Light Agricultural and Industrial Structures LIGHT AGRICULTURAL AND INDUSTRIAL STRUCTURES: Analysis and Design G. L. Nelson AgricultiIral Engineering Department Ohio State University Columbus, Ohio H. B. Manbeck Agricultural Engineering Department Pennsylvania State University University Park, Pennsylvania N. F. Meador Agricultural Engineering Department University of Missouri Columbia, Missouri avi An Book Published by Van Nostrand Reinhold Company New York AN AVIBOOK (AVI is an imprint of Van Nostrand Reinhold Company Inc.) Copyright © 1988 by Van Nostrand Reinhold Company Inc. Softcover reprint of the hardcover 1st edition 1988 Library of Congress Catalog Card Number 87-37263 ISBN 978-1-4757-0413-6 ISBN 978-1-4757-0411-2 (eBook) DOI 10.1007/978-1-4757-0411-2 All rights reserved. No part of this work covered by the copyright hereon may be reproduced or used in any form or by any means-graphic, electronic, or mechanical, including photocopying, recording, taping, or information storage and retrieval systems-without written permission of the publisher. Van Nostrand Reinhold Company Inc. 115 Fifth Avenue New York, New York 10003 Van Nostrand Reinhold Company Limited Molly Millars Lane Wokingham, Berkshire RG11 2PY, England Van Nostrand Reinhold 480 La Trobe Street Melbourne, Victoria 3000, Australia Macmillan of Canada Division of Canada Publishing Corporation 164 Commander Boulevard Agincourt, Ontario MIS 3C7, Canada 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 Library of Congress Cataloging-in-Publication Data Nelson, G. L. (Gordon Leon), 1919- Light agricultural and industrial structures. "An AV I book." Bibliography: p. Includes index. 1. Farm buildings-Design and construction. 2. Structures, Theory of. I. Manbeck, H. B. II. Meador, N. F. III. Title. TH4911.N45 1988 690'.892 87-37263 Contents Preface vii 1. Analysis and Design Concepts 1 2. Fundamental Concepts of Stress Analysis 7 3. Stress Analysis of Coplanar Statically Determinate Trusses 33 4. Bending Deformation 63 5. Analysis of Statically Indeterminate Coplanar Frames 91 6. Load Analysis 127 7. Fundamentals of Structural Connections 185 8. Structural Steel Design 209 9. Cold-Formed Steel Design 283 10. Light Timber Design 359 11. Reinforced Concrete Design 455 Index 545 Preface This book is an outgrowth of a much earlier book, Farm Structures, by H. J. Barre and L. L. Sammet, published by John Wiley & Sons in 1950 as one of a series of textbooks in agricultural engineering spon sored by the Ferguson Foundation, Detroit, Michigan. Light Agricul tural and Industrial Structures: Analysis and Design will be useful as an undergraduate student textbook for junior- or senior-level compre hensive courses on structural analysis and design in steel, wood, and concrete, and as a reference work for practicing engineers. Emphasis is on basic analysis and design procedures. The book should be useful in any country where there is a need to design structures for agricul tural production and processing. It is assumed that readers have had prerequisite course work in engineering mechanics and strength of materials as typically taught to undergraduate engineering students. The scope of this book is wide; it might be difficult for instructors and students to cover all of the chapters in a typical three credit-hour course. The instructor will need to assess his own situation and scheduling constraints. More or less time could be spent on chapters one through five, depending on the capability the students already have in analysis of statically deter minate and indeterminate structures. Two to three weeks might then be allocated for study of each of the last six chapters dealing with design in steel, reinforced concrete, and wood. We suggest that instructors help their students who may not have a major interest in structural analysis and design to understand that the content of this book is not limited in usefulness to light building structures. For example, students interested in farm machinery design vii viii Preface must be able to analyze statically indeterminate welded steel frames used to a wide extent in farm machinery. Students whose primary interest is in soil and water engineering need capability in analysis and design of reinforced concrete culverts and related structures. Stu dents interested in processing and food engineering will encounter structural analysis and design problems in relation to crop and food storage, handling, and transport facilities. The timber, steel, and reinforced concrete chapters are based upon recent structural design specifications. However, it should be noted that design specifications are not static. Undoubtedly, one or more structural specifications will change with time. The authors suggest that the instructor may use the text to present basic concepts and methods of structural component design, but supplement the text by reference to the most up-to-date specification for each structural I material. English and SI units have been used interchangeably throughout the book to reflect the present level of adoption of SI units. The timber and steel construction industries are moving slowly toward adoption of SI units. Reinforced concrete construction seems to be leading in adoption of SI units. It is almost inevitable that errors in the text will come to light. We hope they are few in number. The authors will be indebted to anyone who identifies errors or the need for clearer meaning that could be remedied in future printings or editions. 1 Analysis and Design Concepts Light structures are used for many agricultural and industrial needs. They are characterized by one- or two-story configuration, moderate spans, and light to moderate superimposed loads. Construction costs must be carefully controlled to maintain a profitable relationship be tween capital cost and income from use of the structures. Often, they are built according to standard plans and utilize mass-produced frames and other structural components. A high degree of precision in structural analysis and design for light structures is often necessary. For example, a single standardized design may be used for several hundred or thousand buildings. This multiple use of one basic design can justify considerable analysis and design effort. A high degree of refinement is desirable for multiple-use plans for low-cost structures. 1.1 ANALYSIS AND DESIGN OBJECTIVES One important and basic objective in structural analysis and design is to produce a structure capable of resisting all applied loads without undue deformation during its intended life. For some structures, pres ervation of the original shape may be a critical requirement. Even a minor amount of deflection or deformation may be objectionable. An example is a structural frame supporting a long run in a screw conveyor system. Here misalignment would cause excessive wear and operating difficulties. In other instances relatively large deformations may be permissible without impairing the usefulness of the structure. An ex ample is a structural frame for a greenhouse enclosed with plastic 1 2 Analysis and Design Concepts sheeting. In all instances, the risk of collapse or destruction must be controlled by adequate analysis and design. Production of an adequate design is the engineer's basic responsi bility. The ubiquitous demand for economy, often severe in light struc tures, should not take precedence over structural adequacy. An owner seldom gives an engineer much credit for a structure that collapses or a machine frame that comes apart in the field, even though the struc ture or machine was inexpensive to build. 1.2 DESIGN PROCEDURE Structural analysis and design are based on preliminary drawings previously prepared to meet functional requirements of the building. These drawings reveal gross dimensions and configuration ofthe build ing, and how space within the building will be used. Information on the preliminary drawings helps to establish spacing offrames, columns, and other primary components. Building width, height, and cross-sec tion shape are also shown. The type of frame may be specified. Structural design starts with load analyses. These are computations to estimate loads which the structure must carry. Information from the preliminary drawings plus knowledge of weights of building ma terials, wind forces, occupancy loads, pressures from stored materials, and other loads are used in preparing the load analyses. The objective is accurate estimates of the loads which may reasonably be expected to occur on the structure during its lifetime. These are the design 100~ds. Next, stress analyses are completed. These are computations to pre dict total stresses that will occur in the structural members when the design loads are applied. Total stress is the force resisted by a member subjected to axial or shear load; or bending moment resisted by a member subjected to bending. Unit stresses, such as pounds per square inch, either direct or bending stress, cannot be calculated until the cross-section properties of the members are known. Usually a separate stress analysis is made for each kind of load. For example, in a simple shelter frame, separate analyses may be made for wind loads, dead loads, and snow loads. Then, the stresses produced by likely combinations of loads are investigated to determine which produce the critical stresses that control the design. Structural design is done after the critical total stresses have been estimated. It includes selection of materials, member sizes, configu rations, and fastenings for the load carrying parts of the building. For example, a beam is designed for bending by dividing the total bending 1.3 Design Philosophy 3 moment obtained from the stress analysis by the allowable unit bend ing stress for the beam material to be used. The result is the section modulus, an index of the beam size and cross-section configuration required. Structural design is often a "cut-and-try" process of successive se lection, calculation, and refinement or modification of the original se lection until satisfactory agreement is obtained between the selected size and the required size as revealed by design calculations. For ex ample, in the general case of a statically indeterminate frame, a stress analysis cannot be made until a trial design is available to use for estimating stiffnesses of the frame members. Final design drawings are prepared after completion of structural design. These, in combination with written specifications, should give all the information needed by the builder to erect an actual structure, which in all important respects is the same as the one created on paper by the designer. A novice in structural design is often at a loss to decide what is required in design drawings. The general requirement is to describe by drawings and specifications everything the builder needs to know beyond the dictates of standard practice or knowledge common to the trade, to erect the structure. Much helpful information can be gained by careful study of standard or typical drawings for the kind of structure to be designed. An activity closely related to structural analysis and design is check ing or investigation. This consists of verifying the adequacy of an ex isting design to carry specified loads; or in determining what stresses will result when specified loads are applied to a structure shown in an existing design. For example, various plan services offer complete con struction plans for many kinds oOight buildings. A structural designer or analyst often finds it necessary to check the structural design, be cause it may have been prepared for loading conditions that are not the same as for the locality or use being contemplated for the building. 1.3 DESIGN PHILOSOPHY An engineer must make many decisions based partly on judgment and partly on calculations to complete a design for a building. Exercise of judgment requires assessment of the consequences and penalties of failure of the structure. Iffailure such as complete collapse would result in loss of human life, the designer makes conservative selections of allowable stresses. He selects live loads based on extreme conditions. He exercises every precaution to make the likelihood of failure ex- 4 Analysis and Design Concepts ceedingly remote, even though these precautions may markedly in crease the cost of the structure. In contrast, the designer or owner of a simple shelter for cattle may tolerate a greater risk of failure to obtain worthwhile savings. Struc tural collapse might not be disastrous. If the designer is employed by a commercial organization or public service agency that publishes and distributes standard designs in large quantity, he must balance the extra cost imposed on all who use an overly conservative design against the potential penalty of failure in a few instances with a less conser vative design. The probability of failure always exists in any structure. The engi neer's task is to produce a design whose probability of failure is com patible with consequences of failure and the added cost of making it less probable. Engineering design is essentially an organized process of rendering judgment and making decisions. A skillful engineer utilizes all avail able means to support his judgment and decisions. In formalized courses, the emphasis is on analysis and design calculations, since these are among the most useful and powerful aids to judgment. 1.4 DESIGN GUIDES, AIDS, AND SHORTCUTS The design of any structure requires many detailed computations. Some of these are of a routine nature. An example is computation of allowable bending moment for standard sizes, species, and grades of dimension lumber. Numerous tables and graphs are available to minimize or eliminate such routine and repeated computations. Standard construction and assembly methods have evolved through experience and need for uniformity in the construction industry. These have resulted in standard details and standard components for building construction published in handbooks or guides. Many designs are for structures that must meet local or area building codes. These often specify design loads, quality of materials, standard construction details, and other design and construction requirements. These must be met, and therefore serve as design guides. Since structural design is often a "cut-and-try" process, the number of "cut-and-try" sequences often can be reduced by using information from previously executed designs for structures with comparable con figurations and loadings. Therefore, designers should be familiar with the sources for standard plans and designs. One source with which designers of farm structures should be familiar is the Midwest Plan