Cavity Wall

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Seminar Report

On

C0MPOSITE MASONARY WALL AND CAVITYWALL

NAME:

ROLL:MIT/CE/S6-

MRS!IDA"AD INSTITTE O# TEC!NOL$Y


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INTRODCTION

Simply stated, a cavity wall is two wythes of masonry, separated by a cavity ofvarying dimension. The masonry wythes may consist of solid brick, structural clay tile,

or concrete masonry units and are bonded together with masonry ties. The cavity(ranging from 2 inches to 4 12 inches in width! may or may not contain insulation.

See "igure 1. #ombining these elements with a sound structural design, appropriatedetails, $uality materials and good workmanship will result in high performance cavity

walls

!ISTORY

#avity walls are not new, they have been observed in ancient %reek and &oman

structures. 't the %reco &oman town of ergamum, on the hills overlooking theTurkish town of )ergama, a stone wall of cavity type construction still

e*ists. Sometime in the early part of the 1+th century, the cavity wall was probablyreinvented by the )ritish. lans dating as early as 1- suggest a type of

construction, featuring two leaves of brickwork, bonded by headers spanning across a/0inch cavity. 'n early )ritish publication (dated 121! suggests the use of cavitywalls as a means of protection against moisture penetration. The use of metal tieswas introduced in Southern ngland sometime after 1-. These original ties were

made of wrought iron.


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#avity walls were first built in the nited States late in the 1+th century. "igure 2illustrates an alternate type of cavity wall system originally featured in an 1++ te*tbook assembled for people engaged in the engineering professions and constructiontrades. 3owever, it was not until 1+5 that this type of construction gained officialacceptance by any building or construction agency in the nited States. Since then,interest in and use of cavity walls in this country has increased rapidly. This has

resulted in e*tensive testing to determine cavity wall properties and performance.

The early use of cavity walls in this country was limited primarily to e*terior load0

bearing walls in low rise construction. 6n the 1+4-s, designers began to recogni7e theadvantages of cavity walls in high0rise buildings. Today, masonry cavity walls are usede*tensively throughout the nited States in all types of buildings. The primaryreasons for their popularity are superior resistance to rain penetration, e*cellentthermal properties, e*cellent resistance to sound transmission and high resistance tofire.

PROPERTIES O# CAVITY WALLS

RESISTANCE TO MOISTRE PENETRATION

8o single unreinforced 49 wythe of masonry is totally impervious to moisturepenetration. ' cavity wall is designed and built as a moisture0deterrent system. Thissystem takes into account the possible moisture penetration through the outer wythe.:oisture will penetrate masonry walls where hairline cracks e*ist between masonry

unit and mortar. ;ater which runs down the e*terior wall surface will be drawntowards the inner cavity due to wind pressure e*erted on the e*terior of the wall and

the negative pressure present within the cavity. roviding a clean air space will allowthis moisture to flow unobstructed down the cavity face of the outer wythe. "lashing

installed at recommended locations will then divert this moisture back to thebuilding


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't one point in time, energy conservation was not a ma=or consideration in buildingdesign. #avity walls were primarily built for their structural and moisture diverting$ualities. >uring the mid 1+5-


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moisture, and its foil back enclosure creates a reflective air space that increase thewalls overall & value by appro*imately 2.. The & value of a typical cavity wall mayrange from 14 to 2/ depending on the type and thickness of insulation selected

Ta%&e ' - R Va&(e o) "ri*+ an, "&o*+ Cait. Wa&&

*terior 'ir "ilm -.15

49 )rick -.44

& of reflective air space 2.-

29 olyisocvyanurate 1/

/9 cmu -.4

1 129 air space -./

T@T'A & B'A 22.

#IRE RESISTANCE

&esults of the 'ST: 011+ "ire &esistance Tests and the contents of both the "ire

rotection lanning &eport (#:6"#!2 and the "ire &esistance &atings. &eport ('6S%!clearly indicate that masonry cavity walls have e*cellent fire resistance. 'll cavitywalls have a fire rating of 4 hours or greater.

STRCTRAL PROPERTIES


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:asonryead load C -Dft * 242 C +/- Dft/9 #: C *2/ C 21- Dft

Aive load C 4- Dft2 * 242 C 4- Dft

se Aive Aoad &e$uirement&oof Aoads?


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Aet drainage fill F roofing C 2- Dft2>ead load C /- F 2- C - Dft2 * 242 C +/-Dft

Aive Aoad C -*242 C /- Dft;all >esign

se '#6 -0++'S# 0++T:S 4-20++!'ssume?G ;all height < -9

G 9 concrete plank bears fully on /9 #:'t &oof H C .+/I1 F ./I1 C 1.2I1

e C ./ 2 0 ./ C .+9 19'llow. load. C /./4I1 H 1.2 @I

't 2nd "loor H 1 C 1.2 C (.21! C 4(.+/! F 4 (.5 * .4! C 5./ I1

e C .+92 C 1.44I1 * (.+! C 1.4I1

1 F 2 C +.-+I1

ev C 1.4 +.-+ C -.191 F 2'llow load C +.4I1 H +.-+I1 @I


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'fter the cavity wall has been designed to meet the structural re$uirements,connections between the precast concrete plank and the masonry wall must bedetailed. @ther details, such as flashing, must

also be developed. The wallfloor connections provide the wall with lateral bracingagainst wind loads. This connection should also assist in the transfer of shearstresses, and in the case of bearing walls, transfer gravity loads to the foundation

CONNECTION #OR LOAD "EARIN$

@ne way to anchor precast concrete plank into load bearing concrete masonry is tocreate a positive tie with reinforcing bars bent at +- degree angles, see "igure . 'structural engineer should determine the si7e and spacing of the reinforcementre$uired.

The reinforcing bar is set into the layway formed between the concrete planks and

grouted solid. The e*posed portion of the reinforcement fits into the cell of theconcrete masonry unit. 6n the ne*t course, a positive connection is formed when the

cell is grouted.

6f lateral forces are low, an alternative connection should be considered,see "igure /.This connection bonds the precast concrete planks to the masonry with a solidlygrouted =oint. lugging the cores of the precast concrete planks creates a continuousgrout cavity. ;hen the grout is poured it flows into the grout pocket formed at theend of the planks. 'fter the grout cures a positive key connection is formed between

the planks and the concrete masonry units. 'll the precast planks should be in place

and the grout fully set before the wall construction continues. )ecause this detailrelies on the bearing pad


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CONNECTION #OR NON-"EARIN$

8on0bearing walls (which span parallel to the floor planks! must also be laterallybraced by the concrete plank floor system. @ne method re$uires holes to be broken inthe top of the plank at designated intervals, see "igure 5. Specify the plank ad=acentto the wall to bear on the wall a minimum of inches. The cures of the plank areplugged on both sides of the hole with inserts to form a grout packet. ' strap anchoris installed so that one end pro=ects down into the grout pocket and the other endpro=ects up into the cell of a concrete masonry unit. The grout pocket and cell of theconcrete masonry unit are grouted solid. This connection transfers sear stresses

through the floor diaphragm to interior shear walls while providing lateral support forthe e*terior wall. 'n alternative connection re$uires cutting or breaking the precast

concrete plank continuously and butting the plank against the wall,see "igure .

&einforcement is aligned and set into the head =oints of the concrete masonry andbent at +- degrees into the core of the precast plank. The core of the precast plank isthen grouted solid when the grout cures it forms a positive connection. Thesignificance of base flashing can never be over emphasi7ed. The success of any cavitywall system depends on proper flashing details at the base of the wall. "igure +

illustrates a properly flashed cavity wall at the foundation. ;eepholes are re$uired at1/9 or 249 on center to divert moisture from the cavity to the e*terior of the building.

"igure 1- suggests one method of construction for a window0head condition. ' bondbeam is used in lieu of a steel angle lintel. "lashing should be e*tended beyond the

=amb lines with both ends damned. Solid masonry =ambs should be avoided. 3owever,for steel windows, the =amb must be partially solid to accept most standard =ambanchors. Stock si7es of windows may be used in cavity walls, although sometimesadditional blocking is needed for anchorage. ;indow spans may be limited for thistype of construction.


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CAVITY "EARIN$ WALLS

GENERAL

Cavity walls have been successfully used in midrise and high riseconstruction. Buildings in excess of 40 stories have been utilizingcavity walls and a structural concrete frame. There are two methodsof support for cavity walls cladded to concrete frame structures. Oneis by means of shelf angles the other is to bear the wall directly onthe outer slab edge. !ach system has advantages anddisadvantages.


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TALL T!IN CAVITY WALLS

There are two methods for determining the ma*imum unbraced wall height, empiricaldesign or rational (engineered! design. The empirical design method is discussed in

#hapter of the )uilding #ode re$uirements for :asonry Structures ('#6 -0++'S# ++!. The code establishes 1* the nominal wall thickness as a limitingfactor for the distance between lateral supports.

The "piegel warehouse # office is constructed of reinforced cavity walls $4% &'%&(%)built to a height of '*+&0%.


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COMPOSITE MASONARY

REIN#ORCED MASONRY

:asonry is an e*cellent material to resist compressive forces, but is relatively weak intension. 3owever, steel is sub=ected to buckling under compressive loads but ise*cellent when used to resist tension forces. #ombining these two materials willproduce a homogeneous structure capable of resisting substantial lateral and verticalforces.

&einforced masonry performs because the materials work together. &einforcing steelplaced within a masonry system must be capable of being stressed. The mechanism

used to provide this capability is grout. Solidly grouting a cell of a concrete masonryunit which contains reinforcing steel, creates a bond between the interface of steel,

grout and concrete masonry. ;hen a wall is laterally loaded, it deflects producingcompression in the masonry. The forces are transferred through the masonry and into

the grout and by bond into the reinforcing steel, thus the steel is stressed in tension.

Structural engineers are now encouraged to use the ultimate strength design methoddesignated by )@#' &esearch &eport /01, when designing masonry walls. This

report e*pands the limit on deflection to -.--5h. ' masonry wall can now be designedto limit lateral deflection under service loads. This limit on lateral deflection insures

that the steel will be stressed below yield strength conditions. The wall will rebound toits normal vertical conditions when the lateral load is removed because the stress in

the steel is within its elastic limits. >esigning a cavity wall with this method wouldenable the builderowner to construct a cost effective system.

DETAILS

>etails for reinforced cavity walls are similar to non reinforced walls. "lashing isre$uired at all typical locations (e.gJ over openings, under openings, at the top and at

the base of the wall!. Special provisions are re$uired for flashing walls which aregrouted. ;hen flashing e*tends into the cells of #: that are reinforced, the ends

must be trimmed. (See "igure 15 below!. This will allow grout to flow freely throughthe cells when it is poured. 6f the flashing is allowed to obstruct grout flow, problemssuch as grout hangups, honeycombing or cold =oints will develop. >isrupting thecontinuity of the grouted cell will affect its structural effectiveness.

CONNECTIONS

Special consideration should be given to connection details. "or bearing walls, pocketsshould be formed to house steel beams. There should be some ad=ustability for the

method of attachment, to accommodate for construction tolerances. See "igure ne*tpage.


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"earin 1a&& *onne*tion

,f steel -oists bear on the masonry wall attachment to the -oists could be weldedto a continuous steel member which is embedded into a bond beam. "ee igure/(b above and to the right. or non&bearing walls a method of attachment mustbe detailed which adeuately braces the wall against movement due to windloads.


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Latera&&. "ra*e, Wa&&


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BOTTOM LINE

1side from the finished product being of considerable beauty and formidable

strengthother initial and long&term benefits are gained when cavity wall systemsare coupledwith the structural entities previously cited.

INITIAL BENEFITS

2 The statement that %3asonry is too expensive% is -ust a myth. Cavity wallsystemsare initially lower in cost than many glass curtain walls metal panelcurtain walls granite panels marble panels and architectural precast concretewalls. 1dd to this a reduction of '5.00 per lineal foot of shelf angle deleted andadditional savings occur.2 6imiting the number of crafts involved promotes rapid construction resulting insavings due to early occupancy.2 1ll materials reuired are usually available locally which eliminates costlyshipping charges and untimely postponements.

LONG TERM BENEFITS

2 Cavity walls are energy efficient when considering the life cycle cost of abuilding. 1typical %7% value can be increased if greater energy&efficiency isdesired.2 1 structure built with the type of systems previously discussed provides a built&in

'&to&4 hour fire&rated barrier. 1nnual fire insurance premiums can be reduced bynearly /#* depending upon the type of construction chosen and its occupancy.2 3asonry construction is very economical with respect to long&term maintenance.

REF:

/. Technical 8ote /5 7ev 9ire 7esistance: Bric; ,ndustry1ssociation 7eissued Oct. /

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Cavity Wall