Laserfiche WebLink
<br />49 <br /> <br />-~ <br /> <br />6.4 Impact Loads <br /> <br />6.4,1 General <br /> <br />An impact load results when any object or material I which is carried by the flood water <br />strikes a structure. It is very difficult to accurately prl'dict the exact effect of impact loads. <br />However, an estimate must be made of these loads iwhen designing a structure in the flood <br />plain. Impact loads may be classified as normal, spe~cial and extreme, <br /> <br />6.4,2 Application <br /> <br />Normal impact loads are caused by such things as logs, ice blocks, or floatable objects of a <br />size that are often encountered in flooding, The normal impact load that should be con- <br />sidered for design purposes is a concentrated load alcting horizontally at the BFE or at any <br />point below it, equal to the impact force produced ~y a 1,000 pound mass traveling at the <br />velocity of the flood water and acting on a one squ'1re foot surface of the structure. <br /> <br />When large conglomerates of floatable obje<:(s, suc~, as accumulation of ice floats or debris, <br />strike a structure, it is known as a special impact loall. In an area where these loads may <br />occur, the design intensity should be 100 pounds pJr foot acting horizontally over a one-foot <br />wide horizontal strip at the BFE or at any level belo4- it. Special impact loads may be ignored <br />if natural or man-made barriers exist which would efkctively prevent the loads from occur- <br />ring on the structure. <br /> <br />loads which relate to large objects, such as runaV\la~ barges or collapsed buildings that are <br />being moved by the water, are classified as extreme limpact loads, In the vast majority of <br />cases, it is impractical to design structures to with,;tapd the", loads. Therefore, these loads <br />usually are not considered, <br /> <br />Methodology <br /> <br />The methodology is quite simplified using the crile~ia stated in section 6.4.2. Once the <br />object or mass to be designed for is established, thei impact due to the mass is calculated as <br />the mass times velocity divided by the duration of irf,pact. The duration of impact is usually <br />assumed to be one second, Depending on the exacti flood plain, characteristics in relation to <br />the amount and type of debris present, a safety fact~)f of 1.5 can be used. Calculation of <br />impact loading is shown in the following example, ' <br />