Column and Rafter

If the tank diameter is small enough, and the roof material stiff enough, you may design a self supported roof (no rafters or columns). This is usually an umbrella, or self supported cone (cone slope of a self supported roof must be at a minimum 2:12). The max allowable thickness of a self supported roof, per API 650, is 1/2"; a self supported roof may incorporate roof rafters in compression, utilizing a center compression ring and a tension ring at the tank shell, thereby increasing the span while keeping the roof plate thickness down.

When it becomes unreasonable/undesirable to design a self supported roof, you may incorporate a roof structure composed of center column and rafters. In this case the roof slope is usually held at 3/4:12.

Hope this helps.

Thank SDK801 for your help. Today, my lead said over 6 meter diameter tank will be used rafter, under 6 meter diameter tank will be self support tank, and over 12 meter diameter will use column. But, he is not sure, because depending specification of licencor. Is he right? Because many small tank used rafter. I very confuse.

I agree with what your Leadman stated. I have about 400 13-ft / 4-meter tanks with self-supporting cone roofs, with only a 1:12 roof slope/pitch. They work fine, but I would be 'happier' if they were 2:12 per API.

Nothing wrong with adding rafters; but when they are not needed, the customer is wasting his money and creating a lot of small shelves to catch and hold material.

Thank Duwe6 very much