Innovative, stylish and ‘green’ Lakeside offices for Exxaro

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By Ilana Koegelenberg and Brandon Huddle from Aurecon
The development team’s efforts surpassed its targets to achieve a 5-Star Green Star SA Design certification which will be confirmed with an ‘As-Built’ rating application once the building is fully functioning.

DPS

Growthpoint Properties has completed its R600-million development of Exxaro’s leading edge new head office in Centurion, opposite the Centurion Gautrain Station.

Growthpoint’s highly skilled development and workspace design team created an innovative green building, integrating P-grade aesthetics and finishes and high-performance workspace.

This development consists of a new nine-storey building, with four parking and five office floors above. The first two levels of basement parking are naturally ventilated while the bottom two parking levels are mechanically ventilated. The office buildings have open plan spaces along with some enclosed spaces surrounding an atrium.

Exxaro, one of the largest and foremost empowered South African-based diversified resources companies, commenced its long lease over all the new building’s 18 500m2 of gross lettable area on 1 May 2019 after six weeks of early occupation. Over a three-month period, Exxaro will relocate all its operations to the new building, which can accommodate up to 1 000 staff members.

Exxaro’s new corporate headquarters consolidate its current offices in Pretoria and Johannesburg into a single thriving workspace, at a convenient midpoint in a superbly accessible location.

The two-year development of Exxaro’s headquarters broke ground in February 2017 to become the second phase of Growthpoint’s redevelopment of the prime Lakeside site on West Street, Centurion.

A major challenge in the building’s construction was the site’s dolomitic bedrock. Over 100 boreholes were drilled to create a three-dimensional model of the bedrock. High-lying dolomite pinnacles were blasted and drilled down, upper layers of soil were compacted and a steel-reinforced concrete ‘raft’ foundation, complete with sensor monitoring, was laid.

As a prominent feature of the development, organic dolomite was chosen to inspire several building design elements. Its forms are subtly expressed in acoustic wall panels, bollards and other thoughtful features. The building’s innovative safe foundations are brought to life with a special glass covered manhole, which provides a view underground to a dolomitic pinnacle beneath the site’s parking area periphery.

Exxaro’s new energy-efficient building will foster a healthy, energising work environment and high productivity levels in a cost-effective setting. Green measures include a performance glazed façade, zoned lighting, energy-efficient building services and systems, water-efficient fixtures, rainwater harvesting, water-wise landscaping, low VOC interior finishes, abundant fresh air and natural light, as well as a hydro panel that generates water from air. It has dedicated parking for more fuel-efficient vehicles, such as hybrids, electric cars and scooters, as well as secure bicycle racks.

While the building is owned by Growthpoint, it has been tailored to Exxaro’s specific requirements. The building creates a variety of experiences throughout, all custom designed to support Exxaro’s business goals, ethos, values and vision.

Client brief

The client had the following requirements in terms of the design criteria:

Outside design conditions

Internal design conditions offices

Building thermal envelope

Energy

Commissioning

Basements

Offices

Control system

HVAC system overview

The selected HVAC system for this building consists of a ceiling-concealed chilled water fan coil unit system with a centrally, treated CO2-controlled fresh air system.

Chilled water is generated by three air-cooled chillers located on the roof. Each has 33% of the total design capacity.
Chilled water Air Handling Units (AHU) provide treated and filtered fresh air to the building. The air is either cooled or heated to supply the air to the fan coil units at 20°C. Floor CO2 monitors are provided to reduce the fresh air supply when possible.

Ceiling concealed fan coil units (FCUs) provide cooling and heating to each thermal zone. The number of fan coil units depends on the number of thermal zones. Generally, a FCU is provided per 40-70m² of office space. An FCU can serve multiple offices in a thermal zone and the temperature controller will be located in a representative office.

The FCUs are connected to an insulated chilled water reticulation system. Fresh air is ducted and supplied into the ceiling void. Supply to the space is via flexible connections and constant volume ceiling diffusers.

Return air is via a ceiling mounted hinged aluminium return air grille with a washable filter. A plenum box on top of the grille is connected to the fan coil unit with flexible connections.

HVAC system detail

There are three identical chillers located on the roof. These machines have scroll compressors and operate with refrigerant R410A. The machines are each equipped with integral vertical in-line primary water circulating pumps. A supply and return water header for both chilled and hot water is located adjacent to the chiller set. The primary pumps will circulate water through these headers.

The chillers have integral capacity control which will ensure that each supply header will have the design temperature, i.e., 6°C chilled water.

The chillers have a secondary chilled water pump set, comprising a run and standby pump. The secondary pumps draw water from the supply headers to distribute water through the air handling equipment and then returning to the return water header.

The secondary pumps are of the end suction vertical discharge type, long-coupled with motor and pump on a common base plate. The pumps have stainless steel shafts, bronze impellers and cast-iron volutes. Chilled water pumps have stainless steel drip trays with galvanised drain pipes to the nearest drain points.

The air handling equipment has control valves, which throttle to meet cooling demand. The cumulative flow requirement for the secondary pumps thus fluctuates with overall demand.

To compensate, the pumps are fitted with VSDs. The speed is controlled by the pressure differential between supply and return water.

Since the primary pumps and secondary pumps will seldom be delivering the same water flows, a de-coupler is provided between the supply and return of each header set, to by-pass excess water in either direction.

Constant pressure is provided to the system by means of pressurisation pump sets on chilled water storage vessels. Make-up water is supplied to the chilled water system via galvanised piping from a make-up tank, and the expansion of the chilled water is taken up by means of hydrosphere tanks located on the roof.

A dosing pot is also provided in the system for insertion of the chemicals required for the water treatment of the system.

A motor control panel is provided in the plantroom with the necessary switchgear and controls and high and low voltage wiring for the chillers, secondary pumps, booster pump, air handling units and fans. The plant is operated according to adjustable time schedules provided by the BMS.

There are four air handling units (AHUs) located on the roof that provide conditioned fresh air (at 20°C) for all office areas. Galvanised pipes from the AHU drip trays to full-bore drain outlets provide for condensate waste.

Conditioned air is conveyed from each AHU to the respective spaces via insulated sheet metal galvanised ducting and distributed to the space. Return air is ducted back to each unit via galvanised sheet metal ducting generally from ceiling return air grilles.

Fire dampers are installed where ducts penetrate fire walls. These are of the fusible link type. The links will melt and the fire dampers close when temperatures exceed 74°C.

FCUs are used in localised office areas. The units have relatively short supply duct runs and supply air to the space via ceiling diffusers or side-wall grilles. Return air is via ceiling return air grilles with flexible ducts connected to the unit inlets.

Each FCU serving individual meeting rooms and spaces has its own wall controller for temperature set-point or on/off or time clock setting. FCUs serving open plan areas have wall stats and are controlled by the BMS. Galvanised pipe condensate drains are run to the nearest plumber’s drain point.

Occasional specialist or remote localised areas are equipped with DX split units. These are totally independent of the central system providing the user with full autonomy over temperature set points and on/off or time clock control for 24/7. Typical applications are for server rooms, or individual offices.

The canteen is air conditioned by FCUs. In addition, extract canopies were required for cooking areas. The public toilets are mechanically ventilated by means of axial flow fans and the air is conveyed via galvanised sheet metal. The air is captured via ceiling mounted disk valves and is discharged to the atmosphere by means of a weather louvre.

The parking basements are ventilated with a combination of natural and mechanical ventilation. The mechanical ventilation is achieved with smoke rated axial extract fans controlled with variable speed drives. The speed is determined by CO sensors which speed up the fans as CO levels increase. The fans also double as smoke extract fans, as during a fire condition the fans will ramp up to full speed.

Cross-ventilation of the basements is supplemented by means of impulse fans.

In the event of a fire condition on any of the office levels, the roof mounted smoke extract fans will be activated to exhaust air from four vertical full height masonry shafts. On each level a motorised damper is installed between the shaft and the ceiling void. These will automatically open the level that receives a fire signal.

Ceiling-mounted smoke extract grilles, strategically placed, allow air to be exhausted from the entire office area to the extract shaft.

In the event of a fire condition on any of the office levels, the four pressurisation fans will be activated to pressurise the three fire escape stairwells, and lift shaft via VSDs to maintain 45Pa. Transfer grilles located every second level, introduce air into the stair wells.

Control

The HVAC system is controlled by a PC-based direct digital control system in terms of its building management system (BMS).
The client required that this system be able to control the chiller operation, pumps, fans, fan coil units and air handling units on the roof and also the basement ventilation fans.

In addition, the client required that the BMS can be controlled remotely to switch the fan coil units on or off, adjust the set point, and indicate the room temperature.

Challenges

The WELL-rating was a challenge, in that the open ends of each duct entering the site had to be covered and protected from dust ingress for purposes of the WELL-rating. Those not covered had to be vacuum cleaned inside to remove dust.

Congested ceiling services made co-ordination a very intensive process, including design, shop drawing and construction stages. These were overcome with detailed, co-ordinated Revit models at design and shop drawing stages. Construction stage went well as a result of the above, but required much skilled labour to get the job done on time due to the fast-track construction programme for this type of system.

Environmentally sustainable strategies

Computer modelling has been used to estimate the energy usage of the building in line with the Green Star SA Office Energy Calculator and Modelling Protocol Guide. The results of the analysis were then used to determine the potential emissions of the proposed office building once operational.

Based on the Green Star Office v1.1 energy calculator, the annual base building benchmark for the Lakeside Offices Building is 191.5kgCO2/m²/year. The expected annual emissions value for the Lakeside Offices Building is 101.3kgCO2/m²/year. This results in a 47% reduction in base building annual CO2 emission.

Other sustainable aspects incorporated in the design, include:

Prof list

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