FLOOD03449 - Laserfiche WebLink

Home Browse Search

41 ---- VI. WATER lOADINGS &.1 General Before any significant structure is built, investigation should be done to determine the expected loads that will be placed on it. Not just the magnitude, but also the point of application and the direction of the forces must be determined. The most common loads that are considered are dead load and gravity live load. Dead load includes the weight of all permanent construction materials, equipment, and for- ces resulting from prestressing. Live loads act when the structure is in service and vary in magnitude and location. Another type of load, restraint, results from such things as swelling and shrinkage of structural components, differential settling, and creep. This chapter will define and explain the additional forces which may be placed on a struc- ture by the environment, specifically a floodwater environment. The forces can be of con- siderable magnitude, As a comparison with hurricane winds, a 100 mile per hour (mph) wind blowing on the side of a house has the same dynamic loading effect as 12 inches of water flowing at a velocity of 10 mph (14.7 fps). It is also possible to have a combination of forces, wind and water, for example, working against a structure at the same time. The structure must, therefore, be able to withstand the sum of all these forces and the normal structural loads already mentioned. The following sections presen, the additional loading conditions and forces that must be considered when a structure is located in a flooding environment. &.2 Hydrostatic Loads Water at rest will exert a pressure against a submerged surface, The resulting force is called a hydrostatic load and can occur above or below ground. The magnitude of such a load is equal to the water pressure times the surface area on which the pressure acts. Pressure at any point is equal to the specific weight of water (&2.4 Iblfp for average conditions) times the height of water above the point or times the height to which unconfined water would rise above the point. This convention uses atmospheric pressure as the zero datum and is valid for the problems dealt with in this manual. Hydrostatic pressure at any point is equal ,n all directions and acts normal 2 (perpendicular) to the surface of an object. A simple classification of hydrostatic forces groups them into vertical, lateral, and uplift forces. &.2.1 Types of Forces Vertical loads are simply caused by the weight of water and act downward on horizontal or inclined surfaces. Lateral hydrostatic loads act in a horizontal direction on vEertical or inclined surfaces, They tend to cause lateral displacement or overturning of buildings or other objects. Uplift loads act vertically upward on the unders,de of objects, The net result of vertical and uplift force is called buoyant force and it always acts upward. &.2.2 Application Certain types of structures will be greatly affected by one kind of hydrostatic load and may not be affected significantly by other kinds of hydrostatic loading, Consider first a structure elevated on piers above the BIT The net hydrostatic effect on the piers and structure is negligible, At the other extreme would be a relatively watertight structure totally submerged, All three forces defined above would be contributing greatly to the loading on the structure. A similar example would be an empty storage tank in a flooded basement. If such a situation is anticipated, the anchoring andlor bolting down of the tank may quite easily be designed to withstand the buoyant force, Another type of structure, a floodwall, may have only the lateral hydrostatic load on it if the ground below is not saturated, The purpose of these examples is to show that it is possible to anticipate the kind of hyd- rostatic loading that will occur. Once that is determined, a design can be made that will accommodate those loads.