Joints between prefabricated components

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Joints between prefabricated components

CANAL Ser THZ B92 no. 4c c . 2 G, K, Garden

Reprinted,

with

pemnission, from "Building

with B e f a b r i c a t e d Components," Proceedings

of a Seminar h e l d in conjunction with t h e Third B u i l d i n g N a t e r i a l s show organized by

Construction IncPustry Exhibitions, M o n t r e a l ,

NOV. 1962, p, 31-35.

-Y -5

3 2 qq; fL?

OEVISION OF BUILDING RESEARCH * N A T I O N A L RESEARCH COUNCIL * OTTAWA, C A N A D A

G.

K,

Garden

Building components, l a r g e or

small,

musL be assembled

t o produce the whole b u i l d i n g with e a c h component having a joint between it and each adjofning component. This i s true in both

conventional and p r e f a b r i c a t e d construction. The major differences

between these methods of construction a r e t h a t t h e l a r g e r the

component, t h e greater is t h e differential movement a t the joint and considerably less lineal f o o t a g e of join%, The

' o i n t is n o m a l l y the weakest Link and the success of b u i l d i n g

G6th p r e f a b r i c a t e d components depends on t h e success of t h e

=otnts,

!The elements . t h a t can be p r e f a b r i c a t e d a r e so diversi-

f i e d t h a t t h e f u n c t i o n a l requirsments of joints are many, with s p e c i a l features for each classification.

In

view o f t h e

complexity, this paper w i l l be confined t o a discussion of t h e joints most commonly producing p r o b l e m s

-

those in t h e exterior

enclosure of a building.

These joints must be so designed as t o be inexpensive

and easy t o f a b r i c a t e and assemble; be n o t easily damaged d u r i n g handling and erection, and should a l l o w f o r removal o f a n

i n d i v i d u a Z component. They should a l s o presen-b a good appearance.

&:aterials used to fulfil

the

joint functions and requirements m s t be d u r a b l e , and a f f o r d l o n g l i f e and l o w maintenance c o s t s . It is also d e s i r a b l e t h a t a joint be designed in such a w a y

t h a t manufacturing aria h s t a l l a t i o n inaccuracies do

n o t

seriously affect i t s success.

Joints in

an exterior closure of a building m u s t

permit the joining of components to transmit l o a d s a s necessary,

compensate f o r d i f f e r e n t i a l movements, tolerances, and c l e a r a n c e s , and p e r f o m t h e functions of the w a l l . Joints between external

w a l l p a n e l s , irrespective o f their many other requirements and functions, must permit differealial m o v e m e n t s b e b e e n panels and y e t prevent r a i n penetration and xesis* air leakage end

h e a t loss. A successful join% need n o t be a m a t t e r of l u c k ,

it can be achieved by good d e s i g n based on a thorough r a t i o n a l

analysis.

The magnitude and natuf-e of d i f f e r e n t i a l movements between companents being joined must be determined. It w i l l

seldom be p o s s i b l e t o make a precise determination but

can be c l a s s i f i e d either as structural o r those o r i g i n a t i n g

within t h e component due t o dimensional i n s t a b i l i t y of m a t e r i a l s .

~?.lovements

in

joints t a k e many f o m s which a r e mast e a s i l y

explained by considering t h e n a t u r e of stress which would

o c c u r in a jointing m a t e r i a l . These s t r e s s e s a r e longitudinal s h e a r , transverse shear, t e n s i o n , compression and b o t h tension

and compression by bending due _{to r o t a t i o n of components a b o u t}

%he joint ax about an a x i s perpendicular to t h e plane of the

components,

BTa jor dlff e r e n t i a l movements c a u s e d b y accidental structural settlements can s e l d o m be compensated for by joints between components and s h o u l d never occur with ttodayls engineering technology. Anticipated settlement should b e controlled by

major control joints not b y c l a d d i n g joints, Sway in t a l l

b u i l d i n g s , aeflection o f beams t o which components a r e anchored,

creep in c o n c r e t e columns, creep deflections and t e m p e r a t u r e v a r i a t i o n in t h e structural frame will a l l cause d i f f e r e n t i a l movements b e t w e e n p a n e l s , the magnitude o f which can and must be determined,

Irreversible dimensional changes in components a r e caused b y c u r i n g shrinkage, post-hydration, c a r b o n a t i o n , c r e e p , e t c

:

,

whereas reversible movements are mainly due t o l e a d

varsations and t e m p e r a t u r e and r n o i s t u ~ e changes. In o r d e r ts

gain an appreciatlsn of t h e dimensional changes due t o tempexa- t u r e or moisture, it i s e s s e n t i a l t h a t a thermal. analysis o f

t h e proposed w a l l be made, \Vith Wowledge of t h e range of

temperature v a r i a t i o n and t h e coefficients of thermal expansion f o r t h e various m a t e r i a l s , t h e magnitude of differen%fal

movements and panel warpage can be determined a s well a s _{t h e}

dimensional changes of the component, In b a t h composite and

homogeneous panels, differential expansion t h a t occurs between

the

i n s i d e and outside faces will e x e r t forces striving

to

bend the unit.

In

a n a l y z i n g a joint some c o n s i d e r a t i o n should

be given t o the f a c t ? h a t t h e components a r e seldom fabricated

or installed at either of t h e extreme temperatures, consequently the joint will b e opened or c l o s e d by temperatwr~ fluctuations

from the width existing when t h e joint is made.

The magnitude of movemen3s due to changes in m o i s t u r e content fs n o t so e a s i l y determfned since values f o r d i m e n s i o n a l changes with r e s p e c t t o moisture content changes are n o t

a v a i l a b l e f o r all m a t e r i a l s and those t h a t are, a r e not r e a d i l y a v a i l a b l e due t o t h e lack of appreciation f o r this s u b j e c t ,

R e l a t i v e humidity is a r a t i o of the a c t u a l v a p a u r pressure of the air t o the saturated v a p o u r pressure and t h e saturation v a p o u r pressure is l o w e r e d by a lowering of t e m ~ e r a t u r e , Thus

an appreciation of t h e r e l a t i v e humidity change- caused by a

t e m p e r a t u r e gradient can be gained f r o m the thermal analysis of the wall. It can be s t a t e d t h a t , exclusive of d i r e c t

wetting, t h e moisture content of materials increases wLth

an

increase

in

r e l a t i v e humidity Which produces an expansion in t h e m a t e r i a l , Conversely, a decrease in r e l a t i v e humiditp w i l l cause a shrinkage in the m a t e r i a l ,

Compression and tension stresses can o b v i o u s l y

develop in a joint by thermal ox moisture changes, Transverse

s h e a r c a n occur from panel warpage o r deflections due t o wtnd l o a d s . L o n g i t u d i n a l shear can occur where panel joints do n o t

make a f our-wag - i n t e r s e c t i o n or where p a n e l s a r e hung between

mullions which in turn aye hung f r o m spandrel beams whtch

deflect, With an appreciation of the magnitude of differential movements caused by structural movements and dimensional

instability of m a t e r t a l s and k n o w i n g t h e range of e x t e n s i b i l i t y

of joint s e a l i n g m a t e r i a l s , the d e s i g n e r can determine the minimum satisfactory width of Joint and the s e a l a n t o r gasket n e c e s s a p j to prevent f a l l u r e .

Tolerances between s t r u c t u r e and components a r e n o r m a l l y compensated f o r by a three-z~ay adjustable anchorage

and by s u f f i c i e n t width, o v e r l a p , or interlock sf joint members,

The width o f t h e ja5n-t required t o compensate f a r t h e h s t a b i l i t y

of m a t e r i a l s w i l l n a m a l l y be sufficient t o a l l o w slight varrEa-

t i o n s to take up those small dimensions n o t resolved b y the

anchorage ,

A i r leakage can be minimized by a t a r b u o u s coMTgura-

tion o f the joint b u t is best stopped by a p o s i t i v e seal f r o m

component t o component by a g a s k e t or bead o f sealant, W t h

f i e l d a p p l i e d g a s k e t s and s e a l a n t s , s m a l l imperf ectluns occur ox w i l l in time d e v e l o p p e r m i t t i n g minute air l e a k a g e s which

w i l l normally n o t be a problem if w a t e r does n o t come in

contact with them, A i r leakage frsm t h e interior

of the

building can be s e r i o u s because in c o l d weather it is normally accompanied by condensation within the w a l l , resulting in

water in the j o m t with t h e risk of accumulation and damage,

Thermal bridges can seldom be completely overcame, but it is important t o provide s u f f i c i e n t resistance to h e a t

flow s o t h a t interior surfaces can be k e p t w a r m enough to prevent. surface condensation,

Rain leakage can b e prevented by a complete and positive s e a l t h r o u g h o u t

the

entire l e n g t h o f a l l joints b u t

absolute perfection is essential, The r e q u i r e d perfection is d i f f i c u l t t o achieve because of inaccuracies in job s i t e

rvorkmanship, men more difficult is the maintenance of a

perfect s e a l over a r e a s o n a b l e p e r i o d of time due to the

constant flexing, stressing, and aging of t h e SointFng

m a t e r i a l s . A x a t l o n a l a n a l y s i s of joint design, r e g u F r e s an examination o f the mechanism of r a i n p e n e t r a t i o n , From this one finds t h a t leakage can be easily controlled except where a n a i r pressure difference forces w a t e r t h r o u g h a crack

that

is b r i d g e d by water,

If

the

control of

r a i n penetra-klan can

be separated from the plane of a i r pressure d r o p , t h e force

necessary L o produce l e a k a g e mill

no

l o n g e r e x i s t ,

Thfs

can

r e a d i l y be accomplished by permitting t h e air pressures b o t h

inside and outside the crack t o e q u a l i z e , Thfs is achfeved

by providing an air space behind the f a c e of the joint w i t h

sufficient opening to the face,

It

is usually more d i f f i c u l t to control r a i n peae-

tration at vertical joints than a t horizontal joints, 1t

can r e a d i l y be seen from examination of PLgure 1 t h a t when b r i d g e d with ~ a t e r t h e slightest failure of the s e a l a% the

outside f a c e of the wall will a l l o w r a i n t o p e n e t r a t e , _By

moving the s e a l t o t h e back o f the joint t h e r e will be a

chamber of air at a pressure e q u a l t o t h a t on the face of t h e

b u i l d i n g and watex is not f o r c e d back to the a i r s e a l , T h e o n l y w a y it can ge-t t o t h e air s e a l is b y the kinetic energy

of the d r o p or by the a c t i o n of g r a v i t y and surface fxsegularities,

It

might be u s e f u l t o mention a t this p o h t t h a t r a i n w a t e r on the face

of

a b u i l d i n g tends t o accumulate

in

v e r t i c a l

i m e g u l a r i t i e s and further m o d i f i c a t i o n s in d e t a i l are necessary,

A loose-fittfng b a t t e n will prevent w a t e r e n t r y by kinetic

energy b u t may permlt t h e v e r t i c a l f l o w of w a t e r in tne joint

due to water accumulation t o become s o g r e a t t h a t it reaches

the a i r seal, The introduction of a loose-fitting baffle in

the jofnt will be more successful in controlling bo-t;h t h e s e

problems as w e l l a s permitting the air pressures on bath s i d e s o f t h e joint t o e q u a l i z e , V e r t i c a l i r r e g u l a r i t i e s on t h e

surface will minimize t h e volume of w a t e r t h a t accumulates at the f a c e o f the joint.

It _{should be r e s t a t e d t h a t , providing the air s e a l}

does not g e t m e t , minor a i r leakages will n o t be accompanied

by r a i n penetration,

This principle of r a i n penetration control h a s been studied by the Nomegian Building Research Institute, o r i g i n a l l y with r e s p e c t t o w i n d o w l e a k a g e b u t it has been extended t c a l l

problems o f r a i n penetration.

It

is being g e n e r a l l y accepted

t h r o u & o u t E u r o p e and is proving very successful, There axe

o f c o u r s e a f e w r e f i n i n g considerations but t h e essential principles have been outlined,

It

s h o u l d be pointed o u t ,

however, t h a t t h e r a i n p e n e t r a t i o n control provided by a

s h i n g l e d ma11 h a s been appreciated f o r h m a r e d s o f y e a r s and

is t h e simplest expression of this principle. The success o f

masonry c a v i t y -zvalls is a l s o e x p l a i n e d t o some extent by this

prtncfple,

Joints be%ween components w i l l always be of concern

OUTSIDE

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PRESSURE

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