I need a method to design a steel structure for an academic project. The general schematic of the structure is like the below figure which includes braced frames. The effective load on the structure includes cyclic load resulting from the performance of the unit. What method should be used for designing these structure? I am a MSc Student of mechanical engineering and i have not enough knowledge about using designing methods such as LRFD that are related to civil engineering. Is there a method that is integrated with knowledge of mechanical engineering? Please introduce appropriate source for these methods. Thanks
Hi Ehsan,
You have two types of loads
Static (constant and doesn’t change with time)
Dynamic (cyclic loading).
For you design you need to satisfy these limit states
1. Limit state of strength (yielding and collapse)
2. Limit state of serviceability (controlling deformation)
3. Limit state of fatigue
For the first two limit state find the static equivalent of the cyclic loading (if it is not a very slow cycle) and add to the static load and design the structure using your choice of code; AISC LRFD if you wish. The static equivalent of a dynamic load is determined by multiplying the peak dynamic load by a dynamic amplification factor (DAF). DAF is based on the natural period of the structure and the cyclic loading; see any textbook on the dynamics of structures.
The limit state of fatigue is slightly involved. For this you need to know the number of cycles during the life time of the structures. If you have just one type cyclic load, then the job is simple. Your structure has two principle directions. Apply the calculated load as you found for the strength in x direction (as above) and then apply the same load in –x. Do the same for the y and –y directions. The difference between them at a point is the stress amplitude which causes fatigue. Next you must calculate how many of such cycles the structure must endure. You can find it by diving the structure’s life with the load period. Finally you need a code for fatigue design to choose a suitable S-N curve for the joints. Use miner’s rule to accumulate damage.
Your structure is similar to an offshore jacket structure which is subjected to the wave action.
Please check references below.. hope it will be helpful for you…
Regards…
http://digital.lib.lehigh.edu/fritz/pdf/332_1.pdf
http://jestec.taylors.edu.my/Vol%208%20Issue%204%20August%2013/Volume%20(8)%20Issue%20(4)%20416-427.pdf
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