surface structure

1.0 INTRODUCTION

1.1 General Overview of Surface Structures:

Surface structures are structural elements that are able to transfer load basically through

membrane stresses. Surface structures have small thickness compare to their other dimensions. .

Sometimes the material is very flexible and can take the form of the tent or air-inflated structure.

In both cases the material acts as a membrane that is subjected to pure tension. Surface structures

may also be made of rigid material such as reinforced concrete.

Rigid surface structures are sometimes called rigid shells. As such they may be shaped as folded

plates, cylinders, or hyperbolic parabolas and are referred to as thin plates or shells. These

structures act like cables or arches since they support loads primarily in tension and compression

with very little bending. In spite of this, plate or shell structures are generally very difficult to

analyze, due to the three-dimensional geometry of their flexible surface structures include

mechanically or pneumatically pre-stressed membrane such as reinforced concrete. . In both

cases the material acts as a membrane that is subjected to pure tension.

In civil engineering, structures where the dominant loading is usually substantially static are the

most common cause of collapse is a buckling failure. Buckling may occur locally in a manner

that may or may not trigger collapse of the entire structure, such as outstanding flanges, in flange

or web plate’s compression members, or in the web or compression flange of girders.

Today, the structural engineer may be involved in providing special structures for launching or

servicing space vehicles, or again he may use his knowledge in assisting with the structural

design of the vehicle itself.

A unique example of surface structure is a glass. One of the unique features of glasses is their

structural relaxation.

The structure and properties of a glass can change with time at a constant. The phenomenon

becomes particularly important in the glass transition temperature range. Traditionally at low

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temperature such as room temperature, the structural relaxation was assumed non-existent.

Recently, it was discovered that the surface layer of glasses exhibits much faster structural

relaxation than the bulk of the glasses. This was demonstrated by measuring the rate of shift of

IR peak wave number of silica structural band. It was found that the surface structural relaxation

is promoted by water vapor and applied tensile stress. By extrapolating both temperature

dependence and stress dependence of the relaxation time, it was estimated that the surface

structural relaxation can be observed in a practical time scale in wet condition under a moderate

tensile stress even at room temperature.

The observed faster surface structural relaxation is expected to have important influence on

nano-sized amorphous materials. Some of the phenomena, which are likely to involve the surface

structural relaxation of amorphous materials, are 1) mechanical strength degradation of silica

glass optical fibers and 2) oxidation of silicon. (Minoru Tomozawa)

In both cases the material acts as a membrane that is subjected to pure tension.

FIG 1. TYPICAL EXAMPLE OF SURFACE STRUCTURE

Such surface are true uniform-stress structural forms which are structurally stable and adequate,

and capable of supporting distributed loadings with no changes of shape except the small

deflections associated with the deformation of the material. Of course, concentrated loadings on

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a flexible membrane-being discontinuous in the loading and can be supported only by violent

localized changes of shape of the membrane similar to those changes required of a cable.

1.2. Types & Characteristics of Surface Structures:

Generally, surface structures consider the nature and tensile or compressive strength of their

member since they support load primarily in tension or compression. They are usually shaped

like cylinders or folded plates and are basically called thin plates or shells.

Their shapes hence give rise to the types of these structures now determined (further research is

still being carried out in this field and other structural elements are being considered) namely:

i. membrane or thin structures: these usually referred to structural members made of thin

membrane fibers made of fiber glass. Membrane structures carry load primary through tensile

stresses. Pre-stressing can be either by compression members, pressurized gases, fluids,

foundations or counter-stressing members. Thin elements are typically curved and are assembled

to large structures. They are use mostly for roof structures since they are economical and can

greatly improve the aesthetics of a building and span over large areas.

ii. Shell structures: these are thin rigid, curved & lightweight elements similar to those of thin

structures but are made of other materials like steel, wood, concrete etc. These structures have

different element types like hyperboloid structures, geodesic domes, etc.

iii. Plate Structures: plate structures are also light weight and thin elements that are curved. They

are made of thin plate elements that are assembled to form large structures.

These structural members are discussed in the next chapter.

2.0. TYPES OF SURFACE STRUCTURES:

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2.1. Membrane Structures:

As earlier stated membrane structures are thin layer structures that have flexible surface. They

primarily transmit load through tensile stresses. In modern construction today, the two main

types commonly use are pneumatic structures, which are air supported and stabilized by

compressed air and tent structures.

FIG 2.0 MEMBRANE STRUCTURE (TENT).

2.1.1. Brief History:

The structural concept of membrane structures can be traced as far back as to Russian engineer

named Vladimir Shukhor who was one of the first to develop practical calculations of stresses

and deformations of tensile structures, shells and membranes. A very early large-scale use of a

membrane-covered tensile structure is the Sidney Myer Music Bowl, constructed in 1958. Anton

Tedesko (1903-1994) also is attributed with much of the success of membrane structures in the

U.S.

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Luigi Nervi (1891-1979) of Italy later give more structural integrity to the subject with his work

on the Orbetello aircraft hangar, the Turin’s Exposition Hall and the Spaniard Torroja, a race

track in Madrid.(1951).

Also accredited for the development of the concept and establishment of much of the first

principle is Wright Bird who further use air supported membranes as covering for several

different kind of structures. His work was in the late 1940s.

However, tents have been the dwelling places of most of the world's nomadic peoples, from

ancient times until the present. The traditional Bedouin tent consists of a rectangular membrane

of strips of woven camel hair that is strained on webbing straps and secured with guys over a

rectangle of poles. The American Plains Indians developed the conical tepee. The Central Asian

nomadic pole dwelling, or yurt, uses skins and textiles as its covering. These are all ancient

origins of membrane structures.

FIG 3.0. TENT DESIGN

Pneumatic structure:

This type of membrane structure is stabilized by air or internally air supported. A network of

cables is used to stiffen the fabric and their assembly is supported by rigid ring at the edge. . The

air pressure within this bubble is increased slightly above normal atmospheric pressure and

maintained by compressors or fans. Air locks are required at entrances to prevent loss of internal

air pressure.

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FIG. 4.0. EXAMPL OF PNEUMATIC STRUCTURES

Tent structures:

These structures on the other hand use mast or poles or tensile membranes enclosures.

Examples of these are animal skin or fabrics. Tent structures are prestressed by externally

applied forces so that they are held taut under anticipated load conditions.

2.1.2. Strength and Behavior or Membrane structures:

In discussing the strength and behavior of a membrane structure it is important to consider the

design characteristics/variables of membrane structures. These variables may include (a)

geometrical quantities, like the thickness and the dimensions of the structural members, (b)

topological parameters, which define the location and the connectivity of such elements, and (c)

the characteristics of the eventual prestressing system, described by the prestressing forces and

the cables profile.

Four main sub classes of membrane structures is discussed in this report namely:

i) Air-supported structures in which an enclosing membrane is supported by a small differential air (or

fluid) pressure, example storehouses,

ii) Inflated structures in which highly pressurized tubes or dual-walled mats are used as structural

members in a space structure, example is dual wall shells,

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iii) Pre-stressed membranes in which fabric or rubberlike sheets are stretched over rigid frameworks and

columns to form enclosures or diaphragms, e.g., tents, masted roofs.

iv) Hybrid systems in which membrane panels span between primary load-carrying members such as pre-

stressed cables and rigid members, e.g., reinforced fabric roofs, fluid storage tanks. Various materials

can be used in the fabrication of a membrane structure, such as hyper elastic (rubberlike) materials,

fabrics, composites etc.

Hybrid Systems like cable membrane structures:

One of the main characteristics of cable membrane structures is that they have no stiffness

against loading perpendicular to the line of the cable or the surface of the membrane. Common

materials for doubly-curved fabric structures are PTFE coated fiberglass and PVC coated

polyester. These are woven materials with different strengths in different directions. Warp fibers

(those fibers which are originally straight—equivalent to the starting fibers on a loom) can carry

greater load than the weft fibers or fill fibers, which are woven between the warp fibers.

Other structures make use of ETFE film, either as single layer or in cushion form (which can be

inflated, to provide good insulation properties or for aesthetic effect as on the Allianz arena in

Munich. ETFE cushions can also be etched with patterns in order to let different levels of light

through when inflated to different levels. They are most often supported by a structural frame as

they cannot derive their strength from double curvature.

PV Flexibles can, for example, be used for single-layer roofs and facades without an additional

supporting structure. They can also be substituted for the upper layer of pneumatically supported

cushions. Used on situations such as these, photovoltaic elements serve not only to generate

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electricity; they can also provide shade, which may often be an essential requirement.

FIG 5.0. Anticlastic shape of an integrated photovoltaic- PTFE-membrane structure with applied

laminates

The development of the thin-film technology used in this context took place at the University of

Neuchâtel in Switzerland. In the meantime, it has undergone further refinement by the

established Swiss company VHF Technologies, which also manufactures the resultant product.

In a continuous (roll-to-roll) production process, the photovoltaic cells are applied to the polymer

bearing material in a sequence of layers, whereby the solar cells ultimately have a total thickness

of only about 1 µm.

The use of economic polymers as a substrate results in a very high deposition speed in

comparison to that of alternative bearing materials such as glass or metal foil. This, in turn,

results in moderate substrate temperatures. Otherwise, a thermally induced distortion of the

substrate material would inevitably occur, which would present a further obstacle.

According to a study carried out by the Q-Cells company, the potential of this technology in

terms of cost savings is superior to that of any other technique. In 2010, the overall costs could

be as much as 70 per cent lower than those of comparable modern systems.

Photovoltaic rolls manufactured in this way are cut to length, aligned and joined to form

laminates that meet the specific requirements of each project. The photovoltaic membrane is then

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bedded between two ETFE layers of different thicknesses. This process of lamination ensures

that the photovoltaic cells are effectively protected against loads and stresses, as well as against

moisture and weathering.

At present, the size of the modules is still limited by the dimensions of the available laminating

equipment (approxiately 3 m ≈ 1.5 m). Depending on whether they are used in roofs or facades,

in a single-layer form of construction or as part of a multilayer membrane cushion, the individual

laminae may have to be joined to form larger areas.

When the photovoltaic elements are used as an intermediate layer or inside a cushion, they are,

of course, optimally protected. In such cases, however, the light-refracting effect of the upper

film layer and the thermal gains that occur in the heat-absorbing middle layer would result in a

diminution of the energy yield. For that reason, it is clearly preferable to integrate the

photovoltaics in the outer layer of the cushion.

FIG 6.0.PNEUMATIC MEMBRANE STRUCTURE

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2.2.0 Plate Structures:

Basically, a plate structure is one of the most difficult surface structure to analyses. Analysis of

their bending or buckling abilities is use to determine their structural capabilities. An example of

a plate structure is a flat concrete slab.

Plate structural usually bend in two directions. Plates are commonly used as cover plates on

wide-flange beams, as the flanges and webs of plate girders, and as the sides of tube-shaped

beams and columns. In all these cases, serious consideration must be given to the fact that the

plate may buckle when compressed. But the plates have edge supports in the direction of the

stress, so they function as panels rather than as beams. Their ratios of length to width are large

enough that the resistance to local buckling of the plate element depends upon its width-

thickness ratio, practically independent of its length. Because of the length of the overall section

it is still significant in determining the member's capacity.


The concept of plate theory was actually conceived long ago in the 1700s but it was Kirchhoof

who in 1850 publish the first notable thesis on plates and steted two basic assunptions which in

later years came to be accepted as plate-bending theory and are known as Kirchhoff’s hypothesis

In his work, he pointed out that there exist two boundary conditions on a plate edge..

Kirchhoffs other significant contributions are the discovery of the frequency equations of the

plates and the introduction of virtual displacement methods in the solution of plate problems.

It was not however until the end of the 19th century that the application concept of the plate

theory began to catch on when ship builders change their construction method by replacing wood

with structural steel.

This change in structural material was very fruitful in the furtherance of the plate theory and

through the contributions of various other scientist the fundamental assumptions of the behavior

of plate theory was developed.

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The Crymlyn Viaduct over the Ebbw Alley which opened in 1857 as Welsh coal mining

expanded is one of the earliest work of plate structures with relation to the application of the

plate theory concept. It was constructed of wrought and cast iron, and remained the highest

railway viaduct in the UK until its closure in 1964 due to increased locomotive weights (1908

photo). The advance from masonry to the slender metal compressive members which make up

each column requires substantial bracing to prevent buckling.

F IG 7.0. Crymlyn Viaduct over the Ebbw Alley (1908)

2.2.2 Strength and Behavior of Plate Structures:

In the general behavioral analyses of plate characteristics, fundamental assumptions of the linear,

elastic, and small-deflection theory of bending of plates may be stated as follows:

the material of the plate is elastic, homogeneous, and isotropic,

the plate is initially flat,

the defection, (the normal component of the displacement vector ) of the mid plane is

small, compare with the thickness of the plate. The slope of the deflected surface is very

small and the square of the slope is a negligible quantity in comparison with unity.

The straight lines, initially normal to the middle plane before bending remain straight and

normal to the middle surface during the deformation.

One type of plate structure is the sill plate.

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http://school.mech.uwa.edu.au/~dwright/DANotes/buckling/intro/CrymlynBIG.jpeg%00%E5%A1%B9%EF%92%81%E1%B4%BB%E4%A1%BF%E2%B2%AF%E5%B6%82%E8%97%84%E6%8C%A7%00%00%EA%AE%A5

Sill plate

A sill plate or sole plate in construction or architecture is the bottom horizontal member of a wall

or building to which vertical members are attached. Sill plates are usually composed of lumber.

It usually comes in sizes of 2×4, 2×6, 2×8, and 2×10. In the platform framing method the sill

plate is anchored to the foundation wall. The bottom of the sill plate is kept 6 inches above the

finished grade. This is to prevent the sill plate from rotting as the finished grade contains

chemicals.

FIG 8.0. TYPICAL

SILL PLATE

Considering also another unique example

of the plate type, the folded plate.

Folded Plate

A folded plate is an example of a 3-dimensional or space structure. To consider the effectiveness of the

folded plate look at the following experiment. When a thin sheet of paper rests between two supports it

will bend due to the fact that it has insufficient strength to carry its own weight.

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http://en.wikipedia.org/wiki/Foundation

http://en.wikipedia.org/wiki/Framing_(construction)

http://en.wikipedia.org/wiki/Lumber

If the same piece of paper is folded then it will be capable of supporting one hundred times its

own weight.

If the load is increased past this point then the structure will fail and the folds will flatten

out. his problem can be rectified by using transverse stiffeners at the ends. The folded plate acts

as a beam and can support even greater loads.

Folded plates consist of straight pieces joined with sharp edges. It cannot be made as thin as a

shell due to the fact that it is subjected to bending. The result of this bending can be seen in the

animation below. Folded plates can be seen as a space version of a rigid frame. Folded plates are

best formed from reinforced concrete due to the fact that they can be easily cast. Folded plates

can take the form of frames as seen above or domes.

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The last varying-stress form to be considered is the slab or plate element. The structural

behavior of a plate may be visualized as being similar to that of two layers of beam strips, each

layer consisting of strips which are cemented together side by side. The layers are then oriented

so that the directions of the strips are perpendicular as the layer placed one on the top of the

other. If the edges of such as a plate are supported and it is then subjected to a transverse loading,

both layers will collaborate in resisting the load, each layer bending and transmitting the load

into the pair of edge supports at the ends of its beam strips.

FIG 9.0. Example of Folded plate roof

2.3.0 Shell Structures:

Shell structures in building refers usually to a thin, curved plate structure shaped to transmit

applied forces by compressive, tensile, and shear stresses that act in the plane of the surface. A

thin shell is defined as a shell with a thickness which is small compared to its other dimensions

and in which deformations are not large compared to thickness. A primary difference between a of 23

shell structure and a plate structure is that, in the unstressed state, the shell structure has

curvature as opposed to plate’s structures which are flat.

The shells are most commonly flat plates and domes, but may also take the form of ellipsods or

cylindrical sections, or some combination thereof. The first concrete shell dates back to the

second century.


The oldest known concrete shell, the Pantheon in Rome, which was completed about AD 125, nd

is still standing. It has a massive concrete dome 43m in diameter. The structure is monolithic and

it appears to have been sculpted in place by applying thin layers on top of each other in

decreasing diameter. Massively thick at the bottom and thinning at the top, the Pantheon is a

remarkable feat of engineering.

FIG. 10.0.Pantheon in Rome (AD125)

Modern thin concrete shells, which began to appear in the 1920s, are made from thin steel

reinforced concrete, and in many cases lack any ribs or additional reinforcing structures, relying

wholly on the shell structure itself.

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Shells may be cast in place, or pre-cast off site and moved into place and assembled. The

strongest form of shell is the monolithic shell, which is cast as a single unit. The most common

monolithic form is the dome, but ellipsoids and cylinders (resembling concrete Quonset huts) are

also possible using similar construction methods.

Geodesic domes may be constructed from concrete sections, or may be constructed of

lightweight foam with a layer of concrete applied over the top. The advantage of this method is

that each section of the dome is small and easily handled. The layer of concrete applied to the

outside bonds the dome into a semi-monolithic structure.

Monolithic domes are cast in one piece out of reinforced concrete, and date back to the 1960s.

Advocates of these domes consider them to be cost-effective and durable structures, especially

suitable for areas prone to natural disasters. They also point out the ease of maintenance of these

buildings. Monolithic domes can be built as homes, office buildings, or for other purposes.

The Seattle Kingdom was the world's first (and only) concrete-domed multi-purpose stadium. It

was completed in 1976 and demolished in 2000. The Kingdome was constructed of triangular

segments of reinforcements that were cast in place. Thick ribs provide additional support.

2.3.2 Strength and Behavior of Shell Structures:

Membrane action in a shell is primarily caused by in-plane forces (plane stress), though there

may be secondary forces resulting from flexural deformations. Where a flat plate acts similar to a

beam with bending and shear stresses, shells are analogous to a cable which resists loads through

tensile stresses. However the ideal thin shell must be capable of developing both tension and

compression.

There are several types of shell structures that offer advantages in terms of utility and cost and

perhaps in aesthetics.

Long Span Domes

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Constructions of spherical domes by using inflated membranes have proven to be a viable

solution to many problems. The largest shells constructed are now less than 300 feet in diameter.

Some uses, such as sports facilities demand greater spans, perhaps as much as 1000 feet.

The design and construction of such a shell is a challenge, and using inflatable forms and a

uniform depth may not be possible. For very large spans, a grid constructed with pans is

necessary so that the dome will be stiff enough and still weigh less than a uniform depth.

For simple geodesic domes we recognize the associated curve to be the surface of a sphere. Here

is how chords of geodesic spheres are generated. We first choose an underlying polyhedron with

equal triangle faces. The regular icosahedrons are most popular. The sphere we use is

specifically the "circumscribing sphere" that contains the points (vertices) of the underlying

polyhedron. The desired frequency of the subsequent geodesic sphere or dome is the number of

parts or segments into which a side (edge) of the underlying polyhedral triangle is subdivided.

The frequency has historically been denoted by the Greek letter "ν" (nu). By connecting like

points along the subdivided sides we produce a natural triangular grid of segments inside each

underlying triangle face. Each segment of the grid is then projected as a "chord" onto the surface

of the circumscribing sphere. The technical definition of a chord factor is the ratio of the chord

length to the radius of the circumscribing sphere. It is therefore convenient to think of the

circumscribing sphere as scaled to radius = 1 in which "chord factors" are the same as "chord

lengths" (decimal numbers less than one).

For geodesic spheres a well-known formula for calculating any "chord factor" is

chord factor = 2 Sin (θ / 2) where θ is the corresponding angle of arc for the given chord, that

is, the "central angle" spanned by the chord with respect to the center of the circumscribing

sphere. Determining the central angle usually requires some non-trivial spherical geometry.

The construction of this structure will require the greatest ingenuity. One method will be to

construct the dome by a form on a track that moves around the inside of the dome, placing

concrete at a short horizontal and vertical section, and then moving around to place the next

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section. After each circle is completed, then the rig would be raised, and the ring above placed.

The grid could be formed by using semi rigid insulation thus ensuring acoustical treatment.

FIG. 11.0.TYPICAL DOME UNDER CONSTRUCTION

Vaults

A vault may be defined as a single barrel shell, supported on its side by walls or columns. The

virtue of the vault is that half of the load on the shell is carried by the walls, and the other half is

carried to the ends and at that point the usual arch and tie are required. The thickness of a shell

can be much less that for a normal arch of the same span because the shell carries loads as a

space structure.

This structural system can be used, for example, for sports facilities with widths up to 300 feet

and lengths to 500 feet. These spans will require a ribbed structure created by pans or insulating

blocks of foam set on the forms. Again ingenuity will be required for maximum economy.

Folded Plate

The usual folded plate structure has been constructed with two or three element folded plates,

with slopes of slightly less than 45 degrees, and tied at the ends by frames and horizontal ties. A

much more interesting system is to use plates with much less slope and use vertical columns for

end support rather than the usual ties.

The spans of the slab elements can be of relatively long, (25 to 30 feet), if the slabs are hunched.

The optimum distance valley to valley would then be, say, 50 ft., and the span of the folded

plates, 50 ft. with a height of 5 ft. The slope of the slabs is the ratio of 1 to 5. The concrete would

be much easier to place than the usual steep slope, thus leading to better economy. In my opinion

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the shape would be more esthetically interesting, leading to a de-emphasis of the repetitious

nature of some folded plate structures.

Funicular Shells

A dome of any size or shape can be designed for any plan by using the differential equation of a

bubble. Finite differences are used to solve the equations. The photo shows a stick model of a

tear drop shaped dome. The construction of this model was possible because the dimensions

were generated by the program.

FIG 12.0. TYPICAL DOME SHAPE

Shell Arches

A shell arch has a longitudinal cross section of a barrel shell or a folded plate, but is a circular

arch or other shape in profile. This arch is suitable for extremely long spans, say to 1000 feet,

and is one of the most efficient structural systems possible. To put it another way, there will be

less concrete in the roof than in the floor system, and the reinforcing will be minimum.

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The most interesting aspect of the design is the selection of the forming system. The usual

method would be to use a curved form moving on a track that would form two or three units, and

then decent, move to the next set of shells, recenter and cast the next unit. The decentering of

such a large structure for would be a real chore and would require expensive manual or hydraulic

jacks.

FIG 13.0 .Lattice Shell of the Shukhov Hyperboloid Tower.

FIG 14.0. St. Louise Airport, Designed by Anton Tedesko, outside view

A Cylindrical Groin Vault

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http://en.wikipedia.org/wiki/Shukhov_Tower

3.0. CONCLUSIONS:

In concluding this topic, it is important to emphasize that surface structures have changed and

added aesthetic beauty to modern architecture design since it was improved and developed over

the years.

It plays today a very significant role in the construction industry and its value and shape have

been advanced over the ages by improve technology and further research in this field. Its

improvement and advancement in the field of engineering have shape and change the beauty and

aesthetics of modern day structures in the most dramatic way.

Unlike earlier times where information about the structure behavior of these members was

limited, today these members are divided into whole new fields of study.

These structures give the engineer much more options on the cost and suitability of materials

especially when the span of the structure and the aesthetics of the structure is of importance.

4.0 REFERENCES:

Books:

1) Thin Plates & Shells, by Eduard Ventsel & Theodor Krauthammer, Copyright, 2001, Marcel

Dekker

2) Structural Analysis, by R.C. Hibbler, 6th edition, Copyright 2006, Prentice-Hall Inc.

Web addresses:

1. http://images.google.com.my

2. www.fstructures.com/category/membrane_structures/

3. http://www.springerlink.com/content/wrvxg2191k401632

4. http://en.wikipedia.org/wiki/Geodesicdome

5. www.britannica.com/Eb

6. www.answers.com

7. www.sciencedirect.com

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http://www.sciencedirect.com/

http://www.answers.com/

http://www.britannica.com/Eb

http://en.wikipedia.org/wiki/Geodesicdome

http://www.springerlink.com/content/wrvxg2191k401632

http://www.fstructures.com/category/membrane_structures/

http://images.google.com.my/

surface structure

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