Classification of Structures Analysis CE 213

Classification of Structures

Classification of Structures

It is important for a structural engineer to recognize the various types

of elements composing a structure and to be able to classify structures

as to their form and function.We will introduce some of these aspects

now and expand on them at appropriate points throughout the text.

Structural Elements. Some of the more common elements from

which structures are composed are as follows.

Tie Rods. Structural members subjected to a tensile force are often

referred to as tie rods or bracing struts. Due to the nature of this load,

these members are rather slender, and are often chosen from rods, bars,

angles, or channels, Fig. 1–1.

Beams. Beams are usually straight horizontal members used

primarily to carry vertical loads. Quite often they are classified according

to the way they are supported, as indicated in Fig. 1–2. In particular,

when the cross section varies the beam is referred to as tapered or

haunched. Beam cross sections may also be “built up” by adding plates to

their top and bottom.

Beams are primarily designed to resist bending moment; however, if

they are short and carry large loads, the internal shear force may become

quite large and this force may govern their design. When the material

used for a beam is a metal such as steel or aluminum, the cross section is

most efficient when it is shaped as shown in Fig. 1–3. Here the forces

developed in the top and bottom flanges of the beam form the necessary

couple used to resist the applied moment M, whereas the web is effective

in resisting the applied shear V. This cross section is commonly referred

to as a “wide flange,” and it is normally formed as a single unit in a rolling

mill in lengths up to 75 ft (23 m). If shorter lengths are needed, a cross

section having tapered flanges is sometimes selected.When the beam is

required to have a very large span and the loads applied are rather large,

the cross section may take the form of a plate girder. This member is

fabricated by using a large plate for the web and welding or bolting

plates to its ends for flanges.The girder is often transported to the field in

segments, and the segments are designed to be spliced or joined together

The prestressed concrete girders are simply

supported and are used for this highway

bridge.

at points where the girder carries a small internal moment. (See the

photo below.)

Concrete beams generally have rectangular cross sections, since it is

easy to construct this form directly in the field. Because concrete is

rather weak in resisting tension, steel “reinforcing rods” are cast into the

beam within regions of the cross section subjected to tension. Precast

concrete beams or girders are fabricated at a shop or yard in the same

manner and then transported to the job site.

Beams made from timber may be sawn from a solid piece of wood or

laminated. Laminated beams are constructed from solid sections of

wood, which are fastened together using high-strength glues.

Columns. Members that are generally vertical and resist axial compressive

loads are referred to as columns, Fig. 1–4. Tubes and wide-flange cross

sections are often used for metal columns, and circular and square cross

sections with reinforcing rods are used for those made of concrete.

Occasionally, columns are subjected to both an axial load and a bending

moment as shown in the figure. These members are referred to as beam

columns.

Types of Structures. The combination of structural elements and

the materials from which they are composed is referred to as a structural

system. Each system is constructed of one or more of four basic types of

structures. Ranked in order of complexity of their force analysis, they are

as follows.

Trusses. When the span of a structure is required to be large and its

depth is not an important criterion for design, a truss may be selected.

Trusses consist of slender elements, usually arranged in triangular fashion.

Planar trusses are composed of members that lie in the same plane and

are frequently used for bridge and roof support, whereas space trusses have

members extending in three dimensions and are suitable for derricks

and towers.

Due to the geometric arrangement of its members, loads that cause the

entire truss to bend are converted into tensile or compressive forces in

the members. Because of this, one of the primary advantages of a truss,

compared to a beam, is that it uses less material to support a given load,

Fig. 1–5. Also, a truss is constructed from long and slender elements,

which can be arranged in various ways to support a load. Most often it is Loading causes bending of truss,

which develops compression in top

members, tension in bottom

members.

Cables and Arches. Two other forms of structures used to span long

distances are the cable and the arch. Cables are usually flexible and carry

their loads in tension. They are commonly used to support bridges,

Fig. 1–6a, and building roofs.When used for these purposes, the cable has

an advantage over the beam and the truss, especially for spans that are

greater than 150 ft (46 m). Because they are always in tension, cables will

not become unstable and suddenly collapse, as may happen with beams or

trusses. Furthermore, the truss will require added costs for construction

and increased depth as the span increases. Use of cables, on the other

hand, is limited only by their sag, weight, and methods of anchorage.

The arch achieves its strength in compression, since it has a reverse

curvature to that of the cable. The arch must be rigid, however, in order

to maintain its shape, and this results in secondary loadings involving

shear and moment, which must be considered in its design. Arches are

frequently used in bridge structures, Fig. 1–6b, dome roofs, and for

openings in masonry walls.

Frames. Frames are often used in buildings and are composed of beams

and columns that are either pin or fixed connected, Fig. 1–7. Like trusses,

frames extend in two or three dimensions.The loading on a frame causes

bending of its members, and if it has rigid joint connections, this structure

is generally “indeterminate” from a standpoint of analysis.The strength of

such a frame is derived from the moment interactions between the beams

and the columns at the rigid joints.

Surface Structures. A surface structure is made from a material having

a very small thickness compared to its other dimensions. Sometimes this

material is very flexible and can take the form of a 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. As such they may be shaped as folded plates, cylinders, or

hyperbolic paraboloids, and are referred to as thin plates or shells.

These structures act like cables or arches since they support loads

primarily in tension or 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 surface. Such an analysis is

beyond the scope of this text and is instead covered in texts devoted

entirely to this subject.