205 COMMON STRUCTURAL ELEMENTS COMMON STRUCTURAL ELEMENTS

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2.05 Common Structural Elements

Common structural elements.

Rigid Structural Elements

Truss and Trussing

Arches

Gothic arch

Domes

Plates

Vault

Bearing walls

Barrel Vault

Beam

Column

Frame

Space Frame

Mass Structure

Non rigid elements

Cable

Membrane systems

Structural loads on Elements

Properties structures must have

Stability

Strength

Ridigity

Sources of loads

Weight of structure

Occupants, users and interior components

Environmental impacts

Load classifications

Static

Dynamic

Permanence of load

Dead load

Live load

Loads from Natural Elements

Design loads
- Building codes
- Impacts of live loads

Rigid Structural Elements

Trussing: To support or brace with a truss.

Truss: a supporting structure or framework composed of girders, rods, or beams commonly of wood or steel lying in a single plane. Trusses usually take the form of a triangle or combination of many triangles, this design makes certain the greatest rigidity. Trusses are used for heavy loads and large spans, especially in roofs and bridges. Their open construction is lighter than, yet just as sturdy as, a beam. The members are known as struts, tie-beams, rafters, and posts; the space over which the truss extends is called the span. The upper and lower beams are connected by web members. X-bracing is a way to help resist wind and earthquakes.

Below is an example of a simple truss.

205 COMMON STRUCTURAL ELEMENTS COMMON STRUCTURAL ELEMENTS

Truss systems: Is combination of many single trusses matched together to form a complete system. An example of this is either a roof system or a flooring system.

Below is an example of an arch.

A rch: A structure forming the curved, pointed, or flat upper edge of an open space and supporting the weight above it. It is composed of voussoirs, which are wedge shaped masonry blocks. A structure, such as the Roman Aqua ducts, is shaped like an upturned U. As the arch’s span increases in relationship to the rise, thrust also increases. Thrust is controlled by the use of buttresses, ties, and span-to-rise ratio modification. More on arches, http://www.civilzone.com/arch.html

Gothic arch: Arch with a pointed top instead of a curved top.

D ome: A vaulted roof structure having a circular, polygonal, or elliptical base and a generally hemispherical or semispherical shaped top.

205 COMMON STRUCTURAL ELEMENTS COMMON STRUCTURAL ELEMENTS

Plates: (slabs) are the structural elements that span areas between beams, columns etc. They are used to enclose framed structures and create surfaces (floors, roofs, and walls). Plates require support that may be supplied through a one-way or two-way support system. The required support is determined by the depth span relationship (as span increases, depth must also increase to support the same weight).

205 COMMON STRUCTURAL ELEMENTS COMMON STRUCTURAL ELEMENTS

Vault: A structure that uses arches usually made of masonry or concrete, serving to cover a space

205 COMMON STRUCTURAL ELEMENTS COMMON STRUCTURAL ELEMENTS

Bearing wall: Are solid walls that hold loads, provide support for each other, and a roof system are much like plates except that they are vertically positioned. They support weight, produce height and enclose space (create rooms). Uneven load can cause a bearing wall to buckle or collapse.

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B arrel vault: Is a roof shaped like ½ cylinder and resembles a series of connected arches.

205 COMMON STRUCTURAL ELEMENTS COMMON STRUCTURAL ELEMENTS

Post and Lintel: is a horizontal beam (lintel) across two posts.

B eam: The beam is a rigid, linear, horizontal structural element used to span distance. Beams can sag when supporting weight. Sag can be prevented through understanding and application of the principal of the depth-span ratio. Types of beams include the girder, joist, rafter, header, and lintel.

Column: The column is a rigid, vertical, linear structural element that supports the weight of

beams and other elements. Crushing and buckling are two potential problems to avoid through application of the diameter-to-length ratio and deliberate “over design”, the practice of using stronger columns than are needed. The pier, pile and pillar are types of columns.

Identify and apply significant proportional relationships

a. Depth to span ratio otherwise known as Hook’s’ Law (Calculating deflection in beams): Is a beam’s tendency to bend while supporting weight and is directly related to the length of the span.

D = PL³/48EI

D = Deflection, P = force in center of span, L = length of beam, E = modulus of elasticity, I = moment of inertia.

b. Diameter to length ratio (Calculating deflection in columns) is a columns tendency to bend under load and is directly related to the length and diameter of the column.

F = P/A

F = compressive stress, P = compressive force, and A = cross-section area

Frame: A frame is a three-dimension assembly of components used to construct a structural skeleton. Any frame has structural tendencies to bend and twist. Stabilization is achieved through reinforced joints, trussing and rigid plates.

205 COMMON STRUCTURAL ELEMENTS COMMON STRUCTURAL ELEMENTS

Space frame systems: Consists of three dimensional truss beams extended to cover a large area. They are strong, lightweight, economical and able to cover large areas of open space with very few supporting columns.

M ass structure: Achieves strength and stability through tremendous weight. Dams, roads, bridges, and buttresses are mass structures. Mass structures have little or no useable internal space.

Nonrigid Structural Elements

C able: The cable is a nonrigid rope like, element used to span distance. Major types of cable

include: guys, used to brace rigid vertical structures like radio towers. Then there are hangers

which are suspended vertically to support weight; and spanning cables, which are draped

steel cables used in cable-stayed bridges and suspension bridges.

As a span increases in relation to sag the thrust increases. (Thrust is the

force that pulls on the ends of the cable)

M embranes: Membranes are thin flexible materials used to enclose or cover an area. There are two methods of supporting structural membranes: pneumatic (air) pressure and stretching. Pneumatic structures are of two types: the air inflated structure and the air supported structure. Air inflated structures are support by pressurized air between enclosed membrane layers. Air supported structures consist of one membrane layer anchored and supported by internal air pressure. Stretching the membrane on a pole and post is the non-pneumatic way. An example is a circus tent.

Structural Loads

A. To resist loads structures must possess:

1. Stability (equilibrium): The state or quality of being stable, especially

resistance to change, deterioration, or displacement.

2. Strength: The state, property, or quality of being strong.

3. Rigidity: The quality or state of being rigid.

B. Loads come from three main sources:

Weight of structure: Dead load.

Occupants, users and interior components: Live load.

Environmental impacts: Natural loads.

C. Loads are classified by:

1. The effects of the load on a structure

Static: Not in motion; at rest; quiescent. Fixed; stationary. Static and dead load are the same. This is the total weight of all attached parts of a structure.

Dynamic: Of or relating to energy or to objects in motion. Dynamic and live load are the same. This is all additional weight from traffic and non-attached objects inside or on a structure.

D. Permanence of load

A. Dead load: The weight of the entire structure and all connected parts.

B. Live load: Live load varies with the coming and going of people, interior

components, and vehicles if the structure is transportation oriented.

C. Loads from natural elements: These include rain, hail, snow, earthquakes,

hurricanes, tornados, and soil types.

E. Building codes specify design loads and safety factors

Building codes are written to reflect the different conditions in different locales. Codes written for California would not be appropriate for the coastal area of North Carolina. Codes for California must consider the frequent earthquakes while the coastal North Carolina codes must consider the yearly hurricane season.

F. Impacts of live loads from natural elements.

Structures must be built to handle natural events along with the need for the structure. Examples of these are the roof pitches on houses in the north and south of the United States of America. In the north, the roof pitches are steeper to keep snow and ice from accumulating and causing compression failure. In the south, roofs have a much lower pitch angle, except in the mountain areas that have large snow accumulation. Expansion and contraction due to heat is thermal load. Most concrete structures contain expansion joints to counter this condition.

Resources

Dictionary.com

http://www.pbs.org/wgbh/buildingbig/dam/

http://www.gossamersf.com/

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