Roof loading

This specialized form of supply air system is often used in large open industrial spaces. It comprises a modular system of components that can be built up into simple systems. A typical system might have a roof inlet cowl, a recirculation damper, a heater battery, a fan, one or two outlet grilles and short sections of connecting ductwork, and would handle airflows up to 3-4m /s, depending on size. A number of individual systems would be used to provide the total airflow required in the space (Figure 28.4). Systems are normally manufactured with aluminum casings to reduce the roof load. 

Increased snow cover increases roof loads and increases building collapses. 

For a building with a flat roof (pitch less than 10°) it is normally assumed that reflection does not occur when the blast wave travels horizontally. Consequently, the roof will experience the side-on overpressure combined with the dynamic wind pressure, the same as the side walls. The dynamic wind force on the roof acts in the opposite direction to the overpressure (upward). Also, consideration should be given to variation of the blast wave with distance and time as it travels across a roof element. The resulting roof loading, as shown in Figure 3.8, depends on the ratio of blast wave length to the span of the roof element and on its orientation relative to the direction of the blast wave. The effective peak overpressure for the roof elements are calculated using Equation 3.11 similar to the side wall. 

The shape of (he rear wall loading is similar to that for side and roof loads, however the rise time ami duration arc influenced by a not well understood pattern of spillover from the roof and side walls and from ground reflection effects. The rear wall blast load lags that for the front wall by L/U, the lime for the blast wave to travel the length, L, of the building. The effective peak overpressure is similar to that for side walls and is calculated using Equation 3.11 (Ph is normally used to designate the rear wall peak overpressure instead of P,). Available references indicate two distinct values for the rise lime and positive phase duration. 

Pi = roof load, weight of pier, pile cap, and soil = 56.4 kips (251 kN)... 

I-beam- A steel beam with a cross section resembling the letter I. It is used for long spans as basement beams or over wide wall openings, such as a double garage door, when wall and roof loads bear down on the opening. 

Rafter- Lumber used to support the roof sheeting and roof loads. Generally, 2 X lO s and 2 X 12 s are used. The rafters of a flat roof are sometimes called roof joists. 

The df si j-n of the nifler is based upon Ihf roof load plus till wei di of Ibc rafter. Sinee the rafter size and weight is unknown, a preh ttiin. iry design l is< d on the nmi load only will Ite made and later will be checked with the weight of the rafter s< lect d. 

Bending moment due to roof loading = 17,872 KNm Therefore tension in the chord = = 438 KN... 

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