Structural Strategy

sTRUCTURAL sTRATEGY Integrated design in architecture III / Parisa Kanabar / 42220004 / Unit 1B

Site, Soil and Location Considerations

Loads Imposed on the Building Loads are generalised into two main types – dead loads, which are constant, and live loads, which vary.

1)

2)

3)

The structure of the building itself as well as fitted furniture act as dead loads and are highly unlikely to change in the liftime of the building. Occupants, wind, and snow act as live loads and will change throughout. For example, the community centre could be used for a small coffee morning but could also be used for a large celebration, which would have a much larger occupancy that the smaller event. Wind load needs to be considered when lightweight structures are used and for large amounts of glazing. Snow load will become increasingly apparent in Scotland, hence the need for a sloped roof to allow for fast removal.

Types of Foundation 1) Pad Foundations Pad foundations sustain concentrated loads from a single point and suit buildings with skeletal structures such as steel frames. Concrete pads are usually spaced 600mm apart and located at corners and under more heavy loads. Steel posts, brick or reinforced concrete is then built on top of each pad. This type of foundation is does not require large amounts of excavation - an advantage for an urban site such as this.

Site Access and Supporting Infrastructure

2) Pile Foundations Pile foundations are concrete columns drilled into bedrock and aided by frictional resistance from the surrounding ground, securely anchoring the building. This type of foundation is suitable for sites composed of made ground or where the bearing capacity of the soil is low, both characteristics of the Cowgate site. The piles are placed under main walls and at corners and intersections. They are topped with caps onto which ground beams are then placed to support the main walls.

3) Raft Foundations Raft foundations consist of a large continuous reinforced concrete slab extending under the whole building. They are suitable for sites with a history of mining and sites with groundwater or clay as they distribute the load over the entire footprint, reducing the load per unit area. These foundations can be less effective when structural loads are concentrated in specific areas.

Geotechnical Considerations Cowgate is comprised of a Sandstone Ballagan Formation and superficial deposits of glacial till. The soil texture is loam clay and is medium – strong, so whilst the bedrock has a high bearing capacity, the soil is unstable. Furthermore clay is susceptible to shrinkage particularly in the presence of vegetation which is a key component of my building, and during heavy rainfall, which is predicted in Scotland in the future, so deep foundations are required, particularly since the scheme reaches heights of 5-6 storeys in some areas.

The site is essentially an island so is well connected to the rest of the city via Victoria Street and Cowgate. The former is one of the more prominent and picturesque streets in Edinburgh so it is important that building works do not affect this side significantly.

The central square is ideal for use as material and machinery storage and can be accessed via Cowgate. The scheme is situated in an urban area so it is important that the structure chosen lends itself to a construction process that causes minimal disruption.

Strata

Sandstone Sandy siltstone

Mudstone Siltstone

ground level

6.40m 9.60m 10.40m 11.00m 11.50m

Site History The maps below show the history of the built environment of the site. It is clear that vacant spaces on the site were once built on. During demolition of these buildings, the ground was most likely excavated and filled making it necessary to dig lower than the area of fill. Existing buildings are also being removed which will require excavation and filling, leading to a ground condition of made ground, which requires pile foundations to securely anchor the building.

1890

1900

1910

1930

1950

Present

Strategy and categorisation Thermally Massive Materials As mentioned in my environmental strategy, incorporating thermal mass into my structure is important in maintaining thermal comfort and creating a sustainable design. A thermally massive material must be internally exposed for it to be effective.

Types of Retaining Wall A retaining wall is a structure that holds soil behind it and is required to make up the subterranean levels of my building. There are several types of retaining wall, some of which are outined below: 1

2

3

4

For this reason, a timber or steel structure is not appropriate as neither of these materials are thermally massive. Concrete and masonry structures are both thermally massive so are potential options. Since a large majority of the building is subterranean, several retaining walls will be needed so a concrete frame is the logical choice of structure for the scheme.

Wall render masonry cavity insulation masonry plaster Floor oak flooring screed sound insulation concrete floor slab plaster Basement Wall Porous boards waterproof plinth render in-situ concrete insulation masonry plaster Basement Floor oak flooring screed insulation DPM concrete ground slab lean concrete

1) Cantilevered retaining wall cantilevers loads to a structural footing, converting horizontal pressures to vertical pressures on the ground below.

2) Gravity wall relies on mass to resist pressure from behind.

3) Piled Wall made by assembling a series of piles and the excavating soil.

4) Anchored Wall made in any of the above styles and then anchored to the soil or rock behind.

1 20mm 125mm 20mm 120mm 125mm 15mm

Wall cladding ventilated cavity insulation fair-faced concrete

wall tie

Floor oak flooring

15mm 60mm 20mm

200mm

15mm

5

screed 60mm sound insulation 20mm concrete floor 200mm slab

200mm 15mm

Basement Wall pre-cast plinth insulation waterproofing in-situ concrete wall

concrete edging 60mm 10mm 180mm 60mm 120mm 10mm

2 20mm 30mm 120mm

waterproof plinth render

15mm 60mm 80mm

DPM

lean concrete

Basement Floor oak flooring screed insulation DPM concrete ground slab lean concrete

100mm 80mm 2mm

6

concrete plinth element

220mm

15mm 60mm 80mm

200mm 50mm

200mm 50mm

Potential Frame Construction (both warm structures) 1) Double Leaf Masonry Wall Using contextual materiality is an option I want to explore. Materiality around the site is predominantly sandstone/limestone thanks to the wide availability in Edinburgh. A masonry wall structure is unsuitable for a subterranean building so a hybrid concrete/masonry structure can be used with a masonry structure making up the overground levels of the building and a concrete frame for the underground levels. Masonry construction provides good wind, fire and water protection, is durable, and energy efficient.

2) Concrete Structure with external insulation A concrete structure is the most logical solution for my project as retaining walls are required for the subterranean level. Concrete comes in many different textures and colours and a materiality could be chosen that makes a subtle link to the surrounding context without being made of the same material. Formwork could also be used to add interest. Concrete is durable and economical due to its thermally massive properties and creates a healthy environment for the occupant as it does not sustain rot or mould. It is also sustainable not only because of its long lifespan but also because it can be made up of waste and industrial byproducts, or used concrete. Furthmore, concrete provides good acoustic protection, key in a space such as this that houses a range of activities.

Contiguous Piled Wall during construction Types of Piled Retaining Wall The ground conditions of the site require pile foundations as not only is the soil fairly unstable but the ground is most likely built on and has been excavated and filled. The piles need to be deep enough to reach the sandstone, 6.40m below ground level in order to securely anchor the building.

5) Contiguous Piled Wall uses adjacent combination piles to form a retaining wall, creating a watertight barrier. This technique is useful for areas with limited working space.

6) Steel Sheet Pile Retaining Wall uses interlocking steel sheet piles to retain earth. They are more cost effective and quicker to install than other retaining walls.

Layout

Load Bearing Walls

5600mm

Pile Foundation Grid - Continuous Flight Auger Pile Foundations Pile foundations are the best option for a scheme such as this as the ground is unstable and has most likely been excavated and filled. This method has been chosen due to its low level of vibration, therefore providing minimal disruption in an urban location. A hollow stemmed auger is drilled into the ground to a suitable depth. Once the desired depth has been reached, concrete is poured through the hollow stem of the auger whilst it is slowly extracted. Pile diameters range from 450mm-1200mm depending on the load they are supporting. This method of installation is quick and can be designed to work in a confined space. Piles are capped and then a ground beam placed on top to support each loadbearing pre-cast concrete wall.

Non-Loadbearing Walls

primary Structure model

Primary Structure pre-cast concrete frame Secondary Structure steel frame curtain wall

Concrete Frame, Beams and Loadbearing Walls A concrete frame makes up the primary structure of the scheme. The frame is constructed from precast concrete to enable certain formworks and finishes and to speed up the building process. This also reduces the labor on site which is important in an urban scheme to reduce disruption.

Pre-cast concrete floor slabs are used with horizonal reinforced concrete beams placed every 5600mm to support large concrete floor spans. This structure was chosen for its thermally massive properties and its structural properties that enable it to form retaining walls. Formwork and finishes will be used to provide a design that fits its context.

Concrete frame and beams

Strategy and categorisation Continued Curtain Wall Construction The front portion of the building forms a light well that allows light to penetrate deeper into the subterranean levels of the building. This section is essentially a glass box and a curtain wall system is required. A curtain wall is non-structural and does not carry any dead load weight other than its own. A steel frame will need to be constructed to support the glazing from wind load, and also to support the roof.

curtain wall

sectional model showing curtain wall

fixed-base steel column with threaded bars cast in before hand

Iterative Steel Column Layout

The system will consist of glazing fixed to aluminium mullions which are then fixed back to a structural steel frame, which is anchored to the primary structure. The priority for many curtain wall systems is to be discreet but since I am designing a centre for wellbeing and health, a visible structure will be employed to subtly convery transparency and openess. The structure should however still be delicate and light, so thin steel columns and beams will be used to form a lightweight frame. The columns will be fixed into the concrete loadbearing wall.

glazing and aluminium fixing bracket detail visual representation of system

Curtain Wall system model

Curtain Wall System Steel columns

480 x 240 at 2500 centres PPC coated aluminium 125mm x 60 mm at mullions 2500 centres Y-shaped steel lateral 90mm x 230mm x support plates, site welded 12 mm to steel columns to transfer wind load back to steel columns.

PPC aluminium transoms 110mmx 60 mm Double-glazed sealed units 8mm toughened glass outer pane, 12mm air gap, 10.8mm lamintated glass inner pane Black structural silicone horizontal joints.

Roof Frame Steel beams cantilevering off steel columns to form roof

356mm x 171mm at 2500 centres

Three steel beams spanning 203mm x 133mm between primary beams above curtain wall

Curtain Wall System The curtain wall system consists of glazing attached to aluminium mullions. This lightweight, nonstructural element is attached to a secondary steel structure attached to the primary concrete structure in order to anchor the curtain wall to the building. The structure is deliberately kept visible to convey transparency and openess, also creating interesting shadows across the site.

References British Geological Society Approved Document A Apps iGeology MySoil Books Bizley, G. (2010). Architecture in Detail II. 1st ed. London: Routledge.

Bowles, J. (1968). Foundation Analysis and Design. 1st ed. New York: McGraw-Hill Book Company.

Deplazes, A. (2013). Constructing Architecture: Materials, Processes, Structures. 1st ed. Basel: BirkhaĚˆuser.

McLean, W., Silver, P. and Evans, P. (2014). Structural Engineering for Architects.. 1st ed. London: Laurence King.

Sobek, W., Schittich, C. and Staib, G. (2012). Glass Construction Manual. 1st ed. Basel: De Gruyter.

Lectures Blackman, T. (2017). Structural Strategy Blackman, T. (2017). Structural Modelling Personal communication with John Ramsay

Websites Aboutcivil.org. (2011). Retaining Wall - Definition, Types and Uses of Retaining Walls. [online] Available at: http://www.aboutcivil.org/retaining-wall-definition-types-uses-retaining-walls.html [Accessed 28 Mar. 2017].

Collins, M. (2012). Contiguous Bored Piles/Secant Piles. [online] Concrete.org.uk. Available at: http://www.concrete.org.uk/fingertips-nuggets.asp?cmd=display&id=351 [Accessed 28 Mar. 2017].

Design Build Network. (2017). Scottish Parliament Building, Edinburgh, Scotland. [online] Available at: http://www.designbuild-network.com/projects/spb/ [Accessed 26 Mar. 2017].

Designingbuildings.co.uk. (2017). Bored piles - Designing Buildings Wiki. [online] Available at: https://www.designingbuildings.co.uk/wiki/Bored_piles [Accessed 26 Mar. 2017].

Gobrick.com. (2017). Why Choose Brick. [online] Available at: http://www.gobrick.com/resources/why-choose-brick [Accessed 28 Mar. 2017].

Holmes, M. (2016). Foundations for Difficult Sites | Homebuilding & Renovating. [online] Homebuilding & Renovating. Available at: https://www.homebuilding.co.uk/foundations-for-difficult-sites/ [Accessed 26 Mar. 2017].

King, I. (2013). Permanent Retaining Walls. [online] Ivorking.co.uk. Available at: http://ivorking.co.uk/services-view/permanent-retaining-walls/ [Accessed 28 Mar. 2017].

Rock, I. (2017). Foundations for Difficult Sites | Homebuilding & Renovating. [online] Homebuilding & Renovating. Available at: https://www.homebuilding.co.uk/foundations-for-difficult-sites/ [Accessed 26 Mar. 2017].