Quade999
Civil/Environmental
- May 29, 2020
- 61
Hi Everyone,
When calculating the stress range for fatigue under live loading I've seen different ways that people do this when the section goes from positive bending to negative bending under passage of the load. For this example, lets say the bottom layer of steel is the one under consideration.
One way I've seen is that people will calculate the section properties when the bottom layer is in tension, and then calculate the stress using this property for both the positive and negative moments.
Another way I've seen is that people will use the section properties for the bottom layer in tension under positive moments and find the stress, then use the section properties for the top layer in tension and calculate the compressive stress in the bottom layer under negative moment.
Option one assumes the same properties when under positive or negative moment. Therefore, if the positive and negative moments were the same, the stress under positive and negative would be calculated as the same. This method uses a large distance from NA to the steel as positive properties are used.
Option two assumes that under positive moment, the positive properties are used where the NA is far from the steel. Then under negative moment, the section flips and the NA becomes very close to the bottom layer of steel, which results in a small compression force in the bottom steel.
Option one comes up with larger stress changes in the steel than option two.
My question would be, is Option one just a conservative method for fatigue analysis. Because based on section properties, Option two seems to be the most realistic as it properties follows the section properties where under positive moment compression is on the top, and under negative moment compression is on the bottom.
Thanks
When calculating the stress range for fatigue under live loading I've seen different ways that people do this when the section goes from positive bending to negative bending under passage of the load. For this example, lets say the bottom layer of steel is the one under consideration.
One way I've seen is that people will calculate the section properties when the bottom layer is in tension, and then calculate the stress using this property for both the positive and negative moments.
Another way I've seen is that people will use the section properties for the bottom layer in tension under positive moments and find the stress, then use the section properties for the top layer in tension and calculate the compressive stress in the bottom layer under negative moment.
Option one assumes the same properties when under positive or negative moment. Therefore, if the positive and negative moments were the same, the stress under positive and negative would be calculated as the same. This method uses a large distance from NA to the steel as positive properties are used.
Option two assumes that under positive moment, the positive properties are used where the NA is far from the steel. Then under negative moment, the section flips and the NA becomes very close to the bottom layer of steel, which results in a small compression force in the bottom steel.
Option one comes up with larger stress changes in the steel than option two.
My question would be, is Option one just a conservative method for fatigue analysis. Because based on section properties, Option two seems to be the most realistic as it properties follows the section properties where under positive moment compression is on the top, and under negative moment compression is on the bottom.
Thanks
