in close proximity to the indoor units, on the open roof slab.

The architectural intent was for all services in the apartment corridors to be fully exposed to view, which presented a challenge to the MEP consultants in terms of neat and functional co-ordination of such services.

{os-gal-225} Images by RACA Journal | Benjamin Brits

Parking basements

There are two levels of basement parking located below the full footprint of the development. These basements are mechanically ventilated, with each having its dedicated extraction fan, both fans being located in a common plant room on the upper basement.

The design of the layout of this plant room proved to be a considerable challenge due to the extremely tight space constraints (see photo below). Acoustic type exhaust air weather louvres were installed due to the location of the offices directly above this plant room.

Figure 5

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The extract fans have variable speed drives which are linked to CO sensors in each basement, thus allowing the fans to operate at lower volumes (and hence reduced energy consumption) in times of low usage of the parking basements. Both fans have Class H rated motors and double up as smoke extract fans in the event of a fire. The fans start up automatically to run at full speed, on receipt of a double knock signal from the fire detection system.

Fresh air is drawn into each basement level via three large shafts located on the opposite side to the extraction systems, with air drawn in (induced) through large weater louvres on ground floor level.

Building Management System

The BMS is essentially an HVAC-only management system, as it does not have any monitoring or alarming functions with regards to electrical plant, hot water generation, access control or vertical transportation services.

The BMS is based on a state of the art Tridium Niagara Framework platform, which resides on an IP backbone which is installed throughout the buildings. Each main plant room has control panels which house controllers, and the points monitored are attached to these controllers.

The following HVAC plant is controlled and monitored by the BMS:

• Chiller plant: critical chiller functions monitoring and alarms, control of secondary chilled water pumps.
• Basement ventilation: CO monitoring and fan speed control.
• Chilled water energy monitoring.
• Hotel Restaurant AHU metering and VAV control.
• Hotel admin. / BOH AHU monitoring and control.
• Various ventilation fans monitoring, control and alarming.

The guest room and office air conditioning fan coil units are all on stand-alone control (not monitored or controlled by the BMS).

{os-gal-226} Images by RACA Journal | Benjamin Brits

Greenstar interventions

Rabie Property Group stated from the outset their intention to achieve a 5-star Greenstar rating for the Bridgewater development. PJ Carew were appointed as the Greenstar consultants, and from the concept design stage, they issued a list of interventions to be targeted by the various services consultants.

In terms of the HVAC design, such interventions included, inter alia:

• Minimum fresh air quantities for the offices and retail tenancies, as well as hotel guest rooms to be 66% above the SANS10400 requirements.
• Minimising of noise levels in hotel guest rooms.
• Use of high efficiency water cooled chillers.
• Thermal energy metering and sub-metering.
• CO sensors to control parking exhaust fans.
• Use of zero ODP refrigerants in chillers, VRV and split DX units.

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