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Forces and reactions |
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Like a bridge, a building must resist the internal stresses induced in its structural members |
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by the dead and live loads acting on it. Dead loads include the weight of the structure itself |
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plus all of the fixed equipment. Internal live loads include the people, moveable furnishings |
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and equipment, and vehicles occupying the building. External live loads include wind, rain, |
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snow, earthquakes, etc. Besides keeping the building from being permanently damaged by |
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these loads, the structure must be strong enough that no part of the building is significantly |
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deformed by them. Spongy floors or sagging ceilings are undesirable, as is significant |
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swaying of the upper levels of high rise buildings. |
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Wind loads exert a positive lateral pressure on the windward side of the building (the side |
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facing the wind), and a negative, sucking, pressure on the leeward side (the side opposite |
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the wind's direction). The height of a building and its profile are important in determining |
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the effective load of the wind. Wind velocity generally increases with height. And the |
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higher up the wind strikes the greater the bending moment it exerts. Thus tall structures |
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are affected by winds more so than shorter ones. Also whether the sides are vertical or |
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sloped affects whether the generally horizontal force of the wind strikes it head-on or just a |
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glancing blow. As shown in the exercise on vector forces, a force perpendicular to a surface |
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exerts a greater load on it than one that is oblique to it. |
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The same can be said for the snow load on a roof. Recall that a load applied to an A-frame |
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with a steeper slant, or pitch, and therefore a larger H/S ratio, induces less compressive |
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stresses in its legs than one with a shallower pitch. |
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Like a bridge, the individual structural members of a building must be selected so as to |
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have the proper materials and dimensions for the stresses they will be subjected to. And |
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they should be arranged together so as to maximize the efficiency of the structure in |
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supporting the anticipated load. The structural members that experience tensile stresses |
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mainly include the lower chords of trusses, rafter ties, and drag struts. Mainly compressive |
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stresses are experienced by posts, wall studs, columns, diagonal struts, and the upper chord |
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of trusses. Members that experience a combination of stresses include beams, joists, rafters, |
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and girders. |
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You can use the load equations for columns and beams presented earlier to estimate the |
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| tensile and compressive stresses experienced by a building's members. Keep in mind | |||
| though that the joints are usually rigid. Therefore you should use the load equations for | |||
| fixed beams and columns. Recall that fixing the ends of a beam to its supports reduces | |||
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the amount it will deflect under load by a factor of four to five compared to a simply loaded |
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beam. And the maximum bending moment experienced by a fixed beam is two to three |
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| times less than that for a simply supported beam. The maximum bending stress allowed for | |||
| a beam depends on whether it carries the load by itself or whether it is part of a support | |||
| framework such as flooring joists or roof rafters. The maximum allowable stress is greater | |||
| for beams that are part of a repeated structure. This is because the load is distributed | |||
| between a number of members via tributary areas and thus the structure has some | |||
| redundancy. Recall also that fixing both ends of a column lowers its tendency to buckle by | |||
| a factor of four compared to a freestanding column. | |||
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Back to Knowhere |
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