Gravity loads gravity loads include all forces that

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each of the types of loads and how these loads are transferred to the other structural components. Gravity Loads Gravity loads include all forces that are acting in the ver- tical plane (see Figure 2). These types of forces are com- monly broken down into dead loads and live loads in a uniform pounds per square foot loading nomenclature. Dead loads account for the anticipated weight of objects that are expected to remain in place permanently. Dead loads include roofing materials, mechanical equipment, ceilings, floor finishes, metal decking, floor slabs, struc- tural materials, cladding, facades and parapets. Live loads are those loads that are anticipated to be mobile or transient in nature. Live loads include occupancy loading, office equipment and furnishings. The support of gravity loads starts with beams and purlins. Purlins generally refer to the roof while beams generally refer to floor members. Beams and purlins support no other structural members directly. That is to say, these ele- ments carry vertical loads that are uniform over an area and transfer the uniform loads into end reactions carried by girders. Girders generally support other members, typically beams and/or purlins, and span column to column or are sup- ported by other primary structural members. Girders may support a series of beams or purlins or they may support other girders. Forces imposed on girders from beams, purlins, or other girders are most often transferred to the structural columns. The structural column carries the ver- tical loads from all floors and roof areas above to the foundation elements. Figure 1. Forces experienced by structures ! Leeward Wind (Suction) ! Use and Occupancy ! Self-Wt. of Structure ! Snow ! Wind ! Seismic Forces ! Thermal Stresses ! Ground Pressure
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SYSTEMS PAGE 10 Horizontal Loads Forces created by wind or seismic activity are considered to act in the horizontal plane. While seismic activity is capable of including vertical forces, this discussion will be based only on horizontal forces. The majority of this section will address wind forces and how they are transferred to the primary structural systems of the building (see Figure 3). Wind pressures act on the building's vertical surfaces and create varying forces across the surface of the façade. The exterior façade elements, as well as the primary lateral load resisting system, are subjected to the calculat- ed wind pressures stipulated by code requirements. This variation accounts for façade elements being exposed to isolated concentrations of wind pressures that may be redistributed throughout the structural system. Design wind pressures can be calculated using a documented and statistical history of wind speeds and pressure in con- junction with the building type and shape. Calculated wind pressures act as a pushing force on the windward side of a building. On the leeward (trailing) side of the building, the wind pressures act as a pulling or suction force. As a result, the exterior façade of the entire building must be capable of resisting both inward and outward pressures.
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  • Fall '19
  • Structural steel

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