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Chapter 7

EARTHEN BUILDINGS

7.1 INTRODUCTIONEarthen construction has been, is and willcontinue to be a reality. Recent statisticsshow that the percentage of earthen hous-ing in the next 15 years will be higher than50%.

Even though this material has clear ad-vantages of costs, aesthetics, acoustics andheat insulation and low energy consump-tion, it also has some disadvantages suchas being weak under earthquake forces andwater action. However, the technology de-veloped to date has allowed a reduction inits disadvantages, stressing its most valu-able advantages.

Earthen constructions are, in general,spontaneous and a great difficulty is thedissemination of knowledge about its ad-equate use.

The recommendations presented hereinare applicable to earthen constructions ingeneral, but they are especially oriented topopular housing, aiming to enhance thequality of the spontaneous, informal or

massive constructions which are the onescausing the greatest loss of life and dam-age during seismic events.

Therefore, it does not include solutionsinvolving the use of stabilizers (cement,lime, asphalt, etc.) to improve the strengthor durability. Also for making the strength-ening very economical, minimum use of theexpensive materials (concrete, steel, wood,etc.) has been indicated to enhance the dy-namic behaviour of the structure.

7.2 TYPICAL DAMAGE ANDCOLLAPSE OF EARTHENBUILDINGSEarthquake experience shows that earthenbuildings may be cracked at MSK IntensityVI, wide cracks and even partial collapsemay occur at MSK VII and collapses arewidespread under MSK VIII. Damage is al-ways much more severe in two storeyedbuildings than in one storeyed ones. Sometypical damages are sketched in Fig 7.1.However, single storeyed houses with flatroofs constructed in good clay have beenfound to be undamaged in Intensity VIII

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Fig 7.1 Typical damages and collapse of earthen buildings (continued on next page)

(a) Corner failure and out of plane collapse of walls

(b) Gables

(c) Two storey house damage / collapse

(d) Split level roof

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(e) L shaped buildings

Fig 7.1 Typical damages and collapse of earthen buildings (continued from previous page)

(f) High walled houses

(g) Awning

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zone whereas at the same location two sto-reyed houses were completely ruined. Themain courses of failure of earthen build-ings in earthquakes are graphically sum-marised in Fig 7.2.

7.3 CLASSIFICATION OFWALLS AND MATERIALPROPERTIESIn earthen construction, the walls are thebasic elements hence it can be classifiedaccording to the wall type as follows:

7.3.1 Classification of earthenconstructionsa. Hand-formed by layers

a.1 Simple forming

a.2 Earth balls, thrown andmoulded as wall

b. Adobe or blocksb.1 Cut from hardened soil

b.2 Formed in mould

Fig 7.2 Graphic summary of causes of failure

b.3 Moulded and compacted

c. Tapial or pise (rammed earth)c.1 Compacted by hand blows

c.2 Mechanized or vibratingcompaction

d. Wood or cane structure, with wood orcane mesh enclosures plastered with mud

d.1 Continuous

d.2 Pre-fabricated panels

Whereas systems (a), (b) and (c) dependfor stability on the strength of earthenwalls, the system d behaves like a woodframe and its construction will be dealtwith separately.

7.3.2 Suitability of soilThe quality of materials, particularly claycontent of the soil may vary somewhat forthe type of construction. But in general thefollowing qualitative tests are sufficient for

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determining the suitability of a soil forearthen construction:

a. Dry strength testFive or Six small balls of soil of approxi-mately 2 cm in diameter are made. Oncethey are dry (after 48 hours), each ball iscrushed between the forefinger and thethumb. If they are strong enough that noneof them breaks, the soil has enough clay tobe used in the adobe construction, providedthat some control over the mortar micro-fissures caused by the drying process isexercised, Fig 7.3(a)

If some of the balls break, the soil is notconsidered to be adequate, because it doesnot have enough clay and should be dis-carded.

b. Fissuring control testAt least eight sandwich units are manu-factured with mortars made with mixturesin different proportions of soil and coarsesand. It is recommended that the propor-tion of soil to coarse sand vary between 1:0and 1:3 in volume. The sandwich havingthe least content of coarse sand which,when opened after 48 hours, does not showvisible fissures in the mortar, will indicatethe most adequate proportion of soil/sandfor adobe constructions, giving the higheststrength.

7.3.3 Strength test of adobeThe strength of adobe can be qualitativelyascertained a follows: After 4 weeks of sundrying the adobe be should be strongenough to support in bending the weightof a man, Fig 7.3 (b). If it breaks, more clayand fibrous material is to be added. Quan-titatively, the compressure strength may bedetermined by testing 10 cm cubes of clay

after completely drying them. A minimumvalue of 1.2 N/mm2 will be desirable.

7.4 CONSTRUCTIONS OFWALLSIn general, the strength of walls is a func-tion of clay content, and its activation byhumidity (promoted by wetting orcompaction procedures) and the control offissuring.

The positive effect of a traditional prac-tice, namely �sleeping� the mud (storing itat least for one day but better for more days)before the fabrication of adobe bricks ormortar was confirmed. It seems that thisprocedure allows for a better dispersionand thus for a more uniform action of theclay particles.

Fig 7.3 Field testing of strength of soil and adobe

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If the soils are clayey, stronger construc-tions could be built, provided an adequatetechnology is used to control the typical fis-sures caused by drying from high moisturecontent. The most economical and simpleform to control such fissures is by addingcoarse sand to diminish the clay contrac-tion or by adding dry straw to the mud tocontrol the micro-fissures.

In general, there are no �recommended�mixing ratios for the soil to be used inearthen constructions. The different per-centages of clay, lime, fine sand and coarsesand will be defined by the most abun-dantly available nearby soil, its clay con-tent (see dry strength test), the type of con-structions required according to the classi-fication and the amount of coarse sandneeded to control or avoid the visible fis-sures and attain a monolithic behaviour.

It may be concluded that:

(i) Soils with low clay content shouldnot be used (see dry strength test).

(ii) Coarse sand needs be added to avoidfissures and straw to control them.

7.4.1 Hand-moulded layeredconstructionThese are the most primitive and weakesttype of constructions because of the lowpercentage of moisture employed to makethe hand-moulding and the poor level ofcompaction attained. For these reasons, allthe clay is not activated, either by moisture,or by compaction.

Even though a small amount of mois-ture is used (depending on the soil), somehorizontal and also vertical fissures nor-

mally appear. These should be controlledby adding straw as much as possible to at-tain a reasonable workabity of the mixture.If this is not possible, coarse sand could beused as additive, in the smallest experimen-tal proportion able to achieve the disap-pearance of visible fissures (try with in-creasing proportions and wait a few daysto check the results). An excess of coarsesand will inevitably reduce the wallstrength.

Generally, it will be necessary to mois-ten the area of the lower layer which willbe in contact with the mud, in order to avoidsudden drying of the contact zone, whichproduces the fissures.

7.4.2 Adobe or blockconstructionIn the case of cut as well as moulded blocks,the strongest units correspond to plastic orclayey soils. However, the block strengthplays a secondary role in the masonrystrength, since the joints between blocksbecome critical. The blocks used should bewell dried in order to avoid futureretractions. Blocks are made in differentsizes in various countries.

It may be stated that the dimensions ofthe blocks, nor the way these are placed,have a serious effect on their strength.

Traditional practices obtain an adequateblock without important fissures, either bymixing sandy and clayey soils or by look-ing after the block so it dries without re-strictions, thus eliminating the fissures. Thesoil to be used should be verified with thedry strength test, Fig 7.3, to ensure a mini-mum strength.

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The mud used to fill the space betweenblocks (the so called �mortar�) requiresspecial attention.

To guarantee the bond between blocksand mortar, the micro-fissures of the lattershould be avoided. The conditions of themortar drying are very severe because ofthe fact that the mortar gets in contact withblocks which readily absorb moisture andalso they restrict the drying contraction.This produces the above-mentioned micro-fissures, which consequently weaken themasonry.

The joint mud should normally be thesame as used to manufacture the block. If itis found to fissure, some straw (nearly 1:1by volume) should be added to the mortaruntil an acceptable degree of workabilityis attained. Some coarse sand could alsobe added, the adequate proportion beinggiven by the fissuring control test, 7.3.2 (b).

For clayey soils, the adobe blocks shouldbe moistened for a few minutes before plac-ing them. Also it will be useful to moistenthe previous layer of blocks before placingthe joint mortar. For sandy soils, it will beenough only to moisten the preceding layerof blocks.

The usual good principles of bonds inmasonry should be adopted for construc-tion of adobe walls, that is,

(i) All courses should be laid level.

(ii) The vertical joints should be brokenbetween two consecutive courses byoverlap of adobe and must be care-fully filled with mortar.

(iii) The right angle joints between wallsshould be made in such manner thatthe walls are properly joined to-gether and a through vertical joint isavoided.

7.4.3 Tapial or pise constructionTapial or pise constructions are rammedearth constructions in which moist soil ispoured in wooden forms of the walls andcompacted to achieve the desired density.

Whilst adobe constructions acquiretheir strength by activation of the claythrough moisture contained in the soil,tapial constructions owe it to compaction,using small percentages of moisture in thesoil.

High strength is obtained by humidityand compaction when clay is present.There are however, practical limitations torestrict the moisture, such as the feasibilityto pound and compact the soil, the exces-sive deformation occurring when the formsare removed and the fissuring problem.

The use of low moisture content (suchas the optimum in the Proctor test or lower)and the control of the amount of clay byadding coarse sand to the soils are requiredto control the shrinkage fissures on drying.If the amount of coarse sand is excessive,the strength diminishes dangerously. It isrecommended to make wall tests with in-creasing percentage of sand, until fissur-ing is reasonably under control.

The compaction or number of blowsapplied to the wall is a function of theweight and shape of the tool used for thispurpose. Higher strengths are obtained

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under higher compaction, but there is apoint at which this is not true any more. Anormal compaction is recommended andthis will be the one under which no mudremains stuck to the form when this is re-moved.

Fifty strokes per 1000 cm2 of wall area,applied with a mallet of about 8 to 10 kgweight is the recommended practice. Therequired height for the blocks varies be-tween 50 and 80 cm, but it is very impor-tant that the compacted layers in the blocksdo not exceed 10 cm each.

The best way to ensure the monolithicstructure of the tapial walls is to sufficientquantity of water at the sub joints at every10 cm. Likewise, between the tapial layers,every 50 to 80 cm, it is necessary to pourplenty of water on the layer before compact-ing further material. The placing of straw

between the tapial layers is not necessary.

The use of excessive amounts of strawin the mud mixture, more than 1:1/4 in vol-ume is selfdefeating, because it causes astrength reduction.

7.4.4 Earthen construction withwood or cane structureThe scheme of earthen construction usingstructural framework of wood or cane isshown in Fig 7.4. It consist of vertical postsand horizontal blocking members of woodor cane or bamboo, the panels being filledwith cane or bamboo, or some kind of reedmatting plastered over both sides with mud.The construction could be done in the ru-dimentary way, building element by ele-ment or by using prefabricated panels.

The behaviour of this type of construc-tion could be very good, as long as the fol-

Fig 7.4 Earthen constructions with wood or cane structures

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lowing fundamental rules are observed:

� Good connections between the woodor cane elements, so as to ensure anintegral behaviour of the structure.The connections are normally fixedwith nails. Their number and dimen-sions should be enough but not ex-

cessive as to split the elements. Theconnections can be also tied withwires, ropes, leather straps, etc.

- Preservation of the wood or cane ele-ments by charring the surface orpainting by coal tar, especially in thepart embedded in the foundation,

Figure 7.5 :: Adequate Configuration

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which should preferably be of con-crete, stone or bricks laid with ce-ment, lime or gypsum mortar.

- Additionally, it is recommended thatthe panel filling material shouldconsist of wood or cane mesh, overwhich a layer of mud and straw (1:1in volume) is placed on each face in

the form of plaster. Very often, themeshes are knit in themselves andaround the structure.

- In houses built as a continuous sys-tem as well as in those made withpre-fabricated panels, an upper ringbeam should be placed, the purposeof it being two fold:

(i) Ensure the integral behaviour ofall walls, and

(ii) Distribute evenly the roofingload.

Only after fixing this upper ring beamand the roof (after completing the nailing),the mud filling must be placed. This willavoid fissuring caused by the strokes of thenailing operation.

In the case of pre-fabricated panels, theframes could have very small and economi-cal sections 25 × 50 or 25 × 75 mm. The

Fig 7.7 Pillasters at corners

Fig 7.6 Wall dimensions

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connection between panels is madethrough nails, but the wood or cane knitmesh over which the mud filling is placed,can be fixed without the use of nails.

7.5 GENERALRECOMMENDATIONS FORSEISMIC AREAS7.5.1 Walls

(a) The height of the adobe buildingshould be restricted to one story plusattic only in seismic zones A and Band to two storeys in zone C.

(b) The length of a wall, between twoconsecutive walls at right angles toit, should not be greater than 10 timesthe wall thickness �t� nor greater than64t2/h where h is the height of wall.

(c) When a longer wall is required, thewalls should be strengthened by in-termediate vertical buttresses,Fig 7.6 (a).

(d) The height of wall should not begreater than 8 times its thickness.

(e) The width of an opening should notbe greater than 1.20 m.

(f) The distance between an outsidecorner and the opening should benot less than 1.20 m.

(g) The sum of the widths of openingsin a wall should not exceed one-third

of the total wall length in seismiczone A, 40 percent in zones B and C.

(h) The bearing length (embedment) oflintels on each side of an openingshould not be less than 50 cm. Anadequate configuration is shown inFig 7.5 for an adobe and tapial house.

(i) Hand-formed walls could preferablybe made tapering upwards keepingthe minimum thickness 30 cm at topand increasing it with a batter of 1:12at bottom, Fig 7.6 (b).

(j) Providing outside pillasters at allcorners and junctions of walls willincrease the seismic stability of thebuildings a great deal, Fig 7.7.

Fig 7.8 Use of longitudinal wood under roof rafters

Fig 7.9 Reinforcing lintel under floor beam

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7.5.2 Foundations(a) The brittleness and reduced strength

of adobe constructions restricts thepossible locations of these buildingsto areas associated with firm sub-soils.

Sandy loose soils, poorly compactedclays and fill materials should gen-erally be discarded due to their set-tlements during seismic vibrations.

Also, soils with very high phreaticlevel (water table) should be avoided.These recommendations are particu-larly important for seismic zones Aand B.

(b) Width of strip footings of the wallsmay be kept as follows:

One storey on firm soil � Equal towall thickness

1.5 or 2 storeys on firm soil � 1.5times the wall thickness

One storey on soft soil � 1.5 timesthe wail thickness

1.5 or 2 storeys on soft soil � 2times the wall thickness

The depth of foundation belowground level should at least be 400mm.

(c) The footing should preferably bebuilt using stone, fired brick using

Fig 7.10 Collar band in walls at lintel level

Fig 7.11 Roof band on pillastered walls

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cement or lime mortar. Alternativelyit may be made in lean cement con-crete with Plums (cement : sand :gravel : stones as 1:4:6:10) or with-out plums as 1:5:10. Lime could beused in place of cement in the ratiolime:sand:gravel as 1:4:8.

(d) Plinth masonry: The wall above foun-dation up to should preferably beconstructed using stone or burntbricks laid in cement or lime mortar.

Clay mud mortar may be used onlyas a last resort. The height of plinthshould be above the flood water lineor a minimum of 300mm aboveground level. It will be preferable touse a waterproofing layer in the formof waterproof mud (para 7.7 (c)) orheavy block polyethylene sheet at theplinth level before starting the con-struction of superstructure wall. Ifadobe itself is used, the outside face

Fig 7.12 Reinforcement in walls of earthen constructions in zone

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of plinth should be protected againstdamage by water by suitable facia orplaster. A water drain should bemade slightly away from the wall tosave it from seepage.

7.5.3 RoofingRoofs have two main parts: the structureand the cover. The roofing structure mustbe light, well connected and adequately tiedto the walls.

(a) The roof covering should preferablybe of light material, like sheeting ofany type.

(b) If thatch is used for roof covering, itshould better be made waterproofand fire resistant by applying watermud plaster, para 7.7(c).

(c) The roof beams, rafters or trussesshould preferably be rested on lon-gitudinal wooden elements for dis-tributing the load on adobe, Fig 7.8.If wood is not used preferably twotop courses of burnt bricks may belaid instead of adobe for resting theroof structures.

(d) The slopes and the over-hangingwill depend on local climatic condi-

tions. In zones subjected to rain andsnow, walls protection must be en-sured by projecting the roof by about0.5 m beyond the walls, Fig 7.8.

(e) The roof beams or rafters should belocated to avoid their position abovedoor or window lintels. Otherwise,the lintel should be reinforced by anadditional lumber, Fig 7.9.

7.6 SEISMIC STRENGTHENINGFEATURES7.6.1 Collar beam or horizontalbandTwo horizontal continuous reinforcing andbinding beam or bands should be placed,one coinciding with lintels of doors andwindow openings and the other just belowthe roof in all walls, in all seismic zones forconstructions of types (a), (b), (c), describedin section 7.3.1. Proper connection of tiesplaced at right angles at the corners andjunctions of walls should be insured.Where the height of wall is no more than2.5 m, the lintel band can be avoided butthe lintels should be connected to the roofband as shown, in Fig 7.11. The band couldbe in the following forms:

(a) Unfinished rough cut lumber in sin-gle pieces provided with diagonalmembers for bracing at corners,Fig 7.10(a).

(b) Unfinished rough cut or sawn (50 ×100 mm in section) lumbers twopieces in parallel with halved jointsat corners and junctions of wallsplaced in parallel, Fig 7 10(b).

7.6.2 Pillasters and buttressesWhere pillasters or buttresses are used, asrecommended in 7.5.1(j) at T-junctions, the

Fig 7.13 Diagonal bracing of wood-structures of earthen construction

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collar beam should cover the buttresses aswell as shown in Fig 7.11. Use of diagonalstruts at corners will further stiffen the col-lar beam.

7.6.3 Vertical reinforcement inwallsa. In mesh form of bamboo or caneIn seismic zone A, mesh form of reinforc-ing will be preferable. Here the whole wallsare reinforced by a mesh of canes or bam-boos as shown in Fig 7.12 along with thecollar beams, which may in this case bemade from canes, or bamboos themselves.The vertical canes must be tied to the hori-zontal canes as well as the collar beam atlintel level and the roof beam at eave level.

b. With collar beams or bandsFor seismic zones A and B, in addition tothe collar beams recommended in 7.6.1 and7.6.2 provision of vertical reinforcement in

earthen walls of earthen constructionstypes (a), (b), (c) will be necessary whereasit can be avoided in zone C.

The most effective vertical reinforcementwill be in the form of wood posts, bambooor cane located at corners and junctions ofwalls. It should be started at the founda-tion level and continued through and tiedto the lintel and roof bands by binding wire,fishing line or rope etc.

7.6.4 Diagonal bracingIn case of earthen constructions type d (Sec-tion 7.3.1), it will be necessary for achiev-ing adequate seismic resistance in zones Aand B to provide diagonal bracing mem-bers in the planes of walls as well as hori-zontally at the top level of walls as shownin Fig 7.13. This can be done by using canesor bamboos nailed to the framing members

Fig 7.14 Good features of earthquake resistant construction

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at the ends and intermediate points of in-tersection.

7.7 PLASTERING ANDPAINTINGThe purpose of plastering and painting isto give protection and durability to thewalls, in addition to obvious aesthetic rea-sons.

(a) Plastering based on natural addi-tives could be formed in two layers.The first one of about 12 to 15 mm, isa mixture of mud and straw (1:1 involume), plus a natural additive likecow dung used to increase the mois-ture resistance of the mud, thus pre-venting the occurrence of fissuresduring the drying process. The natu-ral additive helps to withstand theshrinkage tensions of the restricteddrying process. The second and last

layer is made with fine mud whichwhen dried, should be rubbed withsmall, hard, rounded pebbles.

(b) A technology has been developedwhich consists of plastering wallswith a mud stucco stabilized withcactus as described hereunder:

The main recommendations for plaster-ing adobe walls with this type of stucco areto

(i) Prepare the cactus stabilizer bysoaking cactus chopped piecesuntil the soft (inside) part dis-solves completely leaving theskin only as residue. The ob-tained product is characterizedby gluey consistency, green colorand strong smell of decomposedorganic matter.

(ii) Remove dust from the wall sur-face.

(iii) Apply the stucco in two layers,a first layer of 12 mm thicknessand a second very thin layer (ap-proximately 3 mm). The firstlayer contains straw, and coarsesand in amounts that allow anadequate workability. The sec-ond layer contains straw insmall pieces (approximately50mm) and should not containcoarse sand. The second layercovers the cracks of the first layerand provides a surface adequateto be polished. Both layers aremixed with cactus stabilizer(water is not used).

(iv) Rub the stucco surface with acoarse stone (granitic). Thereaf-Fig 7.15 Axial compression test

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ter, moist the surface with the sta-bilizer and polish it with asmooth stone (basaltic stone).

(v) Paint the finished surface withthe cactus stabi1izer.

(c) To obtain a truly waterproof mudplaster, bitumen may be used in thefollowing way where this materialis found feasible to use: cut-back isprepared by mixing bitumen 80/100grade, kerosene oil and paraffin waxin the ratio 100:20:1. For 1.8 kg cut-back, 1.5 kg bitumen is melted with15 grams of wax and this mixture ispoured in a container having 300millilitre kerosene oil with constantstirring till all ingredients are mixed.This mixture can now be mixed with0.03 m3 of mud mortar to make it bothwater repellent as well as fire pro-tection of thatch.

The exterior of walls may then be suit-ably painted using a water-insoluble paintor wash with water solutions of lime or ce-ment or gypsum and plant extracts.

7.8 SUMMARY OF DESIRABLEFEATURESThe desirable features for earthquake re-sistance of earthen houses are briefly illus-trated in Fig 7.14.

7.9 WORKING STRESSES7.9.1 Unit compressive strengthThe compressive strength of the unit is anindex of its quality and not of the masonry.

It will be determined by testing cubes ofapproximately 100 mm. The compressivestrength fo is the value exceeded by 80% ofthe number of specimens tested.

The minimum number of specimens issix (6) and they should be completely dryat the time of testing. The minimum valueof fo is 1.2 N/mm2.

7.9.2 Masonry compressivestrengthThe masonry compressive strength may bedetermined by:

(a) Prism test with materials and tech-nology to use in the field.

The prisms will be composed by thenumber of full adobes necessary toobtain a height/thickness ratio ofthree.

The minimum number of adobes willbe four and the joint thickness lessthan 20 mm, Fig 7.15.

Special care should be observed tokeep the specimens vertical. They

Fig 7.16 Diagonal compression test

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should be tested after 30 days of con-struction to obtain a mortar com-pletely dry. The minimum number ofprisms will be three.

The permissible compressive stressfm in wall will be:

fm = 0.4R f � m

where:

R = Reduction factor due to wallslenderness.

R can be obtained by analogy toan elastic column but not greaterthan 0.75.

f � m = Ultimate compressive stress ofprism. Two of every three prismsshould have greater values thanthe compressive strength.

Alternatively the following expres-sion can be used:

fm = 0.2 f � m

(b) If no prism test is conducted, the per-missible compressive stress will be

fm = 0.2 N/mm2

The permissible crushing stress willbe: 1.25 fm

7.9.3 Shear strength of masonryThe shear strength of adobe ma-sonry can be determined by:

(a) Diagonal compression test with ma-terials and technology to be used inthe field, Fig 7.16.

A minimum of three specimensshould be tested. The permissi-ble strength of wall (vm) will beobtained from :

vm = 0.4 f � t

Where: f � t = ultimate strengthof specimen tested. Two of everythree specimens will save valuesthat exceed f � t .

b. When no tests are conducted, the fol-lowing value for the shear strengthmay be used

vm = 0.025 N/mm2

7.9.4 Permissible tensilestrength of masonry for loadsperpendicular to its plane ( fa)

fa = 0.04 N/mm2

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