Newsletter 18 - October 2018

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Garden House with Gianni Botsford Architects, our newsletter focuses fully on structural design aspects of this recently completed house in a garden: 

Timber Roof Structure
The timber roof frame is an orthogonal double curvature beam grillage. Support to the roof surface is provided by the end concrete walls, and also by a single intermediate thermally broken stainless steel post that is incorporated within each long glazed edge of the pavilion. The lower edge of the timber roof is framed out with a 300x300x90 L-profile glulam section to provide a constant fixing perimeter to the radial timber ribs.

The double curvature surface directs the spanning forces to perimeter framing elements. Forces are predominantly in compression and tension, though the pronounced offset and asymmetric arrangement of the roof-light means that the beam grillage surface has also to additionally deal with forces in a flexural mode – notably on the wide end slope where predicted flexural deflections were highest, and also the corner ribs which exhibited critical bending and shear.In order to achieve economy in the roof build, fixity of elements became a key driver; following discussions with Zublin the original high fixity model was re-appraised and many elements were moment-released, the upshot of this was the minimum depth ribs increased from 180 to 200mm deep (11% increase), maintaining the 60mm width. This resulted in an efficient 37% increase of major axis bending stiffness in the radial ribs.  The reduction of joint forces when the joints were modelled as notional pins rather than with full moment fixity allowed Zublin to simplify their jointing methodology and hence deliver the roof within acceptable budget constraints.
The ribs and lateral rails were first glued and clamped in a single axis curvature with layers as thin as 4mm, these first pieces were then re-sawn and glued to the final 3D curvature; though full use was made of the digital model to create the clamping jigs, this remained an intensively manual process, the individual ribs were brought to their final smooth faced 200mm depth using a hand held planer thicknesser that was passed along the near-fit 3D glue-ups.

The radial ribs of the whole roof frame were trial assembled before the butt jointed lateral ribs were cut and manually fitted between the radials in an impressive effort of patience and skill. Connection details were agreed to Zublin timbers preferred design, employing skew drilled timber framing screws where possible and also incorporating specially fabricated steel bracketry to carry the higher loads most frequent at the lower ends of the radial ribs.

Excavation and Basement Structural Shell
The garden boundary walls were repaired and underpinned prior to installation of a contiguous piled retaining wall forming the new basement perimeter wall. Internal sacrificial piles were also installed to aid the excavation sequence.
The basement excavation progressed in a top down sequence with the RC slabs being cast onto the ground and connected to the perimeter piles and internal sacrificial piles. As the clay from each floor level was excavated, a suspended slab remained above and the next layer down could be cast and excavated in a similar fashion.

The lowest level slab, at a formation close to 10m below ground level, was supported on piles and incorporated a void former to provide an overburden heave release zone, necessary at this depth.
Permanent reinforced concrete external perimeter liner walls were then formed by specialist concrete spraying sub-contractors.
To support the irregular floor slab geometries internal steel columns were installed in the lowest basement level supporting structural concrete walls in the higher basement level; once these had been installed the internal sacrificial piles were cut away.
Free-standing cantilevered RC walls at each end of the ground floor pavilion were also constructed by spraying into open faced timber formwork.

Timber Stair
The solid Douglas fir stair cantilevers from internally threaded dowels resin fixed flush to the marble clad RC stair wall.
The stair is fixed to these via long loose hex drive bolts which are housed within a resin fixed sleeve set within the stair riser.
Sections of tread riser and balustrade sub-assemblies were made off site prior to site assembly. The tread-riser sections, and the balustrade, incorporate flexible carpentry detailing to accommodate the anticipated seasonal movement.  

Steel Stairs
The 6mm folded profile treads and 8mm plate balustrade were made off site in manageable sub-assemblies that could be
connected together on site. Intense scrutiny was given to the visible bolted joints between the stringer and balustrade sections to achieve visual coordination with the wall connection detail.


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Steve Atkinson                                                 

Structural Design for Contemporary Architecture                                               

Block K, 175 Bermondsey St                 Tel 020 7403 7711    

London SE1 3UW                                                                            


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