Light Agricultural and Industrial Structures: Analysis and Design

558 Pages·1988·14.71 MB·English

by  G. L. Nelson

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.
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Van Nostrand Reinhold
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Macmillan of Canada
Division of Canada Publishing Corporation
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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