We know a machine foundation is usually loaded dynamically which necessitates special design suppressing our conventional design for static
In “Soil mechanics and foundation engineering” books, static design consideration, design procedure, maintenance of existing foundation and sometimes strengthening or retrofitting of structurally or geotechnically weak foundation are discussed elaborately; we also discussed many aspects of such foundation in our previous posts.
The earlier design of foundation that may subjected to vibration includes:
-determination of dynamic factor that was used to increase the static loads
-foundation analysis and design then carried out based on statics using increased load as discussed above.
Another techniques was also used in which foundation mass was increased to absorb vibrations. In this regard, Cozens in 1938 has tabulated a ratio that encountered foundation mass to engine mass. This table provides no reference about
-type of foundation
-properties of foundation
Also this table does not provide any information or any methodology to calculate vibration amplitude and thus not ensure to be in acceptable limit.
But still modern engineers are using this procedure to have initial rough idea about size of foundation and then it is checked for accuracy by any of the reliable methods that are developed through the passing out of time.
The table provided by Cozens is provided below; which shows ratio of mass of foundation to that of engine for different types of machine:
|Si. no||Types of engine||Ratio of mass of foundation to that of engine|
|1||Steam engine(single cylinder)||Not less than 4.0:1|
|4||Gas engine(single cylinder)||3.0:1|
|5||Gas engine(2 cylinder)||3.0:1|
|6||Gas engine(4 cylinder)||2.75:1|
|7||Gas engine(6 cylinder)||2.25:1|
|8||Gas engine(8 cylinder)||2.0:1|
|9||Diesel engine(2 cylinder)||2.75:1|
|10||Diesel engine(4 cylinder)||2.4:1|
|11||Diesel engine(6 cylinder)||2.1:1|
|12||Diesel engine(8 cylinder)||1.9:1|
|13||Vertical gas engine coupled to generator||3.5:1|
|14||Vertical diesel engine coupled to generator||2.6:1|
|15||Vertical stem engine (compound) coupled to generator||3.8:1|
|16||Vertical stem engine (triple-expansion) coupled to generator||3.5:1|
|17||Horizontal stem engine (cross-compound) coupled to generator||3.25:1|
|18||Horizontal stem turbine coupled to generator||3.0:1 ~4.0:1(if of small output)|