Jun 9, 2020

Walking tall

By Alex Carter, Graham Barker and Rob van Zyl

The Birmingham marketplace is on the curve of a change to make its skyline a success. Tall buildings play a key role in cities, acting as way finders, attracting tourists and encouraging an increase in surrounding development. However, they are also complex, risky and visible to design and construct.

In light of this, Cundall has co-authored the guidance document ‘Viability Analysis – Tall Residential Development in Birmingham’ alongside Core Five, Glenn Howells Architects, Knight Frank and Turley.

Below are some of the key engineering considerations when designing a successful tall building in Birmingham.

Structural solutions

The size, proportions and external pressures on tall buildings dictate a significant structural response, which for a residential tenure is almost universally a reinforced concrete frame. Such structures are costly and, built traditionally, slow to construct; their optimisation is central to the viability of tall buildings in the Birmingham market. From a cost perspective, shear walls are significantly more costly than columns, remove flexibility from the end user, slow floor cycles and add weight. Beyond certain tipping points in respect of height, however, they become a necessity. Our research indicates that tall buildings in Birmingham should be structurally led, with architecture reacting to structural layout constraints. Where architectural layouts dictate structural design, this can lead to a sub-optimal frame solution.

Services strategy

MEP services and lifts can account for circa 20-30% of total construction costs. The design solution for heating and cooling the building, mitigating solar gain and providing ventilation where facades are potentially sealed in a noisy city centre environment with air quality issues, is pivotal to a tall building being viable in Birmingham. The major issue typically encountered is thermal comfort, though this can be overcome by introducing purge ventilation to sealed facades. Several developments are now adopting a decentralised all-electric strategy for heating, hot water and kitchen hobs, which improves net: gross efficiency, while offering some cooling to prevent overheating. The cost of electrical distribution also increases with height.

Central plant systems for buildings over 60m high (circa 20 storeys) increase in cost due to pressure breaks through hydraulic separation and a general increase in the pressure rating and relief of pipework. Typically, separation will be at around 60m but could be as high as 100m, though this may not be cost effective. Additional cost is encountered through increased electrical distribution and enhanced protection via sprinklers and wet risers.

Vertical transportation strategy

The benefit of giving early consideration to a tall building’s vertical transportation strategy is twofold; finding the optimum balance between the number of lifts and lift speeds will minimise capital costs whilst offering acceptable waiting times to the building populace, and will have a significant bearing on the attainability of the net: gross ratio of 80%; this should be targeted on a typical floorplate, although this becomes more difficult as a building gets taller.

Whilst lift traffic analysis can be used to determine the optimum number of lifts relative to waiting times, this needs to be balanced with sales values. Lifting strategy (and what is deemed to be an acceptable waiting time) needs to be considered as part of an initial brief due to the significant impact on core sizes and therefore net internal area (NIA): gross internal area (GIA).

Net zero carbon

Birmingham City Council has declared its aim to make the city carbon neutral by 2030. The West Midlands Combined Authority has set a target to have zero carbon emissions by 2041, with interim targets of a 36% reduction by 2022 and 69% by 2027. Any tall buildings being constructed within the region need to consider these targets from the outset of design.

The specification of the façade fabric, materials and HVAC systems will all have a significant impact on the energy demand of a building. However, orientation, form factor and glazing ratio are even more fundamental. A building’s glazing ratio is key to minimising energy demand, reducing winter heating while avoiding summertime overheating and balancing daylighting. The optimum glazing ratios for Birmingham are up to 25% glazed on the southern elevation, no more than 20% on the east and west elevations and as little as possible on the northern elevation. All buildings should be designed to achieve a heating demand of no more than 15 kWh/m2/year, yet this target can currently only be achieved (with triple glazing with U-value of 1.0 W/m2.K) on south-facing apartments.

Image 1 CC

The embodied carbon of the building will have to be reduced by around 40% to less than 500kgCO2/m2.  Timber-frame buildings are limited in height and composite buildings with steel frames and cross-laminated-timber floors and walls might be necessary. Locally sourced low carbon finishes and equipment will be required.

Join the Cundall team for the ‘Walking Tall’ webinar on Wednesday 10 June. Rob van Zyl will present the key considerations when designing building services and vertical transportation. Alex Carter will discuss the complexities of structural design on tall buildings. Click here for more information and to sign up.

To read the full guidance document ‘Viability Analysis – Tall Residential Development in Birmingham’ head over to our website.

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Alex Carter, Graham Barker, Rob van Zyl

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