Masonry in Engineered Construction: Comments on the New S.I.A. Standards no. 113

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Masonry in Engineered Construction: Comments on the New S.I.A.

Standards no. 113

Haller, P.

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Technical Translation 1270

Title: Masonry in engineered construction. ｃｯｮｾ･ｮｴｂ on the new

S.I.A. Standards No. 113.

(Mauerwerk im Ingenieurbau. Betrachtungen zu den neuen

S.I.A.-Normen Nr. 113)

Author: P. Haller

Reference: Schweizerische Bauzeitung, 83 (7): 103-107, 1965

HM30NRY IN ENGJJmERED CONSTRUCTION

ｃｯｮｭｾｮｴｳ on the New S.I.A. Standards No.

113

Is it not astonishing in this age of prefabrication and automation that we find ourselves discussing masonry along side the two "great" build-ing materials of steel and reinforced concrete, at a meetbuild-ing of professional

engineers? Masonry, indeed, is as old as lv'lethuselah and many examples of

it which have survived from distant antiquity bear eloquent witness to great

works. There must indeed be some merit in a method of construction which

even today permits the construction of buildings that are technically quite

remarkable (Fig.

1).

How is this possible? How can masonry, especially brick masonry,

still find a place in the orchestra of competing products, even if it no

longer plays first violin (this role is left to the two "greats")? How ie

artificial stone masonry, and especially the all important brick masonry, still able to find a place in the sun?

For one thing, a small unit is more adaptable than a large one to

different designs. It can be adapted to any ground plan and to any height

of storey. It can be used in interior and exterior walls, and is capable

of taking loads after a few hours, so that construction time can be reduced.

It serves both as a load-bearing element and in-fill panel. Solid brick

masonry is less subject to shrinkage and creep. It acts as an insulator,

dries rapidly, and withstands fire well. Brick masonry has long enjoyed

these advantages.

There are three main points which favour masonry in competition with other materials:

1. Considerable quality improvements make it possible to construct thin

-walls. By suitable choice and thorough mixing of clay materials, by storage

in soaking pits, by more accurate casting, expert drying and more precise burning, considerably higher quality solid bricks can be produced.

2. The greatly improved accuracy in the dimensions of the units in recent

years permits the erection of more accurately dimensioned masonry members

which are consequently capable of bearing greater loads. Moreover, it has

even been possible to increase the output of the masons from 1 - 2

rn?

to

'2

10 m/ per working day, and this is a decisive factor in the economics of masonry.

3.

By detailed testing of the finished walls or piers, by taking into

ac-count absolutely all factors which can influence the bearing capacity, manufacturers have discovered how the product can be improved, i.e. can be adapted to modern requirements.

Tile fact thet the present standard, whLch was accepted, as you know,

in December 1963, by tile meeting of deler,ates of the S.LA. in Solothurn,

is only now replacing its predecessor from the year 1943, is owing to the

lag in the development of testing methodo, and the rapid strides that have

been made towards the manufacture of a solid brick with ｨｩｾｬ ｣ｯｲｾｲ･ｳｳｩｶ･

strenr;th.

The most important results of the numerous experiments are as follows:

1. If the mortar expands laterally more than the brick, parallel to the

course joint, especially over the vertical joints, then the brick is sub-jected to bending stress and local stress peaks occur at the thinner places of the course joints, and thus under breaking load the bricks crack

perpen-diCUlarly to the course joints. Hence the breaking of thick masonry is the

result of exceeding the tensile strength of the brick, the decisive factors being the thickness of the course joints, the nature and size of perforation, the flatness of the brick and crack formation in it.

2. If the briclc is too absorptive, water is too rapidly withdrawn from the

mortar, which then is unable to develop its full bonding strength, and

be-sides this, in slender walls (d < 25 cln) the so-called wobble effect arises.

A layer of mortar from which more water is drawn than the rest tends to be squeezed out by the back and forth movement of the upper part of the ma-sonry . . If the load is well centred the bearing capacity is but little reduced by this effect, but it can be extremely serious if the load is eccentric.

3.

Testa were reqUired in order to clarify the influence of the strength

and deformability of the brick and mortar, the height of the brick, the brick bond in the masonry cross-section, the departure of the axis of the wall from a straight line, the accuracy of construction of the wall, which in turn depends on variations of the dimensions of the brick, and the eccen-tricity of the imposed load.

4.

Thick masonry structures are destroyed by the cracking open of bricks,

whereas slender walls fail even under comparatively small loads. Failure

of thick walls is due to failure of cohesion forces, i.e. the tensile

strength of brick, whereas slender walls become unstable when the buckling load is exceeded, suddenly bend out and break.

Since masonry walls are unable to sustain substantial tensile stresses perpendicular to the course Joints, a new method has to be applied in order to calculate bearing capacity in the strength and buckling region, where the experimentally determined stress-strain diagram of the masonry is taken into account (Fig. 2).

-5-We are confronted with a stability problem if failure of a wall or column occurs before the compressive strength of the masonry is attained

at the critical point. It has already been shown earlier how this

calcula-tion can be ｮｾ､･ (Fig.

3).

With increasing deformation of the critical

extreme fibre in the masonry structure, the bearing capacity rises to a

maxImum (Fig. 4). With still higher fibre strain, or deflection of the

wall, an equilibrium can then be attained only under smaller loads; with still greater deflection, a limit is reached at which equilibrium between the

external forces and internal resistance is no longer possible even under

comparatively small loads. Fig.

5

shows how sensitive the buckling load

of a wall or column is to the stress-strain diagram.

Standards 113 are divided into two main parts: masonry made from

artificial stone and masonry made from natural stone.

The latter has lost some of its importance, because the stones are

expensive to shape, and stone masons can now scarcely be found in sufficient

numbers. Happily, natura 1 stone retaining waL'ls or retaining walls with

natural stone veneers are still being erected in the Alpine cantons. It

remains to be seen whether mechanization (use of cutting discs) can re-vitalize natural stone masonry.

Considerable space is devoted in the standards to this type of masonry despite its minor economic importance, for the simple reason that it was

desired to maintain earlier experience for the sake of posterity, ｡ｮ､ｾ not

least, to ensure proper execution in the case of restoration of existing

natural stone buildings that are worth preserving. Because of insufficient

experimental data, earlier methods of calculating natural stone masonry on the basis of allowable stresses at the edge had to be retained.

In natural masonry a considerable influence can be recognized of the

planeness, i.e. the extent of cutting of the bearing faces, on the bearing

capacity of the masonry. The better the bearing face is cut, i.e. the

flatter it ;is, the thinner the mortar layer will be. This means that

small-er transvsmall-erse tensile stresses are set up in the stone, and thus the bearing capacity of the assembly is increased.

From the knowledge gained from tests on brick masonry, it has been possible also to increase the allowable stresses on natural stone masonry.

The follOWing innovations for artificial stone masonry have been made

compared with earlier masonry standards:

1. The brick manufacturer guarantees masonry strength primarily on the basis

of tests on slender and short test walls erected in laboratories. Only in

this way is it possible to provide for all the pertinent factors whose effects on masonry strength could never be evaluated merely by a test on a

Compressive strength of

uni t in kg!cm2

a brick. Since in the standard brick test only the ｣ｯｮｾｲ･ｳｳｩｶ･ strength

ia determined, whereas in the masonry assembly the tensile strength,and in the case of slender walls especially, the deformability of brick and mortar determine the bearing capacity, these properties are of fundamental

importance. The raising of a particular brick to a higher category, is

possible only on the basis of the masonry strength.

2. Since because of the time involved the masonry is not very suitable as

a control test, bricks and mortars must therefore be tested so that a pre-liminary orientation can be carried out in a simple manner.

The standard brick specifications comprise:

specific absorptivity (mean value and scatter); compressive strength (mean value and scatter); perforation (kind and size);

dimensional tolerances: (a) for the mean value, (b) for the rated value; planeness of bearing faces; susceptibility to cracking;

presence of any unslaked lime grains (danger of splitting).

The mortar standard test includes flexural and compressive strengths on

prisms

4

x

4

x 16 cm after

7

and 28 days.

3.

The standards recognize four classes of masonry for artificial stone

masonry, and three classes for the masonry units:

Masonry quality Strength required in kg!cm2

(example) for a 15 cm wall of

solid brick lid ｾ

8

Standard quality 55 ｾＱＵＰ

High quality 85/125 ｾＲＵＰＯＳＰＰＯＳＵＰ

Very high quality 210 ｾＴＰＰ

4.

The allowable stresses can easily be derived from the guaranteed masonry

strength value, once the degree of safety has been determined. The safety

factor which must take into account

- the approximate load calculation (Hook's Law);

- the unavoidable scatter of the dimensions, the strength and the absorp-tivity values or the water-retention capacity of materials (unit, mortar); - the brittleness of the units;

- the inaccuracies in the laying of the units (departure from rectilinearity and from the vertical axis);

- the deformation of the wall or column with shrinkage and creeping of the mortar,

-7-masonry is inspected three times annually.

The allowable stresses for masonry made from artificial stone are stated

as average stresses, thus simplifying the calculation. In the case of

natural stone masonry, the edge stresses must be stated as before, and

these may be

40%

higher than the stresses at the centre of gravity. The

allowable stresses are substantially higher than in earlier standards. Essential now for the continued maximum utilization of the building materials was a thorough investigation of the interaction of units and mortar, taking into account all the factors affecting the bearing capacity

of the masonry. By examining and evaluating 1600 masonry piers single

in-fluences could also be distinguished.

5.

The allowable stresses are given in the standards for quasi centric and

for equal, eccentric loading on the same side (VSZS sheets). In the case of

interacting eccentric loading unequal at both ends, the bearing capacity of the masonry should be calculated in accordance with the expected deflec-tions by means of a simple empirical formula which has been verified

exper-imentally (Fig.

6).

6.

When a floor deflects, the point of load application on the wall shifts.

Its eccentricity can be calculated approximately and with sufficient

accuracy from the double cross (which is defined in the standards). Obviously

it is necessary to take into account any eccentric loading by walls situated above.

7. The allowable stresses are given separately for single wythe and for

bond (multiple wythe) masonry. In the case of thick masonry walls, owing to

the splitting of the headers above vertical joints in the vertical cross-section, the strength of the bonded multiple wythe masonry is found to be

less than that of the single wythe masonry. Since this weaking is no longer

of importance in the case of buckling, i.e. the wall becomes unstable before the units fail, the rectilinear curve of buckling diminution intersects the abscissa at the same point for both types of masonry.

8. If the walls are stiffened by intermediate walls that are bonded in,

then the maximum permissible stress

a

_s can be applied to the initial wall

section of

3.5

d. The next wall section of 12 d length may be dimensioned

with the mean value ｾＨＰＵ

+

oak)'

ｔｾ･ remainder of the wall may now be

a

loaded only with the buckling load m k. In the case of stiffening of the

wall on both sides the 12 d regions can be made to intersect and the

ap-proximate allowable stresses can be used. Stiffened wall sections can be

loaded with the allowable stress

a

up to a slenderness t/i<17. Slender

s

wall sections should be calculated with correspondingly smaller allowable

can be ･ｮｾｬｯｹ･､Ｌ in which case a moment of inertia referred to the axis

passing through the overall centre of gravity is used (h_max

=

7

d).

9.

The calculations should be based on the S.l.A. Loading Standards No.

160.

In

the calculations it is necessary to take into account not only the

slenderness ratio, but also the eccentricity of load application

equilat-erally or crossed. The useful load, in certain instances, should be applied

in checkerboard fashion.

In

all cases the dimensioning of walls should be

based on the least favourable stressing of the masonry.

In

ordinary straight

masonry, load distributions of 60° can be assumed.

10. Walls erected on floors often exibit cracks which have their origin in the bending of the floors due to their own weight, the imposed load, the

useful load, or to shrinkage and creeping of the concrete. This type of

damage should be avoided by the application of suitable measures.

11. The allowable bending tensile stresses parallel to the horizontal joints and the shear stresses depend on the stresses due to dead weight and on the loading of the wall (friction stresses).

°b all. and ｾ｡ｬｬＮ a ｾＳＰＰｧＮ 005 = 1/10 005

More stringent conditions with respect to calculation, desicn and exec-ution as well as verification of quality must be applied to more heavily loaded masonry made of more brittle masonry units.

In

planning and designing the masonry, the following regulations must

be adhered to:

1. The size of the rooms must be adapted to the dimensions of the brick and

Joints.

2. Windows and doors must be located in such a way that stiffening wall

parts are formed (higher loading

=

more economical utilization).

3.

Horizontal Joints should be laid perpendicular to the direction of force

or to the force "line.

4.

The minimum thickness of bearing masonry is 12 em.

5.

Avoidance of wedging stresses; loading and deformation of walls and

colunms as uniform as possible.

6.

Centric positioning of walls with respect to each other.

7.

Centric imposition of floors and supports above; for walls of less than

25 em thickness, imposed on the entire wall.

8.

The bonding-in length, even in the wall cross-section must be at least

0.45 times the height of a unit.

9.

In

special masonry the units, because of their brittleness must not be

split.

t, t,

t

units must therefore be ordered from the suppliers.

10. Ties, when the inside walls because of the danger of cracking are not

-9-11. Expansion joints should be provided depending on conditions and the

nature of the masonry.

12. In special purpose masonry no slots or perforations shall be made

sub-sequent to erection. Any openings must be shown in the layout. In

high-grade masonry vertical slots may be cut into stiffened walls, but never in

piers.

13. Plans for slots should be seen, checked and inspected by the engineer

responsible for the bearing capacity. Any changes from the engineering

plans should be approved by him.

14. Layouts of angle changes, corners and bindings of inside walls are

indispensable in special purpose masonry, and desirable for high-grade ma-sonry.

15. Cavity masonry with non-corrosive ties at storey height shows a great

many advantages. Corner reinforcements are needed in order to prevent

opening of the walls. The outside wythe must be absorptive, or else an

air gap of at least 2 em should be prOVided.

16. Weather-proof bricks and frost-resistant mortar are indispensable on

the outside.

17. The heat-insulating layer must be laid oveI' the floor; heat bridges

murt be avoided.

18. CI'acks in masonry carrying small loads which are caused by shrinking

roof slabs (top floor), and a Lso the bulging of corners and arching of floor edges (including projections) should be prevented or concealed by Buitable measures.

19. For calCUlating reinforced or prestressed masonry where applicable the

edge stresses may exceed the permissible average stresses by 40,%.

20. Free-standing gable walls should be tied against tensile stressing.

In execution, in addition to the above regUlations 3 to 20, the following

should also be observed:

21. The vertical joints should be staggered in the head also.

22. In high-grade and special-purpose masonry the following conditions must

be satisfied (MH

=

high grade masonry, MS

=

special quality masonry):

Length 250 em MH MS

Departure from straight line in nun 4 3

Departure from plumb in mm 4 3

Departure of course joints from the

horizontal in nun 15 10

Joint thickness in nun 8-16 10-15

Vertical joints 10 mm

23. Bonding-in of masonry seGments every second course: half the length of the un Lt ,

24. In exposed masonry the Joints should be grouted, or at Least smoothed.

25. If Wlder exceptional circumstances masonry is executed at 0 to -5°0,

the following conditions should be observed: mortar temperature +10°0,

laying of dry bricku warmer than O°C; work to stop at 4:00 p.m. The finished

I

ｾ｡ｳｯｮｲｹ should be covered on both sides and over its entire height.

On-site inspection should include the materials (10 units and mortar)

and the masonry work (quality and the rate of construction) (Fig. 7.).

Sununar:¥

Masorir-y can now compete more effectively With o t.her- rnet nods of

construc-tion which are coming into use in domestic architecture, owing to the greater

utilization rendered possible by the new standards. It will contlnue to be

employed even in competition with prefabricated construction me t.nods , at

least as long as qualified masons are available. The greatly improved

uniformity of dimensions of masonry units has enabled the mason to improve

his output. In addition to quality lmprovements, the more uniform unit

dimensions have contributed to a relative reduction in the cost of masonry construction.

Tile present standards have been built up from a large number of test results, and also from experience that has been gained in the construction

of a number of tall bUildings. The requirements imposed especially for

special purpose masonry are stringent, but are necessary if the brittle special quality masonry units showing considerable internal stresses are to be used in relatively thin walls under high stress.

The standards would appear to prove that even an old method of construc-tion like masonry can still compete, in a restricted sector of course, with the two younger methods of construction, since on the basis of recent

knOWledge and through the powerful efforts of the building industry it has been possible to improve it to such an extent that masonry has been elevated

to a status of civil engineering and is even honoured as such. This is an

important point.

Experiments are shortly to be carried out to provide information on the

effect of stiffening by means of walls placed at an angle. The gainlng of

further information is also planned on the bearing capacity of natural stone

masonry. These tests are made possible by the 2000-ton press which has Just

been put into operation. Even the "ancient" craft of masonry continues to

-11-Fig. 1

Brick apartment house in Biel, constructed in 1959/61, 16 storeys Photo by P. Moosbrugger

Internal masonry (bearing): 15 cm brick

Outside masonry:

Inner wythe (bearing) 15 cm brick

Insulation layer 3 cm mineral wool slabs

Outer wythe (veneer,

self-supporting) 12 cm brick

Floors: 17 cm solid reinforced concrete

ｾｾｽ

...

/ ｾ

1

I

4

I I - - - I

/ - - -

_--- I r

ｾ｟ＮＭ

I

we:

_1lI

=-,] I I I I I I I

:

Fig. 2 Stress-strain diagram Load imposed: In strength range Eｾ Eo

u

m Ol

=

f3

(masonry strength)

In buck1 ing region

c

ｾ Eo

Omol<

f3

I: centrically

II: eccentrically

III: null line outside cross-section

ｸＮｾ d s l ight1y eccentrically

IV: null line inside cros$-section

co ..c «JO .&.J 1lO l:l 10 co

...

.&.J m

'0

;>..

...

c _fIl 0 m ｾ II 0 If Slenderness Fi g. 3

Bearing capacities of 12 em brick masonry.

Ｑｾ standard units with round perforations •

ｾ･｡ｮ compressive strength of unit

289 kglcm2; It me cement mortar 29 kglcm2,

width 7R cm, age 29 days, edge hearings.

• and 0 test points. Henvy lines _

theoret-ical curves from the stress-strain diagram from small masonry specimens, calculated with an equilateral eccentricity of point of load application p' _ 1,/1000. Euler

buckling stresses (p' _ 0) I,·)."""" rea a£-rang nary range n-m m (\)

t

m iu 1lO III 6S

...

(\) ｾ rid0Ｂｾｯ , • 100.".",1 I. -I"" m Stress-strain m

ｾ diagram Two examples for the calculation of the

... 0000

ｾ ｾＮＮＮＬｬ bearing capac i ty of a wa11 with equt

Lat.-r

crally eccentric load showing a

compres-ｾ I sive strength of 100 kg/cm2 as a maximum

ｾ I I

ｾ ＮｾＭＱ｟ for increasing de fo rmstl on of fl bre s E: 1•

• - I '0(. S . d' 1, b 1 ( i

Strain (short- tress-straIn Ingram: ra r a 0 a

approx-ening) in %D mation), slenderness ratio "kid,", 20 and 25.

For deformation greater than that corres-ponding to the maximum value, equilibrium is possible only with a load below maximum.

In the imaginary region ("kid == 20, E:l '"'

3.4291.. ) equilibrium is unattainable even

for a sma11 load Strain (shortening) in %D

til _{Strers-strain} til QI d alJrilm H

••

ｾ ·'00 kg/em' ｾ ,ao _{d. 'I,m} Ul p'.I/loO Ul Ul OJ H ｾ til QI bO III H I QI > -< 0 10 ,a 10 Slenderness ratio Lk/ d Fig. S

Comparison of the bearing capacity of brick walls de-termined ｦｲｯｾ the stress-strain diagram with that de-termined from the parabolic stress-strain diagram. Wall thickness 12 cm, lime cement mortar. Eccentricity of load (equilateral) p' 2 L/IOOO slightly absorptive

brick m •.!. II e

o

',00

t •

-y,

- f

o6'K • 811 ora hIe stre sst' s f ot!

centr c oad for Lk/d ｾ 5

...6it.

a lJr,,,ahlellstresses fpr

egut atcf8 V eccentrlG

[ 6: lUadJngｬＮｫＯｾ ｾ｡ｾｬ ｾ･ｮ 0(,Ｍｾ

:6':

(1+rt l+(1-0,1ml(1-'l'J

'1 •

+1bls - f ＧｭｬＯｾ

I

ml

Example r a t i 4 "0,5 ll/Id m.. 1 oUK -+ 1 ｏＬｾ

+Y

z 0,6 o 0,7

-'It

0,8 - t 0,9 Fig. 6

t:mpirical formula for calculation of allowable stresses for oqu l Interally or crossed

-15-600,.,---,---,

_,.,

['I 500 f-u -, ｴｾ .--" Co 400

....

.r:: J.J ｴｾ ｾ 0 \-I JOO J.J til OJ > or! 111 ' 0 ₂₀₀ \-I c. r.; c u m '11 .ｮｈｯ［ＮＬｾ［［ ..［ｩ［Ｎ［［［ｾＬｴ _. Stnd._{compres .•}ｾ ｳｴｲｾｮｧｴ I "Number hUM S 6 s 9 , S. 5 1 , 5 II 1 , 5 5 I I ' ｾ 8 12 ｾ ｦｾ t4 " ｾ , , I/r.1 2 J 4 5· I 7 • , 10 tt I! I] 14 15 II 17 11 !!I 20 21 22 2] 24 25 ZI 27 28 zt ]0 ]1 J2 Fig. 7

Mortar compressive strength values from 32 construction sites from 1958-1964. Prisms furnished in sizes

of 4 x 4 x 16 em, 28 days. Mean values and extreme values. Number of