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DEPARTMENT OF ARCHITECTURE

ABUBAKAR TAFAWA BALEWA UNIVERSITY, BAUCHI

ARC 615: ADVANCED BUILDING STRUCTURES

LESSON 6: SURFACE ACTIVE STRUCTURAL SYSTEMS

6.1 Introduction to Surface Active Structural Systems

6.2 Plate structures

6.2.1 Structural concept of plate structures

6.2.2 Examples of plate structures

6.3 Folded structures

6.3.1 Structural concept of folded structures

6.3.2 Examples of folded structures

6.4 Shell structures

6.4.1 Structural concept of shell structures

6.4.2 Examples of shell structures

6.5 References

6.1 Introduction to Surface Active Structural Systems

These are systems of flexible or rigid planes able to resist tension, compression or shear, in

which the redirection of forces is effected by mobilization of sectional forces. Examples are:

1. Plate structures

2. Folded structures

3. Shell structures

6.2 Plate structures

A plate is a relatively thin rigid (solid) body bounded by two surfaces. It has uniform

thickness and is flat in the unstressed state. These structures sustain loading using bending

stresses. They resist loads by shear and moments. They act like membranes when their

bending stiffness is reduced to zero.

A flat concrete plate is a two-way reinforced concrete framing system utilizing a slab of

uniform thickness, with neither drop panels nor column capitals to resist heavier loads. A

capital is the upper portion of the column, which is usually of conical shape and larger in

cross-section than the remaining portion of the column; a drop panel is a thickened portion

of the slab in the area adjacent to a column.)

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Difference between slabs and plates.

Multi-bay plate structures

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6.2.1 Structural concept of plate structures

A flat plate is a one-way or two-way system usually supported directly on columns or

loadbearing walls. The principal feature of the flat plate floor is a uniform or near-uniform

thickness with a flat soffit.

Advantages of flat plate systems

4. Simple formwork and suitable for direct fix or sprayed ceiling

5. There are no beams which simplifies under-floor services

6. Minimum structural depth and reduced floor-to-floor height.

Disadvantages of flat plate systems

1. They can only be used for medium spans

2. They have limited lateral load capacity (resistance to horizontal thrust) as part of

a moment frame

3. May need shear heads or shear reinforcement at the columns or larger columns

for shear

4. They exhibit long-term deflection

5. They may not be suitable for supporting brittle (masonry) partitions

6. They may not be suitable for heavy loads.

The absence of drop panels and column capitals differentiates flat plate slabs from flat

slabs. The two-way slab is supported directly on columns.

Flat plate slabs do not have drop panels and column capitals.

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6.2.2 Examples of plate structures

Reinforced concrete flat plate system, Eugene, USA. Note the absence of beams, drop panels and column capitals.

Example of flat plate slab structure. Note the absence of beams, drop panels and column capitals.

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Example of conceptual design of flat plate slab structure. Note the absence of beam, drop panels and column capitals.

Example of timber floor slab plates made from Cross Laminated Timber (CLT). Student residence, University of New Columbia.

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Detail of timber floor slab plates made from Cross Laminated Timber (CLT). Student residence, University of New Columbia. Note the use of Cross Laminated Timber floor slab plates, glue

laminated (glulam) columns and steel connectors.

6.3 Folded structures

Folded structures are three-dimensional structures with a folded form of construction,

including structures derived from elements which form a folded structure by their mutual

relationship in space. These elements may be plates or bars.

Folded plate structure

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6.3.1 Structural concept of folded structures

Based on geometric shape folded structures can be divided into:

Folded plate surfaces: all the highest and lowest points of the elements belong to

two parallel planes

Folded plate frames: the elements of each segment of the folds mutually occupy a

frame spatial form. This type of folded structure is spatial organization of two or

more folds in the plane.

Spatial folded plate structures: a spatial constructive set is formed by combining

mutually the elements of a folded structure.

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Classification of folded plates based on geometry.

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6.3.2 Examples of folded structures

Example of folded plate structure. United States Air Force Academy Cadet Chapel, Colorado Spring, Colorado, USA.

Example of folded plate structure. United States Air Force Academy Cadet Chapel, Colorado Spring, Colorado, USA.

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Example of folded plate structure. United States Air Force Academy Cadet Chapel, Colorado Spring, Colorado, USA.

Example of folded plate structure

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Example of folded plate structure

6.4 Shell structures

A shell structure is a thin curved membrane or slab usually of reinforced concrete that

functions both as structure and covering. The term “shell” is used to describe the

structures which possess strength and rigidity due to its thin, natural and curved form such

as shell of egg, a nut, human skull, and shell of tortoise.

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Shell occurring in nature

6.4.1 Structural concept of shell structures

Single curvature shells are curved on one linear axis and are a part of a cylinder or cone in

the form of barrel vaults and conoid shells. Double curvature shells are either part of a

sphere, or a hyperboloid of revolution. The terms single curvature and double curvature do

not provide a precise geometric distinction between the forms of shell because a barrel

vault is single curvature but so is a dome. The terms single and double curvature are used

to distinguish the comparative rigidity of the two forms and complexity of centring

necessary to construct the shell form.

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Single and double curvature shells.

Advantages of shell structures

1. It is a very light form of construction. A shell thickness of 6 cm is sufficient to span

30 m.

2. The lightness of the structure reduces dead load thus economizing foundation and

supporting systems.

3. They take advantage of the fact that arch shapes can span longer distances.

4. They are aesthetically pleasing.

Disadvantages of shell structures

1. Greater accuracy in formwork is required.

2. Skilled labour and good supervision is necessary

3. Labour costs are higher

6.4.2 Examples of shell structures

Sydney Opera House

Shell spans up to 50 metres with shell thickness varying between 7 and 10 cm. The

arches are supported by over 350 km of tensioned steel cable.

Various challenges in finding acceptable construction for the design.

Shells originally designed as a series of parabolas supported by precast concrete

ribs.

The formwork for in-situ concrete would have been prohibitively expensive, and

precasting the ribs would have been more expensive because there was no

repetition in the sections.

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The solution found was to create all arch segments as sections from a sphere. This

allowed arches of varying length to be cast from a common mould.

2400 precast ribs and 4000 roof panels were produced in an onsite factory. The use

of precast units allowed the team to avoid expensive formwork for in-situ concrete.

Cable beams were built and reinforced by steel cables to release the stress of the

weight.

Example of shell structure. Schematic plan and elevation of Sydney Opera House, Sydney, Australia.

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Example of shell structure. Schematic elevation of Sydney Opera House, Sydney, Australia.

Example of shell structure. Schematic section of Sydney Opera House, Sydney, Australia.

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Example of shell structure. Cutaway section of Sydney Opera House, Sydney, Australia.

Example of shell structure. Construction photograph of Sydney Opera House, Sydney, Australia.

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Example of shell structure. Construction photograph of Sydney Opera House, Sydney, Australia.

Example of shell structure. Detail of panel, Sydney Opera House, Sydney, Australia. Source: Photograph by Ogunsote.

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Example of shell structure. Sydney Opera House, Sydney, Australia.

Example of shell structure. Sydney Opera House, Sydney, Australia. Source: Photograph by Ogunsote.

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Pre-stressed concrete-shell

Concrete shell structure: McDonnell planetarium, St Louis, USA.

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Concrete shell structure: McDonnell planetarium, St Louis, USA.

Example of concrete shell structure. TWA Terminal building, JFK Airport, New York, USA by Eero Saarinen.

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Example of concrete shell structure. TWA Terminal building, JFK Airport, New York, USA by Eero Saarinen.

6.5 References

Sekularac, N., Sekularac, J. I. and Tovarovic, J. C. (2012). Folded Structures in Modern

Architecture. Facta Universitatis Series: Architecture and Civil Engineering. 10:1 (1-

16).