The roof is also a dead load. Dead loads are also known as permanent or static loads. Building materials are not dead loads until constructed in permanent position. Live loads, or imposed loads, are temporary, of short duration, or a moving load. These dynamic loads may involve considerations such as impact , momentum , vibration , slosh dynamics of fluids and material fatigue.
Live loads, sometimes also referred to as probabilistic loads, include all the forces that are variable within the object's normal operation cycle not including construction or environmental loads. Roof and floor live loads are produced during maintenance by workers, equipment and materials, and during the life of the structure by movable objects, such as planters and people.
Wind Load. From last two decades, the wind load has considered an important parameter while designing the structure.
Mostly wind load factor has considered when the structure height is above 10m. Environmental Loads are structural loads caused by natural forces such as wind, rain, snow, earthquake or extreme temperatures. A load combination results when more than one load type acts on the structure. Building codes usually specify a variety of load combinations together with load factors weightings for each load type in order to ensure the safety of the structure under different maximum expected loading scenarios.
For example, in designing a staircase , a dead load factor may be 1. These two "factored loads" are combined added to determine the "required strength" of the staircase. The reason for the disparity between factors for dead load and live load, and thus the reason the loads are initially categorized as dead or live is because while it is not unreasonable to expect a large number of people ascending the staircase at once, it is less likely that the structure will experience much change in its permanent load. For aircraft, loading is divided into two major categories: limit loads and ultimate loads.
Ultimate loads are the limit loads times a factor of 1. Crash loads are loosely bounded by the ability of structures to survive the deceleration of a major ground impact. Loads on the ground can be from adverse braking or maneuvering during taxiing. Aircraft are constantly subjected to cyclic loading.
These cyclic loads can cause metal fatigue. From Wikipedia, the free encyclopedia. American Society of Civil Engineers. Eurocode 0: Basis of structural design EN Bruxelles: European Committee for Standardization. Mark's Standard Handbook for Mechanical Engineers 10th ed. It is important that the design team understands these factors and deal with them prudently in the design phase.
EG50T9: Structural Vibrations - Catalogue of Courses
Torsion : Objects and buildings have a center of mass, a point by which the object building can be balanced without rotation occurring. If the mass is uniformly distributed then the geometric center of the floor and the center of mass may coincide. Uneven mass distribution will position the center of mass outside of the geometric center causing "torsion" generating stress concentrations. A certain amount of torsion is unavoidable in every building design.
Symmetrical arrangement of masses, however, will result in balanced stiffness against either direction and keep torsion within a manageable range. Damping : Buildings in general are poor resonators to dynamic shock and dissipate vibration by absorbing it.
Damping is a rate at which natural vibration is absorbed. Ductility : Ductility is the characteristic of a material such as steel to bend, flex, or move, but fails only after considerable deformation has occurred. Non-ductile materials such as poorly reinforced concrete fail abruptly by crumbling. Good ductility can be achieved with carefully detailed joints. Strength and Stiffness : Strength is a property of a material to resist and bear applied forces within a safe limit. Stiffness of a material is a degree of resistance to deflection or drift drift being a horizontal story-to-story relative displacement.
Building Configuration : This term defines a building's size and shape, and structural and nonstructural elements. Building configuration determines the way seismic forces are distributed within the structure, their relative magnitude, and problematic design concerns. View enlarged illustration. Soft First Story is a discontinuity of strength and stiffness for lateral load at the ground level.
- Earthquake, Blast and Impact: Measurement and Effects of Vibration.
- Have Souls, Will Travel!
- Earthquake, Blast and Impact: Measurement and effects of vibration - CRC Press Book.
- BLASTING MONITORING.
- See a Problem?.
Discontinuous Shear Walls do not line up consistently one upon the other causing "soft" levels. Variation in Perimeter Strength and Stiffness such as an open front on the ground level usually causes eccentricity or torsion. Seismic designs should adequately separate reentrant corners or strengthen them. Knowledge of the building's period, torsion, damping, ductility, strength, stiffness, and configuration can help one determine the most appropriate seismic design devices and mitigation strategies to employ.
Diaphragms : Floors and roofs can be used as rigid horizontal planes, or diaphragms, to transfer lateral forces to vertical resisting elements such as walls or frames.
Soil Dynamics and Earthquake Engineering
Shear Walls : Strategically located stiffened walls are shear walls and are capable of transferring lateral forces from floors and roofs to the foundation. Braced Frames : Vertical frames that transfer lateral loads from floors and roofs to foundations. Like shear walls, Braced Frames are designed to take lateral loads but are used where shear walls are impractical. Energy-Dissipating Devices : Making the building structure more resistive will increase shaking which may damage the contents or the function of the building.
Energy-Dissipating Devices are used to minimize shaking. Energy will dissipate if ductile materials deform in a controlled way. An example is Eccentric Bracing whereby the controlled deformation of framing members dissipates energy. However, this will not eliminate or reduce damage to building contents. A more direct solution is the use of energy dissipating devices that function like shock absorbers in a moving car.
The period of the building will be lengthened and the building will "ride out" the shaking within a tolerable range. Base Isolation Bearings are used to modify the transmission of the forces from the ground to the building. Base Isolation : This seismic design strategy involves separating the building from the foundation and acts to absorb shock. As the ground moves, the building moves at a slower pace because the isolators dissipate a large part of the shock.
The building must be designed to act as a unit, or "rigid box", of appropriate height to avoid overturning and have flexible utility connections to accommodate movement at its base. Base Isolation is easiest to incorporate in the design of new construction. Existing buildings may require alterations to be made more rigid to move as a unit with foundations separated from the superstructure to insert the Base Isolators. Additional space a "moat" must be provided for horizontal displacement the whole building will move back and forth a whole foot or more. Base Isolation retrofit is a costly operation that is most commonly appropriate in high asset value facilities and may require partial or the full removal of building occupants during installation.
Passive Energy Dissipation includes the introduction of devices such as dampers to dissipate earthquake energy producing friction or deformation. The materials used for Elastomeric Isolators are natural rubber, high-damping rubber, or another elastomer in combination with metal parts. Frictive Isolators are also used and are made primarily of metal parts. Tall buildings cannot be base-isolated or they would overturn. Being very flexible compared to low-rise buildings, their horizontal displacement needs to be controlled.
This can be achieved by the use of Dampers , which absorb a good part of the energy making the displacement tolerable. Retrofitting existing buildings is often easier with dampers than with base isolators, especially if the application is external or does not interfere with the occupants. All items, which are not part of the structural system, are considered as "nonstructural", and include such building elements as:. These items must be stabilized with bracing to prevent their damage or total destruction. Building machinery and equipment can be outfitted with seismic isolating devices, which are modified versions of the standard Vibration Isolators.
Loss arising from nonstructural damage can be a multiple of the structural losses. Loss of business and failure of entire businesses was very high in the Loma Prieta, Northridge, and Kobe earthquakes due to both structural and nonstructural seismic damages.
The principles and strategies of seismic design and construction are applied in a systematic approach that matches an appropriate response to specific conditions through the following major steps:. The location and physical properties of the site are the primary influences the entire design process. The following questions can serve as a checklist to identify seismic design objectives. Consider mission critical or business continuity threats of seismicity on adjacent sites or elsewhere in the vicinity that may render the project site inaccessible or causes the loss of utilities, threat of fire, or the release of toxic materials to the site.
Conduct subsurface investigations to discover loose soils or uncontrolled fill that could increase ground motion.