Glossary

 

Arch: After girders, arches are the second oldest bridge type and a classic structure. Unlike simple girder bridges, arches are well suited to the use of stone. Many ancient and well-known examples of stone arches still stand to this day. Arches are good choices for crossing valleys and rivers since the arch doesn’t require piers in the center. Arches can be one of the more beautiful bridge types. Arches use a curved structure which provides a high resistance to bending forces. Unlike girder and truss bridges, both ends of an arch are fixed in the horizontal direction (i.e. no horizontal movement is allowed in the bearing). Thus when a load is placed on the bridge (e.g. a car passes over it) horizontal forces occur in the bearings of the arch. These horizontal forces, otherwise known as thrust, are unique to the arch and as a result arches can only be used where the ground or foundation is solid and stable.

Architect: The designation reserved by law for a person or organization professionally qualified and duly licensed to perform architectural services.

Beam: A structural member that carries load by bending between points of support.

Building Official: A representative of governmental authority employed to inspect construction for compliance with applicable codes, regulations, ordinances and permit requirements.

Cable Stayed Bridge: A typical cable stayed bridge is a continuous girder with one or more towers erected above piers in the middle of the span. From these towers, cables stretch down diagonally (usually to both sides) and support the girder. Steel cables are extremely strong but very flexible. Cables are very economical as they allow a slender and lighter structure which is still able to span great distances. Though only a few cables are strong enough to support the entire bridge, their flexibility makes them susceptible to large motion due to dynamic forces such as the wind. For longer span cable-stayed bridges, careful studies must be made to guarantee the stability of the cables and the bridge in the wind. The lighter weight of the bridge, though a disadvantage in a heavy wind, is an advantage during an earthquake. However, should uneven settling of the foundations occur during an earthquake or over time, the cable-stayed bridge can suffer damage so care must be taken in planning the foundations. The modern yet simple appearance of the cable-stayed bridge makes it an attractive and distinct landmark. The unique properties of cables, and the structure as a whole, make the design of the bridge a very complex task. For longer spans where winds and temperatures must be considered, the calculations are extremely complex and would be virtually impossible without the aid of computers and computer analysis. The fabrication of cable stay bridges is also relatively difficult. The cable routing and attachments for the girders and towers are complex structures requiring precision fabrication.

Client: The person or organization employing an architect or engineer and to whom these professionals are responsible.

Codes: Regulations, ordinances, or statutory requirements of a government unit relating to building construction and occupancy, generally adopted and administered for the protection of public health, safety and welfare.

Compression: Force causing shortening of a structural member.

Computer-Aided Design: (commonly abbreviated as CAD, or CADD for computer-aided design and drafting): A term applied to systems or techniques for design and drafting that utilize integrated computer hardware.

Construction Document Phase: During the construction document phase, the architect and engineers prepare construction documents consisting of drawings and specifications setting forth in detail the requirements for construction of the project.

Construction Phase: The construction phase of the project begins with the award of the Contract for Construction. During the construction document phase, the architect is usually the representative of the owner and advises and consults with the owner. Instructions to the contractor are forwarded through the architect. During the construction phase, the structural engineer advises and consults with the architect as necessary and when requested. The structural engineer reviews submittals by the contractor of items designed by the engineer or other items affecting the engineer’s design. The engineer makes periodic site observations to assure that the contractor is in general conformance with the intent of the structural design.

Continuous Beam: A beam with more than two points of support.

Contractor: The designation reserved by law for a person or organization qualified and duly licensed to provide contracting services. The contractor is the individual or entity with whom the owner enters into a construction contract to provide the construction services for a specific project.

Cripple Wall: A short height of wall, typically occurring in residential home construction, between the ground floor and the foundation. In older construction cripple walls are typically not covered with sheathing and therefore are unable to transmit shear forces from the walls above to the foundation below. This creates a weak link in the lateral load path that may result in a partial collapse of the wall framing over the unsheathed height.

Dead Loads: Static loads that do not vary, such as the mass weight of a structure.

Deflection: Movement or curvature of a loaded structural member.

Design Development Phase: During the design development phase, the architect prepares design development documents consisting of drawings and other documents to fix and describe the size and character of the entire project as to architectural, structural, mechanical, and electrical systems, materials and other such elements as appropriate. During this phase, the structural engineer identifies the framing system and the space required for the structural portions of the project and those elements that will dictate the strength of the materials.

Feasibility Study: A detailed investigation and analysis conducted to determine the financial, economic, technical, or other advisability of a proposed project.

Foundation bolts: Steel bolts, typically required in residential construction, that connect the wood framing above to the concrete foundation below. In older construction these bolts are generally missing or are spaced to infrequently. This creates a weak link in the lateral load path that may result in a horizontal offset between the structure and the foundation after an earthquake.

Girder: A large beam made of metal or concrete, and sometimes of wood that supports the ends of smaller beams.

Girder Bridge: A girder bridge is perhaps the most common and most basic bridge. A log across a creek is an example of a girder bridge in its simplest form. In modern steel girder bridges, the two most common girders are I-beam girders and box-girders. If we look at the cross section of an I-beam girder we can immediately understand why it is called an I-beam. The cross section of the girder takes the shape of the capital letter I. The vertical plate in the middle is known as the web, and the top and bottom plates are referred to as flanges. The box girder is much the same as an I-beam girder except that, obviously, it takes the shape of a box. The typical box girder has two webs and two flanges. However, in some cases there are more than two webs, creating a multiple chamber box girder.

Gravity Loads: Vertical loads from the weight of static or transient portions or occupants of a structure.

Lateral Loads: Loads from wind or earthquakes that put horizontal forces on a structure.

Live Loads: Transient loads to structural members.

Member: A structural unit such as a wall, column, beam, or tie, or a combination of any of these.

Moments: Force that causes bending, or curvature, of a structural member.

Non-Structural Elements: Elements of a building that are neither primary nor secondary structural elements, such as ceilings, mechanical and electrical equipment, and cladding.

Owner: A person or entity who retains services for design and contracts for construction services. This person or entity typically owns or is the lessee of the building site or project premises.

Pier: Vertical bridge support.

Primary Structural System: The structural engineer prepares the structural design of the primary structural system during the construction document phase.

Prime Consultant: The leader of the design team charged with the design of a new or remodeled facility, either an architect or an engineer, the party hired by the owner.

Professional Engineer: The designation of a registered engineer who provides engineering services. These services may include, but are not necessarily limited to, development of project requirements; creation and development of project design; preparation of drawings, specifications and bidding requirements; and providing professional services during the construction phase of the project.

Railing: A barrier consisting of a rail and supports.

Richter Scale: The scale is a measurement of earthquake magnitude defined by Professor Charles Richter: Richter magnitude has no physical meaning, although some approximations exist with respect to damage. The magnitude scale was defined as follows by Professor Charles Richter, its inventor and original developer: Is intended to be rating of a given earthquake independent of the place of observation. Since it is calculated for measurements on seismograms, it is properly expressed in ordinary number and decimals. Magnitude was originally defined as the logarithm of the maximum amplitude on a seismogram written by an instrument of specified standard type at a distance of 100 kilometers (62 miles) from the epicenter… Because the scale is logarithmic, every upward step of one magnitude unit means multiplying the recorded amplitude by 30… The largest known earthquake magnitude is 9.2, recorded in both Chile and Alaska. In contrast, maximum intensity determinations, such as the Modified Mercalli Intensity scale, relate actual damage to intensity and may take many days due to the field work involved.

Rigid Frame: Rigid frame bridges are sometimes also known as Rahmen bridges. In a standard girder bridge, the girder and the piers are separate structures. However, a rigid frame bridge is one in which the piers and girder are one solid structure. The cross sections of the beams in a rigid frame bridge are usually I shaped or box shaped. Design calculations for rigid frame bridges are more difficult than those of simple girder bridges. The junction of the pier and the girder can be difficult to fabricate and requires accuracy and attention to detail.

Schematic Design Phase: During the schematic design phase, the architect reviews the owner’s program to ascertain the requirements of the project. Based on this understanding, the architect prepares schematic design documents consisting of drawings, outline specifications and other documents illustrating the scale and relationship of the project components. During the schematic design phase, the structural engineer is expected to participate in the selection of a structural system for the building. It is appropriate for the structural engineer to offer alternative choices for the building structure.

Shear: The force causing deformation of a solid body in which a plane in the body is displaced parallel to itself relative to parallel planes in the body. Walls are typically designed to resist shear, the main reason for which plywood walls are sometimes termed “shear” by the construction industry.

Shop Drawing: All drawings, diagrams, illustrations, schedules and other data or information that are specifically prepared or assembled by or for a contractor, and submitted by a contractor to an architect or engineer to illustrate in detail some portion of the work.

Slab: A thin structural member, usually of concrete or stone.

Suspension Bridge: Of all the bridge types in use today, the suspension bridge allows for the longest spans. At first glance the suspension and cable-stayed bridges may look similar, but they are quite different. Though suspension bridges are leading long span technology today, they are in fact a very old form of bridge. Some primitive examples of suspension bridges use vines and ropes for cables. The development of metals brought the use of linked iron bars and chains. But it was the introduction of steel wire ropes that allowed spans of over 500m to become a reality. Today the Akashi Kaikyo bridge boasts the world’s longest center span of any bridge at 1,991 meters. A typical suspension bridge is a continuous girder with one or more towers erected above piers in the middle of the span. The girder itself is usually a truss or box girder, though in shorter spans, plate girders are not uncommon. At both ends of the bridge large anchors or counter weights are placed to hold the ends of the cables. The main cables are stretched from one anchor over the tops of the tower(s) and attached to the opposite anchor. The cables pass over a special structure known as a saddle. The saddle allows the cables to slide as loads pull from one side or the other and to smoothly transfer the load from the cables to the tower. From the main cables, smaller cables known as hanger cables or hanger ropes are hung vertically down and attached to the girder. Some suspension bridges do not use anchors, but instead attach the main cables to the ends of the girder. These self-anchoring suspension bridges rely on the weight of the end spans to balance the center span and anchor the cable. Like an arch, suspension bridges create high horizontal thrust forces where the cable is anchored to the ground.

Structural Element: A single structural member such as a beam, column, wall, brace, truss, or foundation that, when combined with others, forms the structural system.

Structural Engineering: The application of specialized civil engineering knowledge, training, and experience to evaluate, analyze, design, specify, detail, and observe the construction of force-resisting elements of structures. Such expertise includes consideration of strength, stability, deflection, stiffness, ductility, potential modes of failure, and other characteristics that affect the behavior of a structure.

Structural System: An assemblage of framing and bracing members designed to support gravity loads and resist lateral forces.

Strut: A long structural member designed to resist axial forces in the direction of its length.

System Ductility: The relationship of the maximum inelastic drift to the elastic drift.

Tension: Axial force applied along the length causing elongation of a structural element.

Truss Bridge: The truss is a simple skeletal structure. In design theory, the individual members of a simple truss are only subject to tension and compression forces and not bending forces. Thus, for the most part, all members in a truss bridge are straight. Trusses are comprised of many small members that together can support a large amount of weight and span great distances. In most cases the design, fabrication, and erection of trusses is relatively simple. However, once assembled trusses take up a greater amount of space and can become rather complex three-dimensional structures. Like the girder bridges, a truss can have two or more points of support. The small size of individual parts of a truss make it the ideal bridge for places where large parts or sections cannot be shipped or where large cranes and heavy equipment cannot be used during erection. Because the truss is a hollow skeletal structure, the roadway may pass over or even through the structure allowing for clearance below the bridge often not possible with other bridge types.

Value Engineering: The process of suggesting alternative systems, materials, and methods to reduce the cost and/or enhance the value of the project.

Water Heater: A domestic appliance used to heat water. In residential construction these are typically not well braced for lateral movement and can therefore be damaged during an earthquake. Damage to the water heater may result in fire and care should be taken near them following an earthquake. Self-bracing kits are available and should be installed.