Thursday, May 6, 2010

API 650 Addendum 2 includes changes in roof designs by requiring a design for unbalanced snow loads. For column supported roofs this only affects the center column. If the roof slope is no more than 2 in 12, the unbalanced snow load per square foot is the same as the balanced snow load design. This does not affect rafters or girders. Also, the standard says that you do not need to include it for intermediate columns. The only additional requirement is that you have to check the center column for an eccentric load since the unbalanced snow is only on 135 degrees of the roof. This might cause a slight increase in the column size depending upon the loads.

Self-supporting cone and dome roofs have a new formula for checking unbalanced snow loads. Once again it can have an affect on the roof thickness depending upon the loads.

The biggest affect is on the supported cone roof without a center column that we have in our ITSdesign software. First, if your roof exceeds 2 in 12 pitch, then the snow load portion is increased by 50%. This will affect rafter sizes. The compression ring and tension ring are also affected no matter what the snow load is because of the unbalanced loading on the rings. To determine the stresses in the rings from this loading condition, a check at each rafter and between each rafter must be made to determine the location of the greatest tension load and the greatest compressive load on the rings. If you are using this configuration you may want to try to keep the roof slope to the 2 in 12 pitch. You will probably find the compression and tension rings are going to be larger even at this pitch.