You can calculate the wind on a cylindrical tank for the purposes of design using ASCE 7. The current version is ASCE 7-16 Minimum Design Loads, and Associated Criteria, for Buildings and Other Structures. The next most recent version is ASCE 7-10 Minimum Design Loads for Buildings and Other Structures, which is probably still more widely used. However, regarding calculating the design wind loads on a cylindrical tank, each version uses the identical procedure. Before I list the relevant ASCE 7 sections, since you are designing a concrete water tank you will need another document – ACI-350 Code Requirements for Environmental Engineering Structures. The current version is ACI-350-06. This document is used by engineers involved in the design of concrete water holding structures, such as what you describe. Hopefully your professor(s) will provide you with these documents – otherwise I'm sure they would be available in your University Library.
The relevant ASCE 7-16 sections are as follows:
· Section 29.4 DESIGN WIND LOADS: OTHER STRUCTURES. From this section you can calculate the total base shear on the tank, F. You will need the following factors:
o qz = velocity pressure evaluated at height z, as defined in section 26.10, the centroid of the projected area Af.
o G = gust factor from section 26.11. For a rigid structure, such as your concrete water tank, this factor will be 0.85.
o Cf = force coefficient which for a cylindrical tank will come from fig 29.4-1.
o Af = projected area normal to the wind. For an open topped cylindrical concrete tank, fully above grade, this will be the tank diameter x the wall height.
To get qz you will also need some additional factors:
· Kz = velocity pressure exposure coefficient, section 26.10.1. You will have to assume an exposure category B, C, or D to get this coefficient. Exposure C is the typical default, but each category is described section 26.7.
· Kzt = topographic factor, section 26.8.2. This factor will usually be 1.0 unless all of the conditions of 26.8.1 are met.
· Kd = wind directionality factor, section 26.6. For round tanks Kd = 1.0.
· Ke = ground elevation factor, section 26.9. Rarely used, typically 1.0. Values less than 1.0 can be used for higher elevations, so it's conservative to use 1.0 in all cases.
· V = basic wind speed, section 26.5 Determined based on region and structure importance. Review section 26.5 carefully. Maps are provided for the US and its territories, and you will need to determine the risk category of your structure. All of this is discussed in 26.5.
To get Cf you need to assume the degree of roughness of your structure. Figure 29.4-1 gives you three alternatives – moderately smooth, rough, and very rough. Plain concrete is typically considered moderately smooth. However, if you can quantify surface features you might need to use one of the other roughness categories. Obviously, it is conservative to use higher roughness as this will increase the wind force.
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David Chilton P.E., S.E., CP, M.ASCE
Senior Associate Structural Engineer
Burns & McDonnell
Kansas City MO
(503)317-6661
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Original Message:
Sent: 10-03-2018 19:18
From: Siphepheto Sbangwana
Subject: Wind Load for a Concrete Cylindrical Tank
Hello everyone,
I am a 3rd year civil engineering student. We have been given a project to design a cylindrical concrete water tank. Can anyone suggest resources on how to calculate the wind load acting on a cylindrical tank?