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<br /> <br />Impact Forces <br /> <br />Impact forces occur due to floating debris, ice, <br />and other water-borne objects hitting the manufac- <br />tured home or elevated foundation as shown in <br />Figure 4,13, The magnitude of these forces will <br />be dependent upon the mass of debris and the <br />velocity of the water in which they are carried, <br /> <br />~FI <br /> <br /> <br />. <br /> <br />'/ <br /> <br />Figure 4.13 Impact Forces <br /> <br />Impact forces are calculated as follows: <br /> <br />FI _ m x dVb <br />- dt <br /> <br />Where: Fi = impact force in pounds <br /> <br />m = mass of the water displaced <br />by the object in slugs <br /> <br />Vb = velocity of the object in feet <br />per second <br /> <br />t = time in seconds <br /> <br />dVb = acceleration (deceleration) of <br />dt the object in feet per second <br />squared <br /> <br />The impact force acts horizontally at the flood <br />levei and can be assumed to be the impact force <br />produced by a 1000 pound object traveling at the <br />velocity of the floodwater and acting on one square <br />foot of the manufactured home or elevated foun, <br />dation, It can be assumed that the velocity of the <br />object goes to zero in one second and the Impact <br />Force equation can be restated as: <br /> <br />Fj =31 xVb <br /> <br />Table 4,4 provides impact forces as a function of <br />water velocity, For instance, at a velocity of 10 feet <br />per second the impact force would be 310 pounds <br />acting on one square foot of surface, <br /> <br />Table 4.4 <br /> <br />IMPACT FORCES lF~J <br />(Pounds per square foot) <br /> <br /> Yeloci tv 'fos' <br />I 2 3 4 5 6 7 8 9 10 15 <br />31 62 93 124 155 186 217 248 279 310 465 <br /> <br />F~ = 31 x Vb (lb./sq. ft.) <br /> <br />Where: Vb = veloci ty of the object in feet per <br />second <br /> <br />Where floating debris is anticipated, the impact <br />force on the manufactured home elevated foun- <br />dation must also be taken into account in the <br />design, Since the manufactured home itself is not <br />designed to withstand these forces, the impor- <br />tance of elevating above anticipated flood depths <br />is again apparent. <br /> <br />Scour <br />Scour is an important consideration for the <br />manufactured home elevated foundation, <br />The calculation for determining scour depth at <br />the elevated foundation members as shown in <br />Figure 4,14 is based on their shape and width as <br />well as the water velocity, water depth, and type <br />of soil. <br /> <br />Il <br />In <br />711 <br /> <br /> <br />.;.; > <br />:~: { <br /> <br /> <br />---- <br /> <br />Figure 4.14 Scour <br /> <br />39 <br />