Engineering in masonry

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National Concrete Products News, 14, 1, pp. 9-12, 1966-05-01

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Engineering in masonry

Plewes, W. G.

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A N A L Y Z E D

ENGINEERING

IN MASONRY

BY

W. G. PLEWES

R E P R I N T E D F R O M N A T I O N A L C O N C R E T E P R O D U C T S N E W S v o L . 1 4 , N o . I , F I R S T QUARTER t966, p.9. T E C H N I C A L P A P E R N O . 2 2 t O F T H E D I V I S I O N O F B U I L D I N G R E S E A R C H OTTAWA M A Y T 9 6 6

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I { A T I O N A L R E S E A R C H C O U N C I L NRC 9040 P R I C E I O C E N T S

ENGINEERING IN T\4ASONRY

A s h o r t r e v i e w i s g i v e n o f r e c e n t r e n e w e d i n t e r e s t i n m a s o n r y d e s i g n . T h e p r i n c i p a l r e v i s i o n s r e g a r d i n g t h e s t r u c t u r a l d e e i g n o f m a s o n r y embodiedin the National Building Code 1965 are given, particularlythe introductionof an analytical design method for plain maeonry. Greater attention ie also given in the Code to reinforced masonry. The paper emphasizes the importance of including wind and earthquake effects in the structural analysis of masonry.

TECHNIQUES DE LA MAgONNERIE

L r a u t e u r d o n n e u n b r e f e x p o s 6 d e s a m 6 l i o r a t i o n s r 6 c e n t e s b t a c o n c e p -t i o n d e s o u v r a g e s d e m a g o n n e r i e , d u e s b u n r e n o u v e a u d r i n -t 6 r 6 -t e n c e d o m a i n e . I l e x p t r i q u e le s r 6 v i s i o n s p r i n c i p a l e 6 s e r v a n t a u c a t c u l d e s o u v -rages de magonnerie mentionn6s dans lt6dition de 19 6 5 du Code national du bdtiment, en particulier I'introducf,ion drune rn6thode analytique de c a l c u l d e l a m a g o n n e r i e o r d i n a i r e . L a m a E o n n e r i e a r r n 6 e f a i t 6 g a l e -ment Itobjet drune 6tude plus approfondie dans le Code. Lrauteur soul i g n e soul e f a i t q u t i soul e s t i m p o r t a n t d e t e n i r c o m p t e d u v e n t e t d e s t r e m b soul e -m e n t s d e t e r r e p o s s i b l e s d a n s I e c a l c u l d e s o u v r a g e s e n -m a g o n n e r i e .

ENGINEERING

IN MASONRY

by W. G. Plu*ot

A Manitoban, Mr. Plewes is with the Building Structures Section, Division of Building Besearch, National Besearch Council. He took his Master of Applied Science at Queen's Universit5r in 19t1. The article is, in substance, his adilress to the NCPA Convention on January 12th in Calgary, with some later changes by the author.

-Editorial _Note.

In the 1960 and previous editions of the National Building Code, the design method given for plain ma-sonry was that developed from ex-perience over many years. It con-sisted of simple rules concerning minimum thicknesses for walls and maximum spacing of supports, which for most normal situations were conservative enough to take care of variations in materials. work-manship, loads, eccentricities of load, and other factors afiecting masonry strength. Little or no engi-neering analysis was usually re-quired. Allowable stresses for axial compression were given, but the nominal axial stresses rarely govern-ed the design because of the con-$ervative minimum required thick-nesses. No allowable stresses in shear or tension were given as they were not necessary when a general structural analysis was not expected. Over the years the traditional "rules of thumb," or conventional design methods, have gradually be-come more liberal. For example, walls today are thinner than they were 50 years ago. This liberalizing process could only be carried so far, however. if masonrv desisn was to retain iti traditionai simpl-icity. The limit had about been reached. If masonry structures were to be de-signed and built in a more advanced and economical fashion, consider-ably more materials research and en-gineering analysis would be neces-sary.

In a previous paperl, the author mentioned some of the remarkable 15- to lS-storey bearing wall build-ings of thin wall construction built in Europe on the basis of research and analysis. It was surmised that in this country any such departure from conventional desisn would not be-come regular praclice or be reflected in our Codes for some years to come. Since 1963, however. events have progressed considerably faster than

was anticipated. The 1965 edition of the National Building Code.now per-mits new design procedures that offer more freedom.

Parallel with developments in Europe, there has been a growing interest and awareness in North America of the greater potentialities of masonry. Although advanced en-gineering design of masonry does not appear to be widely accepted in American Codes as yet, a number of interesting examples of high bearing-wall buildings have recently been built in the United States. In the main, these were designed on a con-ventional basis, but they nonetheless illustrate a trend. Articles and letters in recent journals have discussed the merits of bearins-wall desisn. A re-cent letter in thJEngineeriig News-Record refers to a masonry building designed using a complete computer analysis of the building. Some of these modern constructions have dm-ployed brick but concrete blocks have also been used. The ASA Com-mittee on Masonry is currently con-sidering specifications that will pro-vide for analytical design of

ma-sonry.

In Canada, too, there has been a similar development. A 12-storey concrete block building of cross-wall construction has been built in the Ottawa area and is, perhaps, a fore-runner of what may eventually be-come commonplace. (It was describ-ed in a recent issue of NCPA News.)

NATIONAL BUILDING

coDE 1965

During the preparation of the 1965 edition of the National Build-ins Code the Revision Committee on Masonry decided at the outset to provide a code that would allow de-partures from some of the restric-tions necessary in the conventional approach to design. As this was not a matter of rnerely copying some

al-ready existing code, a considerable amount of study and deliberation was required. The result was a code. that contains certain innovations which are a pioneer effort at code writing and place it among the most modern in the world. Essentiallv. Part 4.4 of the Code now contairis provisions fot conventional design of plain masonry, analytical design of plain masonry, and a greater em-phasis on reinf orced masonry. These will now be discussed in somewhat greater detail

Conventional design of plain ma-sonry has generally been retained as it was in the previous edition. In all probability this will continue to be the method used for the majority of ordinary masonry construction. There are two changes worth men-tioning at this time.

In Table 4.4.3.4 (National Build-ing Code) the allowable stresses are unchanged, except those for hollow load-bearing units, where the allow-able stress has been raised about 15 per cent for units having a com-pressive strength greater than 1,000 psi. For example, with type M mor-tar the allowable stress is now 100 psi. For weaker units, it is unchang-ed, that is, it is 85 psi.

A second change, intended for clarification, is the insertion of diagrams to aid in interpretation of the minimum thickness require-ments.

Sub-section 4.4.9 contains the most signfficant new development in masonry design. Its title is "Detailed Structural Analysis for the Design of Load Bearing Masonry." To quote from the first clause:

"The design of masonry may be based on a general structural analysis and the requirements of this Sub-section, provided there is special engineering or architectural supervision to en-sure that all joints are filled with mortar and that the

struction and workmanship re-quirements of this Section are satisfled in all respects." What this basically means is that the design of masonry may be freed of the conservative restrictions on thickness and slenderness that apply in conventional design, provided a careful analysis is made of the actual vertical and horizontal loads, the efiective length of members, eccen: tricities of load, load concentrations, stresses, levels, etc., and provided also that there is strict supervision over workmanship in the field. For this purpose allowable stresses are now given for shear and modulus of rupture as well as for compression. In conventional design an allow-able stress is given for compressive str€ngth only. This stress is set low enough so that it can be applied to all walls having slenderness ratios up to 18 for qoncrete block and 20 for brick, which are the maximum slendernesses permitted.

The analytical design method follows a more logical procedure. The basic compressive stresses per-mitted in Table 4.4.9.8 for concrete block under axial load are identical to those for conventioqal design at an h/t ratio of 20. For gr.eater or lesser slendernesses stress decreases or increases are applied. For eccen-tric loads the stresses detennined for axial loads must be reduced by suitable factors.

The stress factors just referred to may be found in Table I. Column 2 applies to the case of zero eccentri-city of vertical load. As just men-tioned, the factor to be applied to the basic stresses is 1.00 at h/t : 20. Above and below this there are de-creases and inde-creases. When the vertical load becomes actually or effectively eccentric, the allowable stressbs must be further decreased in accordance with the factors given in columns 3, 4 and 5. The maximum eccentricity allowed is l/3 of the thickness of the wall. As one would expect, for very slender walls with

TABLD I

NATIONAL BT]Iil)ING CODE OF CAI\IADA

TABLE 4.4.9.C

markedly eccentric load the allow-able stresses approach zero.

Although tests in Switzerlandz in-dicate that higher slendernesses are possible, the maximum slenderness for walls or piers is limited toh/t: 30. For free standing piers and para-pet walls the limits are reduced to 16 and 8, respectively.

These requirements were adopted

after studies of the research and recommendations of organizations such as:

National Concrete Masonry Association,

Structural Clay Products Association,

Building Research Station, Garston, Great Britain, Structural Ceramics Research

Unit, University of Edin-burgh, Great Britain,

Federal Institute for the Testing of Materials, Zuich, Switzer-land,

Central State Research Insti-tute. Moscow.

For this Code. the Committee in-tentionally selected stress factors somewhat on the conservative side, as compared with the research evi-dence, but the factors do in some cases represent a considerable de-parture from past practice.

Within this context a designer will have to analyse each structure, taking into account its actual dimen-sions, supports, openings and forces, and arrange the component members so that the resultant forces remain sensibly axial on the element within the prescribed limits of eccentricity and stress.

The actual required thicknesses for walls in high cross-wall con-struction will depend very much on the height and layout of the build-ing, the floor construction, and the means used to reduce eccentricities to a minimum. There are not as vet details available of an actual con-crete block building designed ac-cording to Sub-section 4.4.9 of the Code. As noted before. much of the background for the new develop-ments was based on research and experience with brick. In develop-ing Sub-section 4.4.9, however, concrete block data were considered. and the method is believed applic-able.

To gain some indication of what might be expected in an actual de-sign, calculations were recently made for an assumed interior cross-wall of a lO-storey building. Twenty-foot spacing of cross-walls, cored slab floors, and 9-foot storey heights were

assumed. The building was taken as

44 feet wide, with corridors down the middle so that a typical cross-wall could be assumed as divided

TABLE tr

C0MPARTSON OF WALL fiTTCKNESS REQTimEMEI\TS

TABLE OF STR,ESS FACTOR;S Stress Factor

Eccentricity of vertical loading as a proportion of the thickness of the member

r/6

0.97 0.90 0.83 0.76 0.70 0 . 6 1 0.56 0.48

r/3

r/4

1 . 3 0 t.23 1 , . t 7 1 . 1 1 1.06 1.00 0.94 0.89 0.83 0.77

o.72

0.86 0.78 0.70 0.63 0 . 5 4 0.47 0 . 3 8 0 . 3 1 0.23 0 . 1 6 0.08

o.73

o.64

0.54 0.45 0.36 0.27 0 . 1 9 0 . 1 0 0.00 0.00 0.00 10 t 2 l 4 1 6 1 8 20 22 24 26 28 30

Linear interpolation between values for the stress factors is permissible.

Thickness Required by Conventional Design, in.

Estimated Thickness Required by Analytical Analysis, in. ( 1 ) Tenth Store.y Sixth Storey First Storey ( 2 ) t 2 1 6 20 ( 3 ) 6-8 Hollow 8 Semi-solid* or 12 Hollow 12 Semi-solid* *Assumed 75 per cent splid.

into two approximately 2O-foot walls 10 storeys high. This is a common and conservative assumption.

Considering the vertical loads only, the calculations indicated that the wall thicknesses shown in Col-umn 3, Table II, would be of the order required for an actual concrete block building. In comparison with the thicknesses required by con-ventional design (column 2), sub-stantially reduced thicknesses appear possible.

It still remains, however, to con-sider the effects of wind or earth-quake. Here again it is impossible to generalize, but to obtain some point of reference further calculations were performed on the above as-sumed wall.

As far as earthquakes are con-cerned there is little doubt that in zone 3 areas reinforcement would be necessary to provide sufrcient re-sistance to lateral forces. Zone '1.., however, implies that earthquake shocks will probably be less severe, and in this case an analysis is neces-sary to determine whether over-stress of plain masonry will be caused.

The calculations indicated that for wind or a zone 1 earthquake, shear and tensile stresses in the assumed wall would be satisfactorv. but the masonry would be overslressed at the base in compression. This could be overcome by increasing the wall thickness by about one third with, of course, some loss of economy.

This example should not be con-strued as typical of all possible buildinss. Refinement of analvsis and mo?ification of the layout might very well alter the picture. (For one thing the assumed building was rather narrower than many build-ings.) In some cases it might be possible to thicken or reinforce only the critical sections. The example does show the order of masnitude of stresses and that desisns ofhieh-rise cross-wall buildings ihould i-ot ig-nore wind or earthquake efiects even in zone 1 areas.

REINT'ORCED MASONR,Y Cases arise in masonry design where the effects of the loads and forces exceed its strength capacity or reduce its economy. Here the design-er can turn to reinforced masonry. Such instances are:

(a) where a plain wall would have to be excessively thicken-ed to keep the stresies within allowable limits;

(b) where the lateral or bend-ing forces are too large to keep the resultant line of action of the vertical load sensibly axial

without requiring an excessive-ly thick wall;

(c) where increased resistance to earthquake or blast efiects are desired.

The term "reinforced masonrv" as used in this paper does not mean masonry with a few steel rods or wires placed in the joints only to re-sist shrinkage or replace headers. Such arrangements have their place, but the masonrv is still essentially "plain". What ii meant is masonry analysed and designed as for rein-forced concrete. In fact, it is stated in Article 4.4.7.6(2) that "rein-forced masonry shall be designed in accordance with the principles and procedures set forth for the design of reinforced concrete in Section 4.5 except as specified in this Sub-section." A certain amount of judg-ment must be exercised in applying the rules of reinforced concrete to masonry, but the same principles apply and the same close attention should be paid to such problems as shear, bond, anchorage, and splices.

The Code gives the allowable stresses for compression, shear, and bearing in terms of the compressive strength of masonry f'-. For ex-ample, the allowable flexural com-pression is 0.33f'-. For a given strength of unit, f'- may be selected from a table of conservative values or, alternatively, it may be deter-mined by a specffied test method.

The minimum size of principal wall reinforcement is specified as 3/8 n. diameter, except that "ap-proved wire reinforcement used as temperature steel may be considered as part of the required reinforce-ment." The maximum spacing of wall steel is 4 ft. on centre, and the minimum percentage in the horizon-tal direction is 0.0015 per cent. Special formulae and reinforcement requirements are given for columns. The construction details of rein-forced masonry are amply described in the brochures and handbooks of the industry. Several difierent schemes are employed, namely:

(a) masonry laid up with rein-forcement placed in the joints;

(b) masonry with reinforce-ment placed between wythes or other built-in internal spaces, which are completely filled with grout;

(c) masonry of hollow units in which some or all of the cells are placed in vertical alignment to form continuous vertical spaces for reinforcement and grout.

The latter arransement is. of course. particularly apfropriate with con-crete block.

Actually, reinforced masonry was

covered in the National Building Code of 1960 bv a brief reference to American Stairdards Association Standard 44I.2 - 1960 Building Code Requirements for Reinforced Masonry. This seems to have missed the attention of some people. The more detailed requirements of Na-tional Building Code 1965 may now encourage its use where it is appropriate.

MATERIALS, WORKMANSHIP

AND CONSTR,UCTION

Materials, workmanship and con-struction have always been impor-tant, even in conventional construc-tion, but if masonry is to be used in a new and more scientific way, they become of even greater concern. In reporting on the modern high-rise bearing-wall buildings already con-structed in Europe, writers empha-size the need for careful selection and testing of materials.

If such buildings are to be built on a regular basis and if the confi-dence of engineers and the building industry is to be gained and main-tained, masonry materials will have to be of consistent quallty and pre-dictable structural performance. All efforts of the industry will have to be devoted to maintaining such factors as high and consistent quality of units and mortar and regularity of dimensions. (This will perhaps en. courage those who work on CSA Specification Committees. )

Construction is important. A masonrv wall has too often been consideied something that is just "thrown up". Control of batching and mixing of mortar, and the hand-ling and laying of units, will have to be of highest quality. This goes hand in hand'with good inspection.

To be more specific: if the analyt-ical design of a thin masonry load-bearing wall is predicted on filled joints and joints of given size, it will be of the utmost importance that these are attained on the job. Simi-larly, reinforced masonry has been found to act much like reinforced concrete. If the spaces around the bars are not completely filled with mortar or grout as called for on the plans, the resemblance to reinforced concrete may rapidly disappear. CONCLUSION

Recent developments in building, in research, and code writing are opening the door to more advanced types of masonry structures such as multi-storey buildings of relatively slender cross-wall construction. As far as strensth is concerned this should be thought of as "engineering in masonry" as opposed to masonry

construution, as it has commonlY been considered in the past. It may be that some people should and will specialize in this work and become masonry designers in the same waY that others are steel. timber and re-inforced concrete designers.

As with all building systems there is still much to learn about advanced

on -the evidence of research and testins.

Th6 introduction of analytical de-sign of masonry may be an instance where research and the codes are a step ahead of the construction in-duitry itself, a reversal of what is normally true. New problems in con-struction may become aPParent, and it is hoped that all those who become engaged in the building of a more advanced multi-storey masonry structure will, particularly during these early stages, treat it as a new experience in building. Lessons ledrned in the office and on the job should be made available for the benefit of the masonry industry as a whole. The behaviour of such struc-tures in service should be observed and reported, particularly responses to moisture and temperature changes and their durability characteristics. (One question to be studied is how well masonry reinforcement resists corrosion.) In this way mistakes once made will not be repeated and a fund of information and experience will be built up to ensure more and rnore adequate construction stand-ards. When this happens engineering in masonry will have come of age. REF'ERENCES

lPlewes, W. G. The Structural Action of Concrete-block Bearing Walls. National Concrete Products News. Vol. 11, No. 2, 1963 (NRC 8 3 3 7 ) .

2Haller. P. The Technological Prop-erties of Brick Masonry in High Buildinss. Schweizerische Bauzei-tung,76(23), 411-419, 1958 (Na-tional Research Council, Technical Translation No. 792, Ottawa 1959).

This paper is a contribution from the Division ol Building Research, National Research Council ol Cana-da, and is published with the approv-aI of the Director of the Division.