SMACNA 1167

Abstract

<strong>INTRODUCTION</strong> <br>This publication is devoted exclusively to vertical, uniform diameter, freestanding stacks founded at ground elevation. Tables are provided for stacks of specific height ranging from 20 to 120 feet; specific wind velocities of 100, 125, and 150 miles per hour; and diameters ranging from 12 to 84 inches. When a desired stack, not listed in the tables, is subject to field conditions that may affect the structural integrity of the stack&#8212;i.e., higher design wind velocity, increased stack height, etc.&#8212; special consideration must be given to the design of the stack to compensate for these conditions. <br>Generally, the third edition stack weights vary from the weights of the second edition, which did not include the weight of the anchor ring and plates. Some are lighter than the second edition, and most are heavier. This edition's weights are the result of several changes that can add or reduce overall stack weight. The reasons are technically sound and important improvements. Previously a conservative design concept was used which added thickness to the stack from the bottom up such that the critical wind velocity was comfortably in excess of the design wind speed (10 or more mph higher than the design wind speed of 100, 125 and 150 mph). This is unnecessary once the critical wind speed is higher than 72 mph because it takes a steady state wind to achieve stack resonance, not the wind gusts which actually exist once the wind speed approaches 60 mph. Beyond that wind speed, winds at ground level through the 100 foot elevation are turbulent and made up of gusts of various speeds and directions, not steady state. This makes a large design difference for stacks between 42 and 60 in. in diameter. Also the previous scheme of allowable stress based on an older document (ASCE 1975 &#8211; Design and Construction of Steel Chimney Liners) was updated resulting in this edition's use of an allowable stress scheme based on the ASME Steel Stack Guide ASME /ANSI STS1, 2006 edition. In many cases both design stress values coincide, but often the ASME allowable stress value is higher resulting in somewhat lighter stacks. There are also a number of stacks affected by the Task Force decision not to weld platforms and ladders onto stack sections thinner than 10 gage. Many stacks that had top sections in 12 gage have been upgraded to 10 gage and all stacks under 24 in. in diameter were eliminated from having permanent platforms and ladders. <br>The Sheet Metal and Air Conditioning Contractors' National Association does not assume any responsibility or liability for the application of the principles and techniques contained in this publication.to the stack eddy zone and may cause down wash. This reduces the effective stack height and may cause the effluent to enter the building cavity, even though the discharge may be well above this cavity. Increasing stack discharge velocity and temperature will increase flume height and thus effective stack height. <br>The stacktowindvelocity ratio should be 1.5 to 1 or higher so that the effluent will break cleanly from the stack, down wash will be eliminated, and the effective stack height will be maximized. In most cases, a stack discharge velocity of 3,000 to 4,000 feet per minute will provide adequate performance.