Technological Properties of Brick Masonry in High Buildings

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In recent years changes have taken place in the manner of using brick masonry and to some

ex-tent in the materials of masonry. These

develop-ments have in some cases resulted in changes in the structural properties and also in the

weather-resisting ｰｲｯｰ･ｲｴｩ･ｾ of the masonry. The need for

technical information to guide designers and builders in the use of such developments is therefore a

con-tinuing one. Accordingly this translation has been

prepared of an account of studies made in Switzer-land of structural properties of brick masonry, particularly as applied to the construction of tall

buildings, and of the methods by which ､･ｳｾｧｮ

infor-mation was derived. It is considered to contribute

a great deal of useful information on the structural properties of brick masonry.

The Division of Building Research wishes to record its thanks to Mr. D.A. Sinclair of the Trans-lations Section of the National Research Council Library for preparing this translation.

Ottawa

February 1959

R.F .. Legget, Director

TECHNICAL TRANSLATION 792

Title: The technological properties of brick masonry in

high buildings

(Die technischen Eigenschaften von Backstein-Mauerwerk fur Hochhauser)

Author: P. Haller

Reference: Schweizerische Bauzeitung, 76 (28): 411-419,1958

HIGH BUILDINGS

A lecture given on the occasion of the Meet-ing on High BUildMeet-ings of the Association of SWiss Tile and Brick Manufacturers of the 1st of March 1958 at the ATH in Zurich by Dipl. Ing. P. Haller, Section Head of the Federal Institute for Material Testing and Research, Zurich.

The fact that a number of 9-otorey apartment houses as well as an 18-storey residential skyscraper are being built at the present time on the outskirts of Zurich is proof of the great advances that have been made in recent years in the

manufacture of brick. The beginning of the entire development

was marked by the purchase of a 6 m. high, 500-ton press which was installed in the newly erected outside station of the EMPA*

in Schlieren, 1946. Before this press was available it was

impossible to carry out the masonry tests which alone could provide a reliable basis for determining the admissible load

capacities of masonry. The first tests with l-storey high

masonry pillars or walls, which were financed and supplied with materials by the Building Inspection Services of Zurich, Berne and Basle, the Swiss Association of Brick and Stone Producers and by the firm AG. Hunziker and Co., have produced results Which made possible a greater utilization of brick masonry and thus also the construction of the high buildings in Basle.

At the same time, great efforts have been made by the

brickworks possessing suitable raw materials. The quality of

masonry bricks has been improved so that the permissible

stresses have been raised still further. The tests have brought

to light a number of hitherto unknown properties of brick which

are of fundamental importance. Only a knowledge of the greatest

The 18-storey brick skyscraper in Zurich-Schwamendingen

possible number of factors affecting the bearing capacity of the masonry and a deeper insight into the complex conditions have made it possible to eliminate undesirable effects and to determine the bearing capacity as well as other properties of importance from the point of view of construction technique as a function of these factors.

First of all we shall illustrate the testing methods and the characteristic forms of distruction of the two-material

sys-tem by a few pictures. A natural stone pier (Fig. 1) in the

500 ton press shows the characteristic cracks over the head

joints. Other cracks, however, developed alongside these as a

result of local stress concentrations produced by irregularities of the bearing faces.

A similar cracking pattern in brick piers and in insulat-ing brick masonry where only slight overlappinsulat-ing guarantees the joint and the working together of the two brick shells is shown

in Fig. 2. In the artificial stone masonry the cracks again

occur always as a continuation of the jOints, alongside other

cracks due to local stress concentrations. In the 38 cm. brick

masonry shown in Fig. 3 cracking over the joints can be recog-nized in three shells, especially in the pier on the left-hand

side. Proper working together of the individual shells can be

obtained only by perfect jointing. Fig. 4, a wall of cement

bricks, shows that in the case of eccentric loading vertical

thrust forces occur which must also be absorbed by these headers. If we load masonry which has no mortar joint, then,

de-pending on the evenness of the bearing faces, the total force will be transferred at a larger or smaller number of points. These point forces produce tensile stresses in the brick which cause it to crack in a direction parallel to the direction of

the force when the latter attains the tensile strength. The

function of the mortar joint is to distribute the forces of contact over a larger area and thus to reduce the tensile stresses in the brick.

both elements expand transversely to the normal strength, i.e.,

perpendicularly to the direction of the force. Since both

materials must expand transversely to the same extent as a

consequence of the adhesive or friction forces, tensile stress-es perpendicular to the direction of the force arise in the brick on account of the generally greater transverse elongation

of the mortar. These tensile stresses, like those due to the

local concentrations of force, also contribute ultimately to the cracking of the brick in the direction parallel to the

force and perpendicular to the bed joints. As a result the

headers in small individual columns will crack on both sides

of the expansion joint. Thus the masonry bricks are not

crush-ed by compressive forces, but are torn open by tensile forces. From these discoveries, the following important conclusions are drawn:

1. The bricks should be as flat-faced as-possible (i.e.,

no tWisted bricks!) in order to keep the jOint thickness to a

minimum. Joints that are too thick increase the transverse

tensile stresses. For high quality masonry, therefore, the

joints should be kept to thicknesses of 10 - 12 mm.

2. The head joints must not be too wide. Not only on the

visible faces, but also in the wall cross section the expansion joints must be covered by headers, or the bricks must be bound

in. Thus it is absolutely necessary to have satisfactory bond

in order to achieve maximum compressive strength.

3. The tensile force cross-section of the brick must not

be reduced ･ｸ｣･ｾｳｩｶ･ｬｹ by perforations or cracks. Restriction

of the perforation areas is necessary, and freedom from crack-ing is absolutely essential in high quality bricks.

4. Tall bricks alongside lower ones are more heavily

stressed, and therefore tend to shatter sooner. For this

rea-son a high degree of accuracy to size in the height of the bricks is necessary.

5. Hard mortar of high compress10n strength compresses

less and its elongation is less. This results in lower tensile

low elongation value with a batching of P 350 to 400 is needed, the compressive strength of which will be over 200 kgm. per

cm.2 _{after 28 days. This means that the sand employed must have}

a good granulometric gradation, that its grain size must not exceed half the thickness of the joint, that the sand must be clean and free of loam and must contain no soft or weathered sand grains.

6. Hard stones are more brittle and show more internal

stresses, so that during pointing they crack easily and without

warning. Their slotting and perforating is therefore ｩｮ｡､ｭｩｳｳｾ

ible. For this reason the locations of all conduits must be

determined in advance.

7. The greater the compressive strength of the brick, the

greater will be its transverse tensile strength as well as the

masonry strength. Masonry bricks used for structural parts

and structures subject to relatively high stress must normally

have a compressive strength of more than 400 kgm. per cm.2 •

The bearing capacity of slender piers and walls is ex-hausted under a smaller load than that of the stout structures, The bearing capacity is thus determined not by the strength

properties, but by the stability of the masonry. Its maximum

strength is attained when an unsteady state of equilibrium is reached between the external forces and the counteracting

forces mobilized in the material. The structure buckles

there-fore at the moment that the material, after considerable

deflection, is unable to mobilize any additional corresponding

resistance. A wall in the buckling range is shown in Fig. 5.

This is a wall four metres high of 12 x Ｙｾ em. brick. The

slender form of the wall can be recognized still better from

Fig. 6. The measuring apparatus comprises gauges which indicate

the deformations of the entire specimen at the four corners of the centrally loaded pillar and two other gauges from which the deflection of the piliar can be measured half way to the

top. Results showing little scatter are obtained only with

Fig. 7 shows the stress-strain diagram of the brick walls

of the first investigation. These then became the basis for

the calculation and designing of the high buildings in Basle

(Ing. E. Geering). Such a diagram does not yield a

mathematic-ally expressible curve. It gives the strength of the masonry

erected with lime cement mortar. The bearing loads can be

calculated from this diagram for every degree of slenderness

and every degree of eccentricity. The initial deflection of

the body axis from the straight line, all flaws in the building materiqls and the unavoidably eccentric application of the

load in the press as well as in the structure itself decrease

the buckling load. Thus all factors which facilitate the

deflection of the structure under load accelerate the buckling and reduce the buckling strength.

Fig. 8 shows the test pOints and the buckling curves on which the high buildings in Sasle were based • . The degree of

slenderness is plotted on the abscissa and the bearing capacity

at various eccentricities on the ordinate. The curves, however,

are calculated from the stress-strain diagram, Fig. 7. They

are thus drawn not only between the test points. If no initial

deflection is assumed the curve does not coincide with the

test points for central loading. A better agreement between

the curve and test points is obtained only when an eccentricity of the order of 1/1000 is introduced into the calculation.

As was to be expected, therefore, the test structures were initially not perfectly straight and thus continued to curve

outward from the very outset. The test points and curves differ

greatly from the Euler curve, which, of course, is calculated from the modulus of elasticity, i.e., from the assumption of

the rectilinear stress-strain law (Hooke). The curves

ill-ustrate the rapid decrease of bearing strength with increasing

slenderness. The bearing capacity decreases With increasing

eccentricity, showing that both slenderness and eccentricity of the point of load application must always be taken into account in the structure.

What, therefore, are the factors which affect the

buck-ling load? These may be divided into two groups: the effects

of the materials employed, i.e., the brick and mortar, and the measures which can be taken by the designer and the mason.

A. The Masonry Brick

A piece of masonry which becomes deformed irregularly,

i.e., one sidedly even under a uniformly distributed load, bends more than one which is deformed uniformly and consequently

shows a smaller bearing capacity.

1. If the orick is uneven surfaced, local stress

concen-trations occur, especially if the brick has been somewhat warped during manufacture.

2. Fig. 9 shows what happens in a wall if either the

brick is not cut square from the extruded column or if the

column curves somewhat on leaving the nozzle. The lack of

rectangularity between the bearing and expansion joints pro-duces greater deflections (and hence a smaller buckling load), irregular deformations as the result of loading and additional deformations due to greater shrikage on one side.

3. Another factor is failure of trueness to size. This

adds to any departure from the wall axis, or wider head joints

occur (Figs. 10 and 11). It may also be said that the higher

the brick the fewer bed joints there will be per unit length

of wall. This should result in smaller deformations and smaller

bUlgings. Against this, however, is the greater effect of the

differences in shape, as Fig. 12 shows.

4. When the brick is laid on the mortar bed it takes up

water of mixing from the mortar due to its capillarity, as a result of which the mortar hardens somewhat and at the same time the adhesion between the brick and mortar is considerably

increased. This is also Why it is difficult to loosen the

brick from the set and hardened mortar. However, if the brick

draws a great deal of ｷ｡ｾ･ｲ out of the mortar, the latter will

hardening capacity and adhesiveness. At the same time the loss of water will result in a loss of workability on the part of the mortar in its fresh state, and as the upper part of the wall moves back and forth the mortar layer will become

rounded off (Fig. 13). After hardening of the mortar or masonry

the bearing capacity under central loading, and especially under eccentric loading, decreases for the simple reason that the

opened joints considerably increase the deflection of the

masonry under load. This effect is definitely noticeable in

the slender pillars and walls of small thickness (12 and 15

｣ｭｾＩＮ For high-quality masonry, therefore, only bricks of

ION absorptive capacity may be used.

Table I shows the results of strength investigations of

bricks from five different brickworks. Under central loading of

only five kgm. per cm.2 the compression increases with

increas-ing absorptive capacity of the brick from 0.022 to 0.102%,

i.e., by about a factor of 5. Under eccentric loading the

deflection increases very sharply from 0.33 to 7.84 mm., i.e., by a factor of approximately 24, once the absorptive capacity

of the brick is increased from Ｗｾ gm. per dm.2 _{min. to about}

40 gm. per dm.2 min. The masonry strength values decrease

considerably for central loading. The decrease of bearing

capacity with increasing absorptive capacity is particularly severe, however, under eccentric loading.

The effect of the absorptive capacity of the brick is clearly recognizable from a comparison of dry laid bricks with bricks which were laid only after soaking in water for two days, so that they had then only a slight absorptive capacity

(Table II). Under central loading the decrease is only 15%

(flattening of cylindrically rounded mortar layer). Under

eccentric loading the wall bulges more as a result of the gaps

｢･ｴｾｬ･･ｮ the ｢ｲｩ｣ｾ and mortar layer, thus causing the

eccentric-ity of the load in the centre part of the test wall to increase

rapidly, so that the buckling load is 83% smaller. The

buck-ling loads of test walls produced With bricks from different

buckling loads decrease hyperbolically, i.c., very rapdily, with the absorptive capacity.

The results of the tests on the 15 cm. and 18 cm. walls

carried out in the EMPA are given in Fig. 15. From the results

the following conclusions may be drawn:

First of all we note the considerable difference between the brick masonry made with lime cement mortar on the one hand

and that with ordinary cement mortar on the other. Under quasi

central loading the bearing capacity of the cement mortar

masonry with increasing absorptive capacity decreases sharply, whereas that made with the softer lime cement mortar decreases

much less steeply. The greater sensitivity of cement mortar

to water loss (decrease of strength and tendency to loosen) must be taken into account in determining the admissible

stresses. In the inspection of bricks, therefore, close

atten-tion must be paid to the absorptive capacity. 'In relaatten-tion to the values for central loading, under eccentric loading equal bearing capacity losses are experienced with increasing absorp-tive capacity on the part of the brick for both types of mortar. In the existing regulations a maximum absorptive capacity of

15 gm. per dm.2 _{min. is required for high quality masonry brick.}

The required compressive strength of 400 kgm. per ｣ｭｾ yields

15 to 20% higher wall strengths than strength of 300 kgm. per

cm.2 _{(Fig. 15).}

In foreign specifications it is often recommended that

the bricks be wetted before laying. Wetting, however, has

various disadvantages, namely, longer drying times, reduced thermal insulation capacity, tendency towards efflorescence, difficulty of control at the building site.

5. The j ointing mortar dries and shrinks, as is known,

from the outside towards the inside (Fig. 16). The gaps which

thus form reduce the bearing capacity for centric and especial-ly for eccentric loading, while at the same time, of course, the creep capacity of the mortar once more provides a certain stress compensation.

6. Gel-type bonding agents, including Portland cement and hydraulic lime, are subject to deformation under loading,

especially when green. That is to say, they creep. Under

centric loading this is somewhat of an advantage, but under eccentric loading a considerable deflection can take place in

the course of time. However, this deflection remains below that

occurring, e.g. in a concrete,wall, for the simple reason that

burnt clay bricks

､ｩｓｐＱｾｾｾｾｾｾｾｲｾ｢ｪＨ･

creep deformation; and the

joint mortar constitutes only a small part of the masonry. Larger eccentricities are therefore to be avoided, especially

in the case of heavily loaded piers. The bearing capacity and

compressibility of the masonry, like the deflection, are as-sociated with the compressibility of the brick and in the buck-ling region can be used as valuable coefficients.

The compressive strength of the masonry brick is affected

by many factors; the kind of raw mater, its preparation and

storage, the deformation, the drying of the precast brick and its burning and cooling in the furnace, the shape and size of the brick, the perforated portion, the kind of preparation, susceptibility to cracking, evenness of surface and finally the manner of producing the test object (mortar layers, loops), before it is placed in the press. For lack of a better standard, the compressive strength of the masonry brick is used as a

quality index for the strength properties, i.e., for the ten-sile strength of the brick, which is a decisive factor in determining the bearing capacity of the masonry and also for

the deformations. It would be incorrect however to employ the

compressive strength of the brick alone as a basis for the

admissible stresses in the quality regulations. In high

build-ings, high quality bricks with a compressive strength of more

than 400 kgm. per cm.2 , and not infrequently up to 600 kgm.

per cm.2 , were used (Tower Building in Schswamendingen:

com-pressive strength of brick 415 to 605 mean value 499 kgm. per

B. The Masonry Work

The designer and mason are responsible for the correct design and expert, craftsmanlike erection of the masonry,

in-cluding corners and splices (joints). The following points

should be observed:

1. The thinner, upper walls must be placed centrically

on the lower ones.

2. For all piers, corners and wall splices, blueprints

should be prepared by the designer. This is necessary not

only in order to conform to the brick formats in the choice of wall heights and ground plan as well as wall openings (for windows and doors), but at the same time to attain maximum bearing capacity and to avoid loss of valuable time by the

mason.

3. Wall sections can be effectively stiffened by means

of other wall sections bonded perpendicularly to them. This

fact should be t.ak en into account in drawing the ground plan and in determining the wall openings, because in many cases

the dimensions of the walls can thereby be reduced. For some

time now investigations have been in progress in the EMPA for

the purpose of ｣ｬｾｲｩｦｹｩｮｧ the effectiveness of such stiffenings.

It is intended that their application shall be facilitated by simplifications.

4. In the team of builders the mason is the all important

anchor man. The best and most true to size bricks, a highly

workable and properly composed mortar, the finest of blueprints are of no avail if the mason for lack of professional knowl-edge and insufficient skill erects a tilted or crooked wall, if he make s the joints of uneven strength and the bed joints

are a.Ll owed to II snake" (item. per m, }, All such

irregular-ities greatly reduce the bearing capacity of the wall. Walls

Which cannot satisfy the conditions laid down are unusable and must be replaced.

5. Brick masonry can at any time be checked for its

quality without great expense and if necessary even be replqced.

It has been shown time and ｡ｾ｡ｩｮ that this is an advantage

not to be underestimated, for again and again unreliable people will get into the construction gangs and their mistakes can be eliminated only by constant, careful checking.

6. Anyone who watches the mason at work will realize that

his output can be increased if the bricks are true to size, e.g. if the key stone can be inserted without a lot of

adjust-ing. Excellent trueness to size is illustrated in Fig. 17.

The question of whether the joints should be completely

filled or not is often discussed. The head joints must always

be filled completely at places where compressive and shearing stresses are transferred in the masonry (concentrated loads: piers, beams, wind screens) or where maximum impermeability

(driving rain against framing m3.sonry) or maximum weight (sound insulation) is considered important.

The ｳｾｦ･ｴｸ ｦ｡｣ｴｯｾ

The inaccuracies of static calculations, the schematic

load assumptions 1 the use of Hooke's 13.w which is applicable

only as a rough approximation, the interaction with other structural elements, the unavoidable material variation and masonry irregularities, the deformations under the permanent load and due to vibrations caused by shrinking and creeping as well as temperature changes affecting one side or all sides, all these influences, which can be assessed only with diffi-culty or with great expenditure of time, make it necessary to introduce a safety factor which must be determined on the basis of experience and by estimating all the factors not taken into

account in ｾｾｨ･ calculation. For brick masonry Which has been

subjected to masonry test, i.e., where the most important conditions of the loaded masonry have already been determined by the test, a safety factor of 4 is regarded as reasonable,

provided the required material qualities for compressive

are observed. The safety factor is raised to 5, however,

8.S soon as general standard values are determined which are

based on the results of investigations from several brickworks, which of course differ much more sharply than the results for

a single brickworks (Fig. 15). It may happen, therefore, that

a given briclcworks can prove the high qual 1ty of masonry pro-duced from its products, but cannot quite attain the qualities of bricks as prescribed.

The admissible stresses calculated from the experiment-ally determined masonry strength divided by the safety co-efficients are applicable for the tested buckling distance and wall strength, on the one hand, and for the tested brick

and mortar material on the other. The most important properties

of these are determined, of course, by obtaining the compressive strength and absorptive capacity, the dimensional tolerances, the specific weight of the brick and the compressive strength

of the mortar after 7 and 28 days. The admissible stresses

are then derived for the remaining slenderness ratios, so that these can finally be stated as a function of the most import-ant parameters affecting the bearing capacity of the masonry. These most important parameters are the compressive strength

of the brick ｾ , the strength of the mortar ｾ , the suction

s m

capacity s, the slenderness ratio hid and the eccentricity

coefficient m

=

elk.

Fig. 18 shows the admissible stresses

for a masonry of high-quality brick. For centric loading the

admissible stress is constant up to a slenderness ratio of 5, then decreases linearly (rather than according to the experi-mentally determined curve) and intersects the abscissa axis

at a slenderness ratio of 55. For higher slenderness ratios

this straight line lies far below the numerical values

obtain-ed from the tests. A comparison with reinforced concrete

standards may be aopropriate here. The reinforced concrete

standard breaks off at approximately hid

=

43; thus, while

reinforced concrete is no longer adequate, a still slenderer bearing structure can be erected with high quality brick

masonry. The same holds even for eccentric loading. This

ｳｴｲ｡ｩｾｨｴ line intersects the abscissa at even smaller

slender-ness ratios; the admissible stresses through the centre of

gravity also decrease with increasing eccentricity. The

bear-ing capacity of the masonry 18 em. thick is about 20% less than

that of the 12 em. and 15 em. masonry. This is so because the

18 em. brick shows greater residual stresses.

The effect of the bond is shown in Fig. 19 and in Table

III. The numerical values of the 12 em. one brick wall are

compared with those of a 25 em. bond wall, in which half of

the bricks are headers. For centric loading and a small

slender-ness ratio, for which the strength is decisive, the 25 ern.

masonry with its 50% header cross-section must have a smaller

bearing capaoity. Under centric loading of the compact

one-brick wall (hid

=

10) the breaking load is 48% higher than

for the 25 em. bond masonry. For hid = 20, however, it is only

17% greater (Table III). Under eccentric loading the weakening

at the header cross-section can no longer play any great part. At higher slenderness ratio, i.e., in the buckling region, the reduced transverse tensile strength has no further effect, and indeed because of the comparatively greater trueness to size the buckling strength of the 25 em. masonry is even great-er than that of the 12 em. masonry.

Table

IV

tells something about the stress conditions of

a 20-storey apartment house with 15 em. unstiffened partitions.

For a safety factor of 4 a masonry strength of 177 kgm. per

cm.::l must be ahown . Test values from certain brickworks have

even exceeded the 200 kgm. per cm.2 limit, so that nothing

ｳｴ｡ｮｾｳ in the way of increasing the number of storeys.

The bases of ｣ｾｬ｣ｵｬ｡ｴｩｯｮ

The simplest method of calculating the eccentricity is to

use a sub s t.Lt ut Lon system in the form of a IIdouble cross" from

which the peak moments can be calculated for the most unfavourable useful load distribution (Checkerboard distribution

Fig. 21 shows the influence of the stiffness coefficient a a = h Jr Er

-t

J

_s

E

_s

for equal spans. The factor a decreases very sharply with

in-creasing stiffness.

The stiffening of unpierced walls can be taken into ac-count by the following expression

o

_permiss. = 0_k + (0_s - a_k) TJ

3

where TJ3 can be obtained from Fig. 22.

The stiffening effect of wall sections perpendicular to the wall being calculated can be taken into account by reduction

of the slenderness ratio. The eccentricity of the point of

load application is most easily determined by graphical means. The corner stiffening, however, reduces not only the slender-ness ratio, but also as a rule decreases the degree of

eccent-ricity. This leads to the important conclusion that in draWing

the ground plan, especially for the case of heavily loaded

pillars, use may advantageously be made of this material saving

fact. For, the less eccentrically the load is absorbed by the

wall, the smaller may be the wall thicknesses chosen. In order

to avoid constraining stresses and cracks resulting therefrom, the greatest possible uniformity in the deformation of all wall sections within a given storey should be striven for in tall

houses. Only in this way, also, can local overloadings and

excessive bending and shearing stresses and their consequences be prevented in the concrete ceilings.

Building ｩｮｳｰ･｣ｴｾｯｧ

This must cover not only the checking of building material

but also the masonry work. The joint thickness, straightness

of joints, cut of bricks, plumbness and curvature of walls, joint filling, spalling and cleaving, perforation, etc., are all factors which definitely affect the bearing capacity of the

permeable in a brick Dense outside layers

masonry. Fi,t?:. 23 and 24 give the br-t ck and mortar strength

values obtained from three bUilding sites. Other constructional eneineering factors

Thermal ｰｲｯｴｾ｣ｴｩｯｮ of outside walls. Denser brick and

denser cement mortar conduct heat more effectively, and there-fore greater wall thickness must be chosen especially for the

upper storeys which are more exposed to the wind. 32 cm.

masonry with cement mortar joints may still suffice in the

centre rooms. In rooms with outside walls, however, where

ventilation is insufficient, water condensation must be expect-ed, especially if fuel is used sparingly.

If the surface layer is sufficiently masonry, vapour permeability is assured. should be avoided.

Driving rain can be warded off either by a suitably applied external plaster, which must be in three coats, or,

in the case of common brickwork, by grouting at the place where the rain penetrates.

Loaded brick walls 15 cm. thick, plastered on both sides, also provide adequate sound insulation for partitions between

apartments. Good stiffening by means of bricked, bonded-in

partitions in each storey, tied with a heavy reinforced con-crete plate would also afford considerable resistance to the blast of bombs, a greater one, at any rate, than facades

stif-fened only by light walls. Concl us i.2.B

Brick, known for several thousand years, has recently been developed into a comparatively strong building material

in a few Swiss brickworks, mak1ng it poss1ble to construct buildings up to 20 and more storeys, assuming, of course, that the partitions can also be used to absorb some of the load.

Masonry erected ｾｬｩ th these bricks requires no forming and no

reinforcement rods, will support loads within a relatively short time and thus makes possible a relatively rapid tempo of

construction. It provides good thermal insulation, absorbs

fire-proof, which are certainly also factors to be considered in

a tall building. It also insulates extremely well against

airborne and solid-conducted sound. With proper designing

of the structure there is no danger of cracking due to con-straining pressures.

The brick industry has thus shown its ability to keep pace with the times and to produce a high quality building material for the construction of high buildings, with which it is possible to execute load-bearing walls with dimensions which doubtless offer an economic advantage 1n addition to their technological qualities.

ｾ｡｢ｬ･ I

Influence of the absorptive capacity of the brick on the deformability and the bearing capacity of the wall

One brick wall 12 cm. of insulating bricks 13i x 12 x 25 cm.

with lime cement mortar HK 250 + P 100, compressive strength

of mortar 30 kgm. per cm.2 ; slenderness ratio hid

=

25, knife

edge suspension, mean value from two piers, age 29 days

Deformation for

0::: 5 kgm. per Masonry strength Ratio

.

_cm.2 >. l=l ｾ ...-l ｾ S

-0 ｾ 0 . .ro", ｾ «l E3 Q)

.

0 o '0 _> 8 ｾ ...-l 8 Q) H rn 0 > Q) rnｶｾ｟ l=l 0 0 ｾ ｾ 0. Q) ｾ 0 ｾ ｾ ｾ H l=l ...-l n H r-f r-f 0 m 0.

.

ｰＬｾ l=l H ｾ II H E! S 0 ｾ ｐＬｉｾ l=l II II 0 bO o l=l ...-l _l=l Q) rn ｯｾ ｾ Q) II 0 S S

a

.0 l=l ro 0 0 0 < ｾ r-fH Q)

=

8 Q) ｲｯｾ r-f ｾｲｮ (,.., 0 Q) 8 ｾ A _> 40 0.102 7.84 39.5 6.0 0.152 B 40 ＰｾＰＸＵ 4.69 59.8 6.2 0.104 C 33 0.059 1.60 65.0 15.8 0.243 D 12 0.028 0.60 82.8 34.8 0.420 E _7i 0.022 0.33 107.2 58.0 0.521

Table II

Influence of absorptive capacity of brick on the bearing strength of the brick

One brick wall 12 em. of insulating bricks 13i x 12 x 25 with

lime cement mortar HK 250 + PlOD; slenderness ratio hid

=

26,

compressive strength of mortar ｾ

=

30 kgm. per ｣ｭｾ , knife

m

edge suspension, age 28 days

Condition of Mean masonry strength Ratio

brick

" centric" eccentric m

=

1 m

=

lim AS0

Dry 51 3.9 0.076

Stored 2 days in

water 60.4 23.2 0.385

Dry/wet 85% 17%

Table III

Ratio of masonry strength of the 12 em. one brick masonry to that of the 25 em. bonded masonry

Influence of the bond by comparison of a 12 em. one brick wall

With one consisting of header and stretcher rows. Brick format

Ｙｾ x 12 x 25 em. compressive strength of brick ｾｳ

=

289 kgm.

per cm.2 , lime cement mortar HK 250 + PlOD, compressive

strength of mortar ｾ

=

30 kgm. per cm.2 , age 29

m

days, knife edge suspension

Slenderness ratio Strength ratio m.

hid 0 _ｾ 1

It

10 1.48 1.17 1.0 0.86

15 1.25 1.15 1.06

High building of brick masonry

Span width: 2 x 5.0 m. Wall: 15 em.

Ceiling, solid 14 em. Useful load: 200 kgm. per m.2

Storey height: 2.8 m,

hid = 2.8/0.15

=

18.6 (without stiffening!)

Storey,

･］ｍｭ］ｾ _ｾ

counting from Pv Pseh Osch av

above P k t./m: t./m: em. kgm./cm.2 _kgm./cm.2 _kgm./cm.2 I 2. 6.11 5.61 1.48 0.59 3.74 4.08 16 N... I 5. 16.19 16.69 0.53 0.21 10.5 10.8 43 5. 16.19 15.69 0.53 0.21 10.5 10.8 43 10. 32.99 32.49 0.26 0.10 21.7 22.0 88 15. 49.79 49.29 0.169 0.068 33.0 33.2 132 20. 66.59 66.09 0.126 0.050 44.0 44.4 177 25. 83.39 82.89 0.100 0.040 55.1 55.7 223

v = full load M

=

crown moment

Fig. 1

Masonry of natural stone after fracture. Crack formation

takes place predominantly above the head joints due to transverse tensile and tensile building

Fig. 2

Masonry of insulated brick from three brickworks after

fracture. Cracks above head joints.

F1g. 3

Br1ck masonry w1th standard br1cks, 38 cm. th1ck, after

fracture. Crack1ng f1rst 1n the three shells

Fig. 4

Cement bricks with cross ties after fracture under eccentric

load. Obvious displacement of the two shells as a result

Fig. 5

25 cm. thick masonry in the 500-ton press with knife edge

suspension on both sides. Steel girders top and bottom

for uniform application of the load to the test body. Length measurement at the four edges and two

Fig. 6

15 em. brick test masonry in the 500-ton ｰｾ･ｳｳＮ Steel beams

and knife edge suspension. meters for. measuring the

uo- ---l:> ell oil

..

L +

..

,... 50'f--f-+--o I.-c o

..

_• I:

I

Cement

ｭｾＬＮｲ｡ＮＬＮ

I I

I

i 445 Fig. 7

Stress-strain diagrams for the three masonries with the three

kinds of mortar: hydraulic lime mortar HK 350, lime cement

mor-tar HK 250 + P 100 and cement mortar P 300. li standard bricks

with 289 kgm. per cm. 2 _{compressive strength. Centric loading,}

slenderness ratio hid ｾ 20, wall thickness 12 em.,

width of wall 80 em., age 29 days

,

30 25 20

｛

Ｍ Ｍ ｾ Ｍ Ｍ ｝ Ｍ - - -

ｾｾｾｾ

-ｾＱＮ 10 5 ｾ 'oj( 60 - - - - ｾＭＭ f._{I "} C l! 40

----...

..,

...

ｾ 20--o oil d £ 0ＡＭＭＭＭＧＭＭＧＭＧＭＴＭＧＮＭＭｌＭＢＭＭＭＭｬＭＬＡ［ＭＭｊｌＮｌｾｾＮＮＮｌＮＮＮｌｾＬＭｌＭＧＮｊＭｊ｟｟｟｟ｩ＼ｌＮＮｌｌＮＮｌＮＺ［ｲＡ［｟Ｇ｟ＭＧＭＭＧＭｾ o

1

'20.---,---,---d -:--.. € .:2ｾｯｯ ---__._l--= j -s ｾ ___ 80

-rr

Slenderne&t. rat-1o lid

Fig. 8

Buckling stresses of a brick masonry 12 em. thick. li standard

bricks 12 x 25 x 9.5 em. with round perforations, mean

compres-sive strength 289 kgm. per cm.2 , lime cement mortar, mean mortar

strength 29 kgm. pOT' ｾｭＮＲＬ l.-J1rith RO crn , ｾ Rge 29 days. Knife edge

suspension. Test points and those calculated theoretically from

the stress-strain diagram for an initial eccentricity p' ｾ 1/1000.

Fig. 9 Fig. 10

Fig. 9: Schematic representation of the masonry with bricks

cut from a crooked string. Increased stresses on the

side at which the joints are thinner. Hence greater

deflections of the wall and a lower bearing capacity.

Fig. 10: Schematic representation of a masonry constructed

with bricks of unequal width. The one-sided

compact-ness of the masonry results in local. eccentricity and stress concentrations.

I

,

I

_ｾ

I

I

1

ｾ

II

I

Fig. 11 Fig. 12 Fig. 11 Fig. 12:

Smaller brick widths relative to the length of the bricks result in wider centre joints, whereby the header is more severely stressed by tensile bending and shearing stresses.

In higher bricks resulting in non-parallelity of the

cut faces, a fairly ｬｾｲｧ･ ｾｩｳｬｯ｣｡ｴｩｯｮ of the local

Fig. 13

When a thin masonry is erected the upper part

moves back and forth. A highly absorptive brick

dewaters the layers of mortar so that the mortar loses plasticity and thus is rounded off towards

the edges (teeter-totter effect). Under centric

loading stress concentrations occur at the centre; under eccentric loading much greater deflections

of the masonry and a reduced bearing capacity result

I ..

O'/Ho' $6·a·400 Hyper bol •• "'auBf'werh ausｬｾｯｉｩｴｩｲｳＯｬｬｩｮ･ｮ J,.:d· ttcm

6 _ • • • JIS:d-tSan

J.,d·,scm

tI• Mlluef'werlt <IUSNOIYnalsleinen N

L./ dｾ 25 (V"""'''''e' hyd' l/oIhmo,./el)

.-

ﾷＭＭＭＭｲＭＭＭＱｾＭＭ

1" "

｟ｾＭ］

20 JO 40 SO {IO 70

of absorbed water/area in

min. (1 em. submerged depth)

t ('oJ

.

ｾｅ 60 .-ｉｴｦｾ

_.

.s:: 8 ｾ eo ｢ｄｾ Q <D I::: s, ..-l #J ｾ (/)

...

M Q) l> II ..-l (/) S (/) Q) ₀ ｾ ..-l P. ｾ 8 .p

..

0 I::: ₁₀ o Q) 0 0 Q) ..., 0 -0 10

"s

::= Wt. gm./dm.:2 Fig. 14

With increasing absorptive capacity of the brick the bearing capacity falls sharply due to the greater deflection of the

masonry which is loaded eccentrically in the core region. The test points scatter about a hyperbola

Mauerwerk aus Isoliersteinen ::= Masonry of insulating brick

Mauerwerk aus Normalsteinen ::= Masonry of standard brick

180 180 401--_ _

[

m ... 0d =18

r:

em _ _ _ _ _ _ hId=17,2, Reduktionsfoktor: 0,8 I ｾ - IpmpntTnorlel ｾ ｉｾＨｬｬＭＭＭＺ［ＧＢＢ､ＭＭＭＭＫＭ｟ • VNI Morlel -" '" ｾ

..

100 ';;; ｾ

..

80 J ... ｾ 60_ o ::; ｾ r-f'---r---j

1-'

d =15em m""O _h/d=1"5 4('1 - - - - + - - - - j - - - 1 I: 1(,0 .. co -;;',,40 _

r,

c ｾ =lemenlmorlel .:. 170I - - - f < - - - - j - - •• veri. Morlel OJ -" go 100 '" .2! '" '" 80I---=::'Olo;;:---+---""'rl'---+--l

!

ｾ 60 o ::; '" £. 0," 1-- + -;:; -t:

..

> 0,31---1---'''''' + -(l,,;I - j ＨｬＬｾ I - - - - n , - .+ - - - - 1 - - - j - - - - 1 co Jo,jqfoh'gkp,1 onq dm' ,"on o _._

'1 ....

7f .__'[__.ｾｌ

...

ｾｏ '"

ｾＱＱＰＬｾｦＭＭＭ

0..0.. n f;O I

Souqoh''Jke,1 If' 9 dm2.m,n 20 .10 40 so I I I I 10

--\

\.

0

ｯｾｾ｜Ｎ

_«!'

ｾｉａ

•

.

_ｾ

_.

1O 20 30

ＴＰｾ

_'I 0,2 0,'

-1

0 " ·0,5 rfc1 Fig. 15

Results of bearing capacity tests of 15 and 18 em. thick

mason-ries of bricks from various brickworks. Reduced compressive

strength of brick ｾｳ ｾ 300 kgm. per cm. 2 • Cement mortar and

lime cement mortar. Strength of masonry under centric loading

as a function of the absorptive capacity of brick and the ratio of the bearing strength under eccentric (in the core

region) and quasi centric loading

Mauerwerksfestigkeit

=

Masonry strength

Zementmortel

=

Cement mortar

verI. Mortel

=

Lime cement mortar

Saugfahigkeit

=

Absorptive capacity

Reduktionsfaktor

=

Reduction factor

o

B

..

Fig. 16

When the joint mortar dries from the outside towards the in-side cracks form which result in an effect which is similar,

but much weaker than the teeter-totter effect

Fig. 17

The bricks from the Ziegelei Paradies used in the Schwamendingen

skyscraper. Width 12, 15 and 18 cm., length 25 cm., height 13.5

cm. Trueness to size: tolerances; width, length, height: 1.0,

1.0 and 1.5%. Mean values may differ at most by 3.3

60 10 N go ..--- _.

r-·-·--_·_-

-1--5 E <, c: Z ｾＳＰＰ ｾ .+--__-+ S =15g/dm . min _ :; {3s =400 kg/emz .L. N ｺ･ｭ･ｮｾｭｩＺ＾ｲｴ･ｬｺｂｦＳｭ =200 ｾ 2' d =12. 15em

1

roo

ｾｾＭＭＧｬｬｲＭｉＭＭＭＬ

ｯＭＭｭＮＭｯＮＭＺＭｾｾＭｬＢＧＧＧＧｏ､ｬｮ

=5_ _ ｾ .b-d6 ｾｦＭＭＭｃｾＮ ＭＭＭｾｕＭ Fig. 18

Mean curves through the test points of the masonries of 12 and 15 em. thickness erected with high quality bricks from various

brickworks. Compressive strength of brick 400 kgm. per cm.2

,

specific absorptive capacity 15 gm. per dm.a min., compressive

strength of mortar after 28 days 200 kgm. per cm.2 , admissible

stresses for various eccentricities m of the point of load

ap-plication approximately 0,

i,

1 and li as a function of the

slenderness ratio

Mauerwerksfestlgkeit

=

Masonry strength

Zementm5rtel

=

Cement mortar

Slcherheitsgrad

=

Safety factor

wo _ _ _ _ _ 0 _ _ _ _ _ _ --_ .._...ＭＭＭＭｾＭ i_I 90 RD <01"-" 70 ｾ 60 -e t-o-, II: 50 41 "--to : . 40 L I: 0 1/1 30 d ｾ 20 /0 0 10 Lp.!Jend: d= 12cm, d:25cm m=0 0 - . m='lz m=1 m=1'/2 Fig. 19

Strength of one brick and bonded masonry with equal

slender-ness ratio of 10 to 20. Lime cement mortar, ｬｾ standard

for-mat, compressive strength of brick 289 kgm. per cm.2 • Under

centric loading the one brick masonry is superior. With

in-creasing slenderness of the test wall and inin-creasing eccentri-city of the load application the bearing strength changes in

favour of the thicker, relatively more accurately constructed bonded masonry

h

0,

= I, k k - J,E, ｾＭ J,E, M, e,= P, M2 e2= P 2 h k ' 2 h I h -- 1C h P 21,2 (1 +r) -122r M,= 4 4 (1+r)2-.:...,.2-M, 21,2 (1+r) --122 r M2 212'l (1+r) ---1,2r 1 +- "I +"2= r p _{2122(1+r)--1,2r} M2c= _{4 4 (1} +_{r)2- r}2 Fig. 20

Simplified calculation of the eccentricities of walls taking

into account the stiffness of the ｣･ｩｬｩｮｾＬ crown and foot

A 0.05 -

--T

ｾＭＭｬＭＭ 0.04 ·1,M=qe'

AJ

--n.nJ

f- --- -_.

__.-o,nnl I 0,02 .... I -0,01 I ''),0092 --- - - - ---

t---·-O,OOJ8 0,001 0,00135 o 10 20 30 o:=Jl....1ic_e J,t, Fig. 21 0.6-1/, O.B

Effectiveness of ceiling and wall stiffness on the crown moment M for equal span Width

r---

r-1-r---

'-l

- N-[1t ( ; , -1)77,jo-;.

r

-

---I

ＨｊＢｾ ="lz·a;

j

I '

1,,,]- -

T

-:::l:

I · I .

1-1 . '"

\1-__'1 lo :;o:.. Z::...,5 -+---.l0.10 I i I I I : a -t--1-j.j;-- 2--ｾｪ ＭｾＭＭＺＭＭＭＭＭＧｬＭｔｉ ｨＬＭＭＭｾＵ Fig. 22

Stiffening by means of partitions increases the bearing capacity (bearing wall without openings!)

'-'" N c-; -ｃ｣ｭｰｲ･ＮｾｳｬＢＣ･Ｎ strengTh kg/cm2 300 Ｍｾ ....

,oo.·n

100 ;:;- '" _ 1.O "" rr'I ... -50

H

ＢＧｹＭＭＭＭＢＧＢ＿ＭＭＫＭＮｊＬＮＮＮＮＭＭＭ［ＭＭＮＮＮＮＮＬＧＭＭＭＭＭ＼ＭＭＮＮｊＮＭＭ＿ＭＭＭ＼ＭＭＭＴｾ o

_.-LL

P400 :...

---P350---Arranged by date of manufacture

Fig. 23

Mortar strength from 4 x 4 x 16 cm. prisms, supplied by the

Altwyler apartment building construction office in Berne. Decreasing mortar strength with decreasing stress on masonry

g/dm2min so 40 Spe.t.IF,t. a..bsoY'ptlve co.po.city 10 20 -o

bricks _JiO.!17 W.!18)1O)l?WJ1RJlOWI1RJfOWllR.!lRl1sW}lRW W

Brickworks f----ｾＭＭ A -- 1 B B eBB B

Arranged by da.te of manufacture Fig. 24

Compressive strength and specific absorptive capacities of

bricks of high quality and of st.anda.r-o quality, "AI twyIer" ,