THE NATIONAL RIBAT UNIVERSITY

Eng. Mazin Elsayed A. Mustafa

Teaching Assistant

Faculty of Architecture

“THE SHELL STRUCURE SYSTEMS”

CONTENTS :

1. Introduction

2. The action of External Forces on a structure

3. The Shell System

4. Shell System Types

5. Shell System Materials

6. Ex. Construction of R.C.C Barrel Vault

7. Advantages & Disadvantages

• The word shell is an old one and is commonly used to describe the hard covering of eggs,

Crustacea (يات ,(قشر tortoises, etc.

• Derived from the word scalus, as in fish scale.

• The development of masonry domes and vaults in the Middle Ages made possible the

construction of more spacious buildings. In more recent times the availability of reinforced

concrete has stimulated interest in the use of shells for roofing purposes.

Introduction:

1. A compression / compressive force will cause a member of structure to be squashed.

2. A tension / tensile force cause a member of structure to be stretched.

3. A Bending force is a complex force state associated with the bowing of a member such as

beam. it causes fiber on one face of the member to elongate and hence be in tension and

fibers on the opposite face to compress

4. A torsion / torsional force cause a member to be twisted.

5. A shear force cause a member to slide/tilt to a side.

The action of External Forces on a structure:

THE SHELL SYSTEM

Definition:

• They are light weight constructions using shell elements.

• Shell structures achieve the most pure synthesis of architectural and structural forms. Also known as

‘surface structures’, shells resist and transfer loads within their minimal thicknesses. They rely upon

their three-dimensional curved geometry and correct orientation and placement of supports for

their adequate structural performance.

Continuity & Curvature: (االستمرارية واالنحناء)

• The essential ingredients of a shell structure in all of the foregoing

examples are continuity and curvature.

• shells are structurally continuous in the sense that they can transmit

forces in a number of different directions in the surface of the shell,

as required.

• Shell structures have quite a different mode of action from skeletal

structures, of which simple examples are trusses, frameworks, and

trees because other structures are only capable of transmitting

forces along their discrete structural.

Fig. The lattice shell of the SageGate shed by Sir Norman Foster

Closed and Open Shells:

• Anyone who has built children’s toys from thick paper or thin cardboard will be familiar with the

fact that a closed box is rigid, whereas an open box is easily deformable (سهل التشوه) . The same sort

of thing applies in shell structure:

1. Close surfaces are rigid.

2. In practice it is not usually possible to make

completely closed structural boxes.

Fig. Shell by ARTechnic Architects. “Being in sync with nature isn't about yielding

to nature – it's about coexistence (التعايش مع البيئة) . The existence of the structure

depends on its power to endure nature.”

• For shell with openings, reinforce the edge of the hole in such a way as to compensate, to a certain

extent, for the presence of the hole.

• The amount of reinforcement required depends on the size of the hole, and to what extent the

presence of the whole makes the structure an open one.

• Large openings are essential in some forms of construction, like in shell roof.

• A main objective in the design of shell roofs is to eliminate those aspects of behavior that spring from

the open nature of the shell.

Fig. Architecture by Santiago Calatrava. “The building's unique architecture comes alive as the lids of the colossal “Eye of Knowledge” opens up to reveal the fascinating (magical) setting.”

Shell Structures Types

1. Folded plate shells:

The peculiar feature of the folded plate is the ease in forming plane surfaces. a folded plate may be

formed for about the same cost as a horizontal slab and has much less steel and concrete for the same

spans.

The principle components in a folded plate structure consist of : • The inclined plates. (االلواح المائلة)

• Edge plates which must be used to stiffen the wide plates.

• Stiffeners to carry the loads to the supports and to hold the plates in line.

• Columns to support the structure in the air.

CYLINDRICAL BARREL VAULTS:

Barrel Vaults : are perhaps the most useful of the shell structures because they can span up to

150 feet with a minimum of material. they are very efficient structures because the use the arch

form to reduce stresses and thicknesses in the transverse direction.

DOMES OF REVOLUTION:

A dome is a space structure covering a more or less square or circular area. the best known

example is the dome of revolution, and it is one of the earliest of the shell structures. excellent

examples are still in existence that were built in roman times. they are formed by a surface

generated by a curve of any form revolving about a vertical line. this surface has double

curvature and the resulting structure is much stiffer and stronger than a single curved surface,

such as a cylindrical shell.

Shell System Materials :

Shell structures can be made of such materials as:

1. Reinforced thin-shell concrete.

2. Glass.

3. Steel.

4. Glass & Steel.

5. Plastic.

1. Reinforced thin-shell concrete:

The material most suited for construction of shell structure is concrete because it is a highly

plastic material when first mixed with water that can take up any shape on centering or inside

formwork. Small sections of reinforcing bars can readily be bent to follow the curvature of shells.

Once the cement has set and the concrete has hardened the R.C.C membrane or slab acts as

a strong, rigid shell which serves as both structure and covering to the building.

Ex. Construction Of R.C.C Barrel Vault

The barrel vault is the most straight forward single curvature shell construction. it is the part of a cylinder or

barrel with same curvature along its length. any number of continuous barrels or continuous spans are possible

except that eventually provision is made for the expansion of the joints in a large structures. the barrel vaults

are used as parking, market place, assembly hall ,etc.

Types of barrel vaults:

1. Short span barrel vaults.

2. long span barrel vaults.

• Short span barrel vault :

Short span barrel vaults are those in which span is shorter

than its width. It is used for the width of the arch ribs between

which the barrel vault span.

• Long span barrel vault :

Long span barrel vaults are those in which span is larger

than its width. Strength of the structure lies at the right angles

to the curvature to that span is linear to the curvature.

* Usual span of the linear barrel vault is from 12-30 m with its

width being about 1/2 the span and rise is 1/5 of the width. To

cover larger areas multibuy ,multi span roofs can be used

where the roof is extended across the width of the vault as a

multibuy .

Stiffening beams and arches:

Under local loads the thin shell of the barrel vault will tend to distort

and lose shape and even collapse if the resultant stresses were more. To

strengthen the shell against this possibility, stiffening beams or arches

are cast integrally with the shell.

The common practice is to provide a stiffening member between

the column supporting the shell. Down stand stiffening R.C.C beam is

most efficient because of its depth, but this interrupts the line of soffit of

vaults, for this upstand stiffening beam is used.

The disadvantage of upstand beam is that it breaks up the line of

roof and need protections against weather.

Edge and valley beams:

Due to self weight and imposed load the thin shell will tend to spread and its curvature flatten out. To

resist this R.C.C edge beams are cast between columns.

Edge beams may be cast as dropped beams or upstand beams or partially as both. In hot climate

the dropped beam is used whereas in temperate climate upstand beam is used to form drainage

channel for rain water.

In multi-bay structures, spreading of the vaults is largely transmitted to the adjacent shells, so down

stand and feather valley beam is used.

Expansion joints:

The change in temperature causes the Expansion and

contraction in concrete Structures, which causes the structures to Deform or collapse (تشوه االنهيار)

To limit this continuous expansion joints are Formed at the

interval of about 30m, along the Span and across the width of the

multi-bay and Multi-span barrel vault roofs. Linear Expansion joints

are formed in a up stand Valley.

Roof lights:

• Top light can be provided by deck light formed in the crown of vault or by dome light. The deck light can

be continuous or formed as individual lights. Roof lights are fixed to an upstand curb cast integrally with

the shell.

• Advantage of the shell is that its concave soffit reflects and helps to disperse light over area below.

• Disadvantage is that top light may cause over heating and glare.

Roof covering:

• Shells may be covered with non-steel or metal sheet metal, asphalt, bitumen felt, a plastic membrane or

a liquid rubber base coating.

Roof insulation:

• The thin shell offers poor resistance to transfer of heat. The need to add some form of insulating lining adds

considerably to cost of shell.

• The most satisfactory method of insulation is to spread a light weight screed over the shell.

Difficulties of providing insulation and maintaining the elegance of curved shape makes these structures

largely unsuited to heated buildings in temperate climate.

Reinforced Concrete Barrel Vault

Advantage

1. Allowing wide areas to be spanned without the use of internal supports, giving an open,

unobstructed interior.

2. No bending moment in shell structures.

3. Shell structures are suited to carrying distributed loads not concentrated loads.

4. Loads applied to shell surfaces are carried to the ground by the tensile, shear, compressive forces

5. Span thickness ratio of 400-500 ------------------ Ex: 8cm Thickness.....30-38m Span

6. Very light form of construction. to span 30.0 m shell thickness required is 60mm

7. Dead load can be reduced economizing foundation and supporting system

8. They further take advantage of the fact that arch shapes can span longer

9. Flat shapes by choosing certain arched shapes

10.Esthetically it looks good over other forms of construction

Disadvantages

1. Shuttering problem

2. Greater accuracy in formwork is required

3. Good labor and supervision necessary

4. Rise of roof may be a disadvantage