The design of masonry structures and foundations

Front matter

FRANKLIN INSTITUTE LIBRARY

PHILADELPHIA, PA.

REFERENCE

Preface

Due to investigations, both analytical and experimental, that have been made in recent years of the properties of masonry materials, the forces to which masonry structures are subjected, and the behavior of such structures, masonry design and construction have largely passed from the status of an art to that of a science, much as did bridge design after the invention of methods for calculating stresses in truss members. The extensive use of concrete with the studies that have been made to improve its quality and to secure economy in its use, and the development of reinforced concrete, a masonry material capable of sustaining tensile strains and requiring a stress analysis in order to proportion the steel, have contributed largely to this changed status in the design of masonry structures. This scientific understanding of masonry design has widened the use of masonry to include many structures for which other materials formerly were used exclusively, although for the most part, notwithstanding their diversity, these structures can be grouped about a comparatively few representative types.

The soft and pleasing hues and outlines of masonry structures as well as their durability have appealed to builders from time immemorial, and masonry still offers the best means of building artistically and strongly with the economy that results from permanence. Moreover, there is an increasing demand in the design of engineering structures to secure not only stability and economy but something of elegance, grace and beauty in addition. Since good architectural treatment can be secured with but slight if any increase in the cost, it is appropriate that this phase of design should be given special attention in planning masonry structures.

The present volume was prepared with a view to furnishing a textbook embodying these ideas. An attempt has been made to offer a mode of analyzing forces and calculating resulting stresses and to indicate an acceptable method of design. Extended discussion of moot questions and of variations in design are purposely avoided with the belief that these are of interest to the

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PREFACE practicing engineer rather than of use in the class room. An effort has been made to keep in mind the aesthetic features of design in the selection of illustrative examples and to mention briefly the underlying principles of good architectural treatment whenever they could be formulated, although no more could be accomplished in this direction, perhaps, than to call attention to the desirability of giving thought to this important phase of design.

A knowledge of mechanics on the part of the reader is assumed. No attempt was made to treat reinforced concrete completely, the brief chapter on this topic being for reference and to provide for those readers who have not previously pursued a course in this subject. The study of the design of masonry structures should preferably be preceded by a course in the mechanics of reinforced concrete.

A brief historical review of masonry materials and construction was included with some recent patent litigation in mind as well as for its intrinsic interest. It is believed that the chapter on built up masonry and the one on plain concrete will serve to give the student the essential facts and principles involved, although the treatment is brief in each case. The consideration of foundations was reserved until after the loads that the superstructures transmit to the foundations had been studied. The arrangement and also the material included are the result of several years teaching the subject as well as some years practical experience in design and construction.

Lawrence, Kan. c. c. w.

January, 1922.

Contents

Page

Chaptek I. General Principles 1

Historical development of masonry construction. — Structures. — Materials. — Factors affecting design. — Adequacy. — Economy. — Aesthetics.

Chapter II. Masonry Laid in Mortar 22

Elements of stone masonry. — Building stones. — Classification of stone masonry. — Laying stone masonry. — Strength. — Stone bearing blocks. — Costs. — Brick masonry. — Manner of laying. — Strength. — Corbeling. — Terra cotta. — Stucco.

Chapter VI. Dams and Sea Walls 205

Forces acting on a dam. — Conditions of stability. — Design of section. — Design of overflow dams. — Dam foundations. — Single arch dams. — Multiple arch dams. — Rock fill and core-wall earth dams. — Architectural treatment of dams. — Breakwaters and sea walls.

Chapter VII. Retaining Walls and Quay Walls 249

Nature of materials retained. — Forces acting on a retaining wall. — Theory of pressures. — Design of section. — Effect of cohesion. — Types of wall sections. — Retaining grillage and slope walls. — Relative economy. — Architectural treatment of walls. — Quay walls.

Chapter VIII. Bridge Abutments and Piers 284

Functions of a bridge abutment. — Elements of design. — Forces acting. — Stability of an abutment. — Types of abutments. — Relative economy. Forces acting on a bridge pier. — Stability. — Hollow piers. — Relative economy. — Appearance.

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CONTENTS

Page

Chapter IX. Concrete Viaducts and Trestles 324

Types of masonry bridges. — Loads on bridges. — Girder bridges. — Three hinged arch bridges. — Slab bridges. — Trestles. — Stresses in trestle frames. — Cantilever concrete bridges. — Waterproofing bridge floors.

Chapter X. Culverts and Underground Conduits 349

Nature of loads and pressures. — Water way required. — Distribution of pressure through earth. — Stresses in pipe conduits. — Stresses in box culverts. — Rectangular culverts. — Arch culverts. — Stresses in a large conduit of curved section.

Chapter XI. Bins and Chimneys 371

Nature of pressures in granular materials. — Shallow bins. — Deep bins. — Janssen's theory of pressures. — Interspace bins. — Design of foundation slab. — Design of silos. — Forces acting on a chimney. — Design of brick chimneys. — Design of reinforced concrete chimneys.

Chapter XII. Masonry Forms and Falsework 407

General requirements. — Action of concrete in forms. — Typical forms. — Movable forms. — Arch centers. — Costs of form work.

Chapter XIII. Foundations on Dry Ground 431

Classification of soils. — Bearing capacity of soils. — Modes of failure. — Investigations of foundation sites. — Wall and column footings. — Grillages. — Pile foundations. — Pile driving. — Supporting capacity of piles. — Concrete piles. — Open well foundations. — Costs.

Chapter XIV. Open Found atinos under Water 477

Coffer-dams. — Sheet piling. — Open caissons. — Solidifying aquiferous strata. — Costs.

Chapter XV. Foundations under Water. Pneumatic Process. . 494 General principles. — Diving. — Pneumatic cylinders. — Pneumatic caisson. — Design of caisson. — Details of caisson. — Launching, sinking and sealing the caisson. — Pneumatic caissons under buildings. — Physiological and pathological effects of compressed air. Costs.

Appendix B 544

Index 547

Chapter I General Principles

Introduction. — Masonry structures are those built of stone, or of stone-like (petrous), materials. In a primitive form, they were among the earliest types of structures erected by man, and the development from the earliest and crudest dolmens to the magnificent masonry edifices and bridges of the present day has been one of slow stages.

The enduring character of masonry structures, the relative simplicity of the processes involved, the pleasing outlines usually obtained, together with the almost universal availability of the materials and the consequent moderate cost, render masonry construction one of the most important of the civil engineer's activities. Moreover, the importance of masonry construction is likely to be enhanced in the future by the growing scarcity of other structural materials, notably steel and timber, and the fact that the ingredients of concrete are almost unlimited in their raw state.

The processes involved directly in the design of masonry structures require a familiarity with the properties of masonry materials and the behavior of the structural elements to be used, a knowledge of the loads to be imposed, the stresses induced thereby and the method of proportioning the parts, and judgment in the principles of good architectural form.

The choice of type of structure, like reconnaissance for a railroad or an irrigation project, is of fundamental importance and requires the exercise of the engineer's best judgment, whereas, after the choice is made, the remainder of the design is largely based on fixed laws of mechanics and of costs. It will frequently happen, of course, that the choice of type cannot be made until after a detailed study of the relative stability, adequacy and costs of several different types has been made, in which event judgment will need to be exercised in assigning proper weight to the merits of the various types considered. In all of these respects, advantages of masonry structures commend this type of construction to the consideration of the engineer.

Historical Development of the Elements of Masonry Construction.— Perhaps the earliest elements of masonry construction were columns and slabs of stone. Even in prehistoric times, the prototypes of our masonry structures were found in the crude dolmens and cromlechs of barbarous man. These consisted of unshaped stones set as columns and partial walls and covered with a slab or slabs of stone without any attempt at attaching one to the other. Mortar was, of course, unknown, but piles of stone with huge natural slabs of stone spanning the intervals are to be found in the ruins at Maidstone and Stonehenge, England, and at various other places in Europe. Gradually these simple elements of support and cover were improved.

The stone masonry column was used by practically all of the early peoples, although it was developed chiefly by the Greeks, and the beautiful stone columns erected by them, copied in some cases doubtless from similar timber structures and suggested by various circumstances of that time, gave rise to the orders of architecture, the term "order" referring to the manner of proportioning and decorating the column. The Egyptian, copying in stone the bulging bundle of reeds that had been used as supports in early reed houses, gave another form of column.

The earliest device in stone construction for covering or bridging the span between supporting columns was the stone slab or beam, and the subsequent progress in masonry construction has resulted chiefly in man's ability to improve his method of spanning the interval between supports. Inclined blocks, forming triangularheaded openings over doorways and other spaces were employed in many instances. Corbel arches, formed by laying successive horizontal layers projecting one over the preceding one until the apex is reached, were employed by the early Persians, Assyrians and Greeks. This form of construction gradually developed into the pointed arch and later into the true arch, which was used on a small scale by these peoples as well as by the Greeks. However, it remained for the Romans to contribute the arch and dome on a large scale. Groined, cloister and other intersection arches are the product of later Roman builders and of the Middle Ages. The pointed arches, or gables, are chiefly of the Middle Age period.

The Chinese made use of the true arch at a very early time also. Probably the oldest known arch is one found in 1893 in the ruins of Babylonia, the estimated date of construction being 4000 B.C. It was constructed of well-baked plano-convex bricks laid as voussoirs with clay mortar. It has a span of 20 in. and a rise of 13 in., and is elliptical in form.1

The practice of using lime and lime mortar as binding agents for stones and bricks used in construction is of great antiquity, just when the custom began being unknown. Evidences of the early use of lime mortar may be observed in the ruins of Europe, Asia, Africa, Mexico and Peru. One of the earliest examples of the use of mortar in masonry construction is found in the Pyramids of Egypt, where the mortar used was similar to that in use at the present time. That the Ancients knew nothing of the essential character of lime and of mortar is evident from a perusal of the explanation given by Vitruvius2 of the action of lime, in which he says :

"The reason why lime makes a solid structure on being combined with water and sand seems to be this: that rocks, like all other bodies, are composed of four elements. Those which contain a larger proportion of air are soft; of water, are tough from the moisture; of earth, hard; and of fire, more brittle. Therefore, if limestone, without being burned, is merely pounded up and mixed with sand and so put into the work, the mass does not solidify nor can it hold together. But if the stone is first thrown into the kiln, it loses its property of solidity by exposure to the great heat of the fire, and so with its strength burned out and exhausted it is left with its pores open and empty. Hence, the moisture and air in the body of the stone being burned out and set free, and only a residuum of heat left in it, if the stone is then immersed in water, the moisture, before the water can feel the influence of the fire, makes its way into the open pores ; then the stone begins to get hot, and finally, after it cools off, the heat is rejected from the body of the lime.

Consequently, limestone when taken out of the kiln cannot be as heavy as when it is thrown in, but on being weighed, though its bulk is the same as before, it is found to have lost about a third of its weight owing to the boiling out of the water. Therefore, its pores being thus opened and its texture rendered loose, it readily mixes with sand, and hence, the two materials cohere as they dry, unite with the rubble, and make a solid structure."

1M. A. Howe, "A Treatise on Arches," p. xiii.

2 Marcus Vitruvius Pollio, a Roman architect and engineer, who lived during the reign of Caesar Augustus, wrote a treatise on architecture, " De Architectural covering the practice of that time. M. H. Morgan, Translation.

Etruscan masonry walls and other structures laid at Rome, Florence and other Italian cities contemporaneously with Roman building were composed of cut stone masonry without mortar, this being a distinguishing characteristic in the ruins at the present time. The use of lime mortar was a decided step in advance over the bedding of stone in clay, as had been the custom by many of the ancient peoples.

The Romans discovered a way to improve lime and to give it the ability to set up or harden under water, and even manufactured an hydraulic cement similar to the natural cement of the present day. Vitruvius, the foremost architect of imperial Rome says in his Ten Books on Architecture (Book II, Ch. 6) "There is also a kind of powder which from natural causes produces astonishing results. It is found in the neighborhood of Baiae and in the country belonging to the towns about Mt. Vesuvius. This substance, when mixed with lime and rubble, not only lends strength to buildings of other kinds, but even when piers of it are constructed in the sea, they set hard under water." From certain formations, chiefly in Tuscany, the Romans obtained a volcanic stone (tufa) which, when calcined and powdered, made a natural cement. The author observed mortar of this type in structures at Rome erected a century or more B. C, although the mortar may have been placed there in re-pointing. In building the Los Angeles aqueduct about a decade ago, a tufa cement was employed,1 and experiments made at the time showed that powdered tufa mixed with lime gave a cement that would set under water and probably resembled the cement used by the Romans of the Republic.

However, it remained for John Smeaton, an English engineer, to discover in 1756 after an extended series of experiments that the lime made from the clayey limestone would harden under water and that a true hydraulic cement could be made by an argillaceous admixture. This material he used in the construction of the famous Eddystone Lighthouse in 1757, which "after being buffeted by the storms of eighty years, the Eddystone stands unmoved as the rock it is built on, a proud monument to its great author."2 To an extent, Smeaton's exploit was a rediscovery of a principle that was known in a measure to the Romans, but had not been utilized in Western Europe. The

2 Proc. Inst. Civil Engineers, vol. 1.

Roman cement was in no sense a "lost art," for there has been no break in the continuity of its use up to modern times.

In 1810, Edgar Dobbs of Southwick, England, obtained a patent for the manufacture of artificial Roman cement by mixing carbonate of lime and clay, molding into bricks and burning. In 1813, Vicat began the manufacture of artificial cement in France as did also James Frost in England in 1822. In 1824, Joseph Aspdin, a bricklayer of Leeds, took out a patent on an improved cement by combining limestone with clay and then burning and grinding. This he called "Portland Cement" because when it hardened it produced a yellowish-gray mass resembling the stone from the famous quarries at Portland, England.

Canvass White, the engineer in charge of the construction of the Erie Canal, began the manufacture of natural cement near Fayetteville, N. Y. in 1818. Later, in 1828, a large plant was built at Rosendale, N. Y. for the manufacture of this kind of cement, and for a while, Rosendale cement became synonymous with natural cement in America.

Since the middle of the last century, various improvements in the processes of cement manufacture have been instituted which have greatly lessened the cost of the product, particularly with regard to portland cement, thus rendering hydraulic cement one of the most available building materials. First England led in the production of cement, then Germany, but since the beginning of the present century, the United States has surpassed all other countries.

Perhaps the most significant development in masonry construction was the introduction of reinforced concrete, by which masonry could be made to withstand tensile stresses comparable with its compressive strength. The first authentic record of the use of reinforced concrete was at the World's Fair at Paris in 1855, when a small rowboat built by M. Lambot of mortar reinforced with wire netting was on exhibition. However, iron bars were used at least a century earlier by architects in Italy for reinforcing the molded statuary in parks and gardens (e.g. statuary groups in Ville d'Este at Tivoli). In 1865, Francois Coignet explained the principles of reinforcing beams, slabs, arches, etc. and in 1869 took out a patent on the process. In the same year, F. Joseph Monier took out patents covering many of the details of this new form of construction, and he has sometimes been called the "father" of reinforced concrete. He was a gardener and used the device for decreasing the thickness of walls, reservoirs, etc. by reinforcing them with a trellis of iron.

While the French and German engineers were developing Monier's method of reinforcement, American engineers began a more scientific study of the new process, and W. E. Ward, who began placing rods in the bottom of his beams and slabs in 1875, was probably one of the first to get the correct conception of the function of the steel. Various systems of reinforcement were developed by different engineers to which the names of the inventors have been attached.

The studies that probably have done the most to place reinforced concrete construction on a scientific basis were the investigations made at the University of Illinois, University of Wisconsin and other laboratories, for they have revealed the character and behavior of beams, columns and slabs, and reduced the principles involved to formulas and laws that can be applied in practical design.

The development of the masonry arch has been one of the notable achievements in engineering. Until the last quarter of the last century, in the main the arch had been built empirically, although various theories had been proposed which purported to explain the nature of arch action. Indeed, arches of considerable length had been built in this empirical fashion, such as the highway arch at Trezzo, Italy, with a span of 251 ft. built in 1377, the Claix, France, highway bridge, built in 1611 with a span of 150 ft., and the Maidenhead railway bridge built in 1838 with a span of 128 ft.

The Elastic Theory of arch action, developed at the close of the last century, is one of the most notable contributions to the design of masonry structures. This theory, based on the equations deduced by Weyrauch in 1879 ("Theorie der Elastigen Bogentrager"), was first applied to steel arches and later to fixed masonry arches, in which application various American as well as foreign engineers contributed. The elastic theory brought confidence in the design of fixed arches with a resultant economy in design and a wider application to the construction of bridges, dams, conduits, and other structures.

It is not necessary in this brief paragraph to point out the contributions to the science of masonry construction made by the many bold-spirited engineers who led the way in constructing the many structures of record size, and of those who rendered possible the splendid achievements in engineering by developing foundation processes that would furnish support adequate to the loads that were unavoidable in the successful carrying out of such construction. Enough has been said, perhaps, to indicate the gradual growth of masonry construction processes and to show the gradual emergence of the science from the empirical art, an evolution that is entirely analogous to that of nearly every other phase of engineering science.

Fig. 2. — An Old Roman Bridge over a Branch of the Tiber River.

Historic Masonry Structures. — The permanence of masonry construction is illustrated in the many structures remaining from the days of the Greeks and Romans, who were primarily masonry builders. Figure 1 shows the famous Pont du Gard, an aqueduct built across the Gard river near Nimes, France, about 17 miles from Avignon. It consists of three tiers of arches, the total height being about 161 ft. and the length about 884 ft. It was built about 15 B.C. and was laid without mortar, although the aqueduct itself is lined with cement mortar, which is still very hard and in fair condition. The channel, which is located above the top tier of arches, is about 2}4 by 5 ft. in cross section and is covered with flat stones about 3 by 12 ft. in size and 14 in. thick.

A well preserved cement concrete bridge along the famous

Amalfi Drive in Italy, near Naples, built in about the sixth century A.D. well illustrates the durability of this type of construction.

The arch ring of the bridge shown in Fig. 2, a bridge over a branch of the Tiber near Rome is nearly 2,000 years old, although the superstructure is of a later date, about 1475.

Figure 3 shows the Cloaca Maxima, or Great Sewer, built about 700 B.C. which still drains the area between the Palatine and Capitoline Hills occupied in part by the old Roman Forum.

Fig. 3. — The Cloaca Maxima.

It is about 10}^ ft. wide and 14 ft. high, although nearly submerged at present by the deposit along the shore of the Tiber. The arch consists of three rings of " peperino " voussoirs carefully shaped and the remainder is built of tufa and peperino.

The permanence of the brick masonry built by the Romans is illustrated in Fig. 4 which shows a corridor of the Palace of Caligula built 37 A.D. These bricks were burned in kilns (a device invented by the Romans) similar to those in use today and were comparable in quality to modern bricks.

Factors Affecting the Design of Structures— A structure is well designed if it meets the needs of the situation completely at a minimum cost. In order that it may do this, all factors and conditions affecting the design must be taken into consideration. As indicated in a previous paragraph, not only the loadings to which the structure will be immediately subjected must be considered, but the function of the structure as related to the primary project of which it is a part, and the possible changes in conditions at a future time, should be considered as well.

Fig. 4. — Corridor in the Palace of Caligula.

In general, the choice of type of structure as well as the general features of the design will be determined by the following considerations:

1. Adequacy (a) Stability

(6) Performance of function

2. Economy

(a) Initial cost

(b) Relative permanence

(c) Maintenance cost

3. Legal requirements

4. Aesthetics

(a) Harmony

(b) Proportions

(c) Ornamentation.

Which of these primary factors is most important will depend upon the circumstances in any particular case. Usually stability will be of primary importance, but where human life is not at stake and possible loss resulting from failure is measurable in dollars, designing for extreme conditions may not be desirable because of the greater cost involved. The possible loss would perhaps not pay for the added cost of construction required to secure complete stability. On the other hand, where a failure would result in loss of life, absolute stability should be made the paramount consideration. In setting off expense against architectural treatment a wider variance of opinion may arise.

Adequacy. — In general, any structure should be designed so as to be stable under the loads and forces to which it may be subjected with a reasonable margin, or factor of safety. This factor of safety is chiefly an "ignorance coefficient" and is intended to provide for the unknown features of the loadings and forces to which the structure may be subjected and also for the variability in the quality of the structural materials used. What this factor should be depends upon circumstances; the greater certainty with which the factors of design are known, the more nearly may the resisting capacity be made equal to the forces encountered. The conditions of stability for different types of masonry structures will constitute the bulk of the succeeding chapters, and the appropriate margin of safety will be discussed in each case.

A structure should be adequate to perform the function required of it, in addition to being stable under the loads to which it is subjected. Masonry structures seldom constitute a project in themselves but are usually a secondary part of a larger project designed for some economic use. Thus, a dam may be a part of a water supply, irrigation system, or river improvement; a retaining wall may support an earth fill on which a factory carrying heavy machinery may rest; a bridge may be a part of a highway or .a railway; a bin may be a part of a large storage plant for grain, coal, ore or crushed stone; a chimney is always a subsidiary part of an industrial plant of some sort; and foundations are inherently a subsidiary part of some principal structure. The design of masonry structures should contemplate, therefore, not only the immediate loading and other conditions, but should take into consideration the complete relationship between the structure and the primary project of which it is a contributing part in order that it may perform properly the function required of it in the realization of the purpose of the primary project.

Whether a structure should be designed for the most severe conditions that may possibly come upon it depends upon the circumstances. Where human life is at stake, of course, nothing should be lacking to make a structure as secure as possible, but frequently a failure or partial failure would involve nothing more than a money or economic loss in the event that it should occur. In such a case, there is a limit to the amount that can be spent to provide for the extreme contingency, which may never happen. In general, from a strictly economic point of view, if L represents the loss that would occur in the event that the extreme conditions should occur, -and 1/N, the probability of its occurrence, then L/N represents the total outlay that would be justified in preventing the loss by providing for the extreme conditions.

The interest on the fixed investment where provision is made for extreme conditions may be so great as to render extreme conservatism unwise. For example, $1 at 6 per cent compound interest amounts to $184.20 in 50 years and $339.30 in 100 years, hence, to provide for a condition that will occur in 50 years, the investment of $1 in the initial cost must save $184.20 at the time the contingency occurs, or $339.30 in the case of a contingency occurring in 100 years. Thus if loads are to be doubled in 50 years, the investment of $1,000 could be made at the time of initial construction as easily as $184,200 at the end of 50 years, according to this basis. However, the added depreciation and maintenance of the structure during the 50 years would greatly alter the above numerical relation, and therefore, only the general reasoning holds true.

Because of the durability of masonry structures, possible future changes in the conditions affecting design should be taken into consideration. These changes may be either physical or economic, the latter being usually related to the function of the major project of which the masonry structure forms a part. For example, the design of a culvert or bridge abutment for a railroad, which would not admit of an extension for a second track would be a serious error when a study of conditions would indicate the necessity of a second track in the near future.

Permanence of Masonry Structures.— When properly designed, to provide for both physical and economic conditions, both of the present and of the future, masonry structures constitute the most permanent type of construction that an engineer can build. Yet an erroneous impression is sometimes created by ascribing the term permanent to masonry or to any other type of construction, because other considerations than that of durability enter into the account. The failures of dams, retaining walls, reinforced concrete buildings, conduits, etc. indicate that one cause of a lack of permanence is the lack of knowledge concerning the behavior of the materials and more particularly concerning the character and magnitude of the forces and other conditions to which the structure may be subjected.