AN INFORMATIVE BULLETIN PROVIDED BY
Warren Environment, Inc.
STRUCTURAL DESIGN
The primary force controlling the design of steel stacks is wind. Unless the stack is a dual wall stack or a refractory lined stack in the highest of seismic zones, seismic loads will probably not control the stack design.
Wind shear and wind moment loads are determined according to applicable Codes and Standards. Stacks are designed as thin-walled cylinders on an allowable stress basis to prevent local buckling. Since the moment increases from top to bottom on a stack, its strength must also increase accordingly so that these allowable stresses not exceeded. This must be done by increasing the stack’s section properties. The section property, or section modulus, is generally expressed as S = π·r^2·t where r is the stack radius and t is the shell thickness. It is apparent that the most efficient way to increase section modulus is to increase r since it is a squared function. This explains why many stacks are flared at the bottom. Any combination of r and t may be used for overall stack design.
A common way to measure a cantilevered structure’s strength is to determine its tip deflection. The less the deflection for a given load, the stronger the structure. Many stack designers feel it necessary to limit a stack’s deflection as a function of its height. By the elementary laws of solid mechanics, when a structure’s stress is limited so is the deflection. It is not necessary, therefore, to add the additional constraint of a minimum tip deflection since limit stress has been satisfied.
It is important to know how much a stack will deflect under a given wind load. This calculation should be performed based on strict adherence to an accurate Wind Code. Increasing the wind load, shape factor, importance factor or exposure category to add conservatism will not produce an accurate estimate of actual deflection. If these increased loads are desired for structural design, they should be properly reevaluated for true deflection.
Static design calculations follow for stack components such as anchor bolts, penetration reinforcement, etc. Special design consideration must be given to the opening reinforcing for gas inlets and inspection doors. The stack is first designed without regard to the openings. The gas inlet is normally quite wide with respect to the stack’s diameter. The reinforcement scheme involves a header or balcony beam above and below the opening with vertical reinforcements on either side. It is important to keep the height-to-width ratio of a stack opening as high as possible for an economical design. When the width of the opening exceeds approximately two-thirds of the stack diameter, the reinforcing required becomes excessive.
