The Dorset House
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Integrated Passive House Technology
1. Introduction
Dorset House is a contemporary hybrid timber and steel frame, detached, 4 bedroom family dwelling located in a rural setting outside Bristol. Its architectural form takes reference from the local vernacular as a contemporary interpretation of a traditional Somerset long house with a simple pitched roof, asymmetrical gable and rhythmical window pattern and low sills to the first floor, whilst its materials and substantial amount of glazing on the ground floor allow a contemporary energy efficient led appearance. A design of its time, together with a single storey flat roofed element that helps to ground the building and relates it to its setting by addressing the south-west facing garden. The external walls are clad in vertical Siberian Larch, blending the house into the surrounding trees and mature vegetation together with grey aluminium clad/ composite timber windows and patio doors. The cantilevered and overhanging upper level appears to be floating above a ground floor glass box providing shade and avoiding overheating, giving the opportunity for an inspirational structural design.
Working closely with the architect we developed a structural system that achieved incredibly slender exposed columns behind the glazing line that seemingly disappear behind the mullions to accentuate the perceived levitation. Furthermore, a double cantilever corner allowed the impressive glass to glass column free projection and the uncomplicated view to the surrounding landscape.
Dorset House is a contemporary hybrid timber and steel frame, detached, 4 bedroom family dwelling located in a rural setting outside Bristol. Its architectural form takes reference from the local vernacular as a contemporary interpretation of a traditional Somerset long house with a simple pitched roof, asymmetrical gable and rhythmical window pattern and low sills to the first floor, whilst its materials and substantial amount of glazing on the ground floor allow a contemporary energy efficient led appearance. A design of its time, together with a single storey flat roofed element that helps to ground the building and relates it to its setting by addressing the south-west facing garden. The external walls are clad in vertical Siberian Larch, blending the house into the surrounding trees and mature vegetation together with grey aluminium clad/ composite timber windows and patio doors. The cantilevered and overhanging upper level appears to be floating above a ground floor glass box providing shade and avoiding overheating, giving the opportunity for an inspirational structural design.
Working closely with the architect we developed a structural system that achieved incredibly slender exposed columns behind the glazing line that seemingly disappear behind the mullions to accentuate the perceived levitation. Furthermore, a double cantilever corner allowed the impressive glass to glass column free projection and the uncomplicated view to the surrounding landscape.
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Figure 1 Architectural images
2. Structural outline
The structural form is a hybrid hot rolled steel and timber frame. The timber frame comprises offsite prefabricated timber stud ‘twinwall’ panels, sawn timber rafters, timber I joist floors, Kerto beams and Glulam beams. A simple vertical steel X braced frame at one end of the building and a series of OSB sheathed timber racking walls at the other provide lateral stability with horizontal forces transfer through a glued Tongue and Groove chipboard floor diaphragm facilitating the simple pin end slender columns. Coupled with the low mass of the timber frame, the columns were too slender for the architect and they were upsized for aesthetic demands.
While the ecological credentials of timber are accepted (although still debateable), there are some aspects of this form of construction that can be missed without a full lifecycle assessment. The steel and timber frame construction is inherently Designed for Disassembly and Recycling DFDR, with even the cladding holding some value at the end of the buildings life. The wood fibre insulation is organic and can be recycled or combusted. Iron although energy intensive to extract from Ore, once refined into steel is infinitely recyclable and the embodied energy locked into the material for future generations. The more problematic materials in the lifecycle analysis, who’s entropy will increase are glazing, concrete and masonry. The end of life use typically forming hardcore or landfill.
The structural form is a hybrid hot rolled steel and timber frame. The timber frame comprises offsite prefabricated timber stud ‘twinwall’ panels, sawn timber rafters, timber I joist floors, Kerto beams and Glulam beams. A simple vertical steel X braced frame at one end of the building and a series of OSB sheathed timber racking walls at the other provide lateral stability with horizontal forces transfer through a glued Tongue and Groove chipboard floor diaphragm facilitating the simple pin end slender columns. Coupled with the low mass of the timber frame, the columns were too slender for the architect and they were upsized for aesthetic demands.
While the ecological credentials of timber are accepted (although still debateable), there are some aspects of this form of construction that can be missed without a full lifecycle assessment. The steel and timber frame construction is inherently Designed for Disassembly and Recycling DFDR, with even the cladding holding some value at the end of the buildings life. The wood fibre insulation is organic and can be recycled or combusted. Iron although energy intensive to extract from Ore, once refined into steel is infinitely recyclable and the embodied energy locked into the material for future generations. The more problematic materials in the lifecycle analysis, who’s entropy will increase are glazing, concrete and masonry. The end of life use typically forming hardcore or landfill.
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Figure 2 Raft bending moments and non linear deformations
The overturning forces from the X braced frame are transferred into a concrete kentledge acting in torsion, this unusual kentledge geometry was driven by the architectural layout.
Wind loads on the gable end are resisted with the classic 'open fronted' stability system. With an engineered diaphragm the minimum number of vertical planes to achieve lateral stability is three, with the building acting in torsion. Therefore with wind 90 degrees acting on the short face of the building the forces are transferring to the rear wall as a shear and the resultant eccentricity resisted as a push pull on the gable end stability walls (X bracing and timber shear wall).
The vertical loads are transferred to the ground via a flat RC raft foundation designed with finite elements and an envelope of subgrade reactions to provide a maximum range of moments and shears. A second order cracked section analysis was then performed with areas of reinforcement input into the model and non linear deflections with creep calculated as this is more efficient than standard span-to-depth ratio checks.
Wind loads on the gable end are resisted with the classic 'open fronted' stability system. With an engineered diaphragm the minimum number of vertical planes to achieve lateral stability is three, with the building acting in torsion. Therefore with wind 90 degrees acting on the short face of the building the forces are transferring to the rear wall as a shear and the resultant eccentricity resisted as a push pull on the gable end stability walls (X bracing and timber shear wall).
The vertical loads are transferred to the ground via a flat RC raft foundation designed with finite elements and an envelope of subgrade reactions to provide a maximum range of moments and shears. A second order cracked section analysis was then performed with areas of reinforcement input into the model and non linear deflections with creep calculated as this is more efficient than standard span-to-depth ratio checks.
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Figure 3 Stability system diagram 0 degrees and 90 degrees
3D finite element analysis (2D plates) was also undertaken on the steelwork to allow eccentric beam-column torsion modelling, stiffener effect and also notching of steelwork for the installation of concealed roller blinds.
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Figure 4 Finite element steelwork (2D plates), 1D member modelling and eccentic column head detail
3. Building Physics
The building itself is simple to the eye, but conceals the cutting edge design principles hidden within that draw on the PassivHaus design principles [5]. The structural design is fully integrating with the architecture and building services and demands that the structural engineer understands the requirements of the thermal principles and ventilation strategy. The PassivHaus principle revolves around 3 simple building physics concepts but extends these to their extreme with careful, conscientious detailing and construction;
The building itself is simple to the eye, but conceals the cutting edge design principles hidden within that draw on the PassivHaus design principles [5]. The structural design is fully integrating with the architecture and building services and demands that the structural engineer understands the requirements of the thermal principles and ventilation strategy. The PassivHaus principle revolves around 3 simple building physics concepts but extends these to their extreme with careful, conscientious detailing and construction;
- Continuous unbroken external ‘jacket’ of high performance insulation
- Continuous and carefully controlled internal air barrier to prevent leakage of internal air
- An air exchange that allows fresh air to be drawn in while blowing out stale inside air and swapping the heat from the two flows, Mechanical Ventilation with Heat Recovery, MVHR.
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Figure 5 Building Physics section and solar PV tiles
The incredibly high specification of external insulation is achieved with exterior wall panels that are a ‘twinwall’ system with separate inner and outer timber stud leaves and Warmcell full fill blown cellulose fibre (newspaper!) insulation. An external layer of woodfibre insulation to the pitched roof and external walls further help towards the thermal stability of the building. These high levels of insulation wrap around the full envelope of the building including walls and roof and into the Isoquick below slab loadbearing insulation to create an uninterrupted jacket minimising heat ingress and escape. The isoquik instulation is precut offsite and simply built like Lego to create a perfect formwork for the slab to be cast on. Complex shapes and inclusions can be made as necessary such as for the kentledge for the stability system.
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Figure 6 Isoquick loadbearing insulation formwork and Warmcell cellulose fibre insulation
Triple glazed windows allow extensive glazing to the ground floor with the overhang of the first-floor cantilever preventing thermal gains in the summer due to high solar incidence. External roller shutters nestle in the overhang shielding the building in summer when solar incidence is fierce. First floor windows are fitted with integrated external blinds preventing overheating the south facing bedrooms. Furthermore, by casting the slab on the loadbearing insulation the reinforced concrete ground floor raft has the dense material inside the building thermal envelope, so acts as both a heat sink and a heat store (thermal mass) to even out peak heating and cooling demand in the winter and summer respectively.
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Figure 7 Architectural first floor overhang detail and groundwork detail
The airtight barrier on the inside of the building envelope prevents air from escaping and also entering the building. Air within a building contains both heat and moisture and with such a high level of airtightness careful control of this air must be maintained to prevent condensation and ensure occupant comfort. This is achieved with a simple unit called Mechanical Ventilation with Heat Recovery, MVHR, whereby fresh air is drawn in from the outside and, with a heat exchanger, the internal air is exhausted past the fresh air without mixing the flows but transferring the heat. This carefully controlled air is then ducted to and from each room via ducts within the floor depth requiring no dropped ceiling service zone. The timber frame I joists can have large cut-outs formed in the OSB webs, along with Vierendeel analysis of the steel web openings. As the structural engineer with an understanding of the building physics we were able to work with the MVHR designers to redirect duct runs to positions that were achievable with the structural layout.
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Figure 8 Ductwork isometric view and principle of MVHR unit
Due to the current health crisis gripping the World, it is anticipated that this form of ventilation will gain traction as further understanding of the important relationship that internal air quality has with aerosolised viral loads and the hygiene risk this can present to occupants. This is a well understood mechanism within medicine and super spreader events tend to cluster around dense occupancy in enclosed, poorly ventilated environments. [1] [2] [3] [4]
The building also utilises discrete Photovoltaic roof tiles that blend into the roof, the excess electricity produced charges a Tesla Powerwall battery bank. Balanced over the year electricity created and outsourced to the grid is greater than the input required on cloudy winter days taking also into account the electricity for a Tesla car, which is substantially more than the electricity required by the house.
The building also utilises discrete Photovoltaic roof tiles that blend into the roof, the excess electricity produced charges a Tesla Powerwall battery bank. Balanced over the year electricity created and outsourced to the grid is greater than the input required on cloudy winter days taking also into account the electricity for a Tesla car, which is substantially more than the electricity required by the house.
4. Contract and construction
The timber frame contract was awarded as a design and build package within a ‘construction management’ procurement route undertaken by the client. The D&B package is common with this form of construction due the specialist nature of the offsite panel fabrication. As a consulting structural engineer we were able to add value by keeping control of the structural design for the client to achieve the architectural needs by providing continuous design support through the RIBA Stage 2 Concept, up to Stage 3 Spatial Coordination, where we were novated to the contractor side to complete the Stage 4 Technical Design. The decision hierarchy for designing with timber requires the engineer to attempt to use timber where possible and not rely on steelwork, this hierarchy does however create additional detailing by the engineer due to the complexity of interfaces between the mixed materials. With practice and determination inventive timber frame details and a personal vocabulary develops.
The timber frame contract was awarded as a design and build package within a ‘construction management’ procurement route undertaken by the client. The D&B package is common with this form of construction due the specialist nature of the offsite panel fabrication. As a consulting structural engineer we were able to add value by keeping control of the structural design for the client to achieve the architectural needs by providing continuous design support through the RIBA Stage 2 Concept, up to Stage 3 Spatial Coordination, where we were novated to the contractor side to complete the Stage 4 Technical Design. The decision hierarchy for designing with timber requires the engineer to attempt to use timber where possible and not rely on steelwork, this hierarchy does however create additional detailing by the engineer due to the complexity of interfaces between the mixed materials. With practice and determination inventive timber frame details and a personal vocabulary develops.
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Figure 9 Steelwork and timber fabrication isometric images
The fabrication detailing of the timber frame panels is a specialist skill and requires intimate knowledge of interfacing the insulation and airtightness between panels. The structural engineers design was prepared in Revit and transferred to the timber framers to continue their detailing. The high level of investment in design time pays off with the timber frame being erecting in just 4 weeks with a mobile crane positioning the panels. The benefit in such a short time onsite are transferred to the client though economy on the contractors’ overheads and preliminaries and therefore bottom line for the client.
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Figure 10 Timber frame in construction
5. Technical details
Architect – PAD Design LTD - www.pad-design.com/
Timber Frame – All Timber Frames
MVHR – SystemAir
Build Cost - £450k
Floor Area – 220m2
Steel mass – 5,900kg
Concrete volume – 42m3
Reinforcement mass – 3500kg
Reinforcement quantity/concrete volume = 83 kg/m3
Architect – PAD Design LTD - www.pad-design.com/
Timber Frame – All Timber Frames
MVHR – SystemAir
Build Cost - £450k
Floor Area – 220m2
Steel mass – 5,900kg
Concrete volume – 42m3
Reinforcement mass – 3500kg
Reinforcement quantity/concrete volume = 83 kg/m3
6. References
[1] https://pubmed.ncbi.nlm.nih.gov/463858/
[2] https://www.hindawi.com/journals/av/2014/859090/
[3] https://www.ribaj.com/intelligence/recirculation-of-air-in-buildings-passivhaus-fresh-air-heat-recovery-beats-covid-19-justin-bere
[4] https://www.erinbromage.com/post/the-risks-know-them-avoid-them?fbclid=IwAR0IH8IExoEIY42g5AFPeDorhNXtZjMBnOF6T9jJt-sR47TUghKT9_4GHhw
[5] https://www.passivhaustrust.org.uk/
[1] https://pubmed.ncbi.nlm.nih.gov/463858/
[2] https://www.hindawi.com/journals/av/2014/859090/
[3] https://www.ribaj.com/intelligence/recirculation-of-air-in-buildings-passivhaus-fresh-air-heat-recovery-beats-covid-19-justin-bere
[4] https://www.erinbromage.com/post/the-risks-know-them-avoid-them?fbclid=IwAR0IH8IExoEIY42g5AFPeDorhNXtZjMBnOF6T9jJt-sR47TUghKT9_4GHhw
[5] https://www.passivhaustrust.org.uk/