Health Monitoring of Structures Using Modern Tools

ENGINEER - Vol. XXXXI, N o . 02, pp. 48-55, 2008 © The Institution of Engineers,.Sri Lanka

Health Monitoring of Structures Using Modern Tools P. B. R. Dissanayake Abstract: T h e h e a l t h m o n i t o r i n g of old structures is a n essential p a r t in Civil Engineering. In Sri Lanka, there are a variety of old s t r u c t u r e s s u c h as stupas, ancient monumei»Js, buildings, bridges, a n d d a m s . S o m e of these s t r u c t u r e s (e.g. Stupas) are m o r e than 2000 years old a n d are national treasures. O t h e r old structures s u c h as b u i l d i n g s , bridges, etc are nearly 100 years old a n d m o s t of t h e m dates backs to British colonial p e r i o d . I n t e r e s t i n g l y , t h e s e a r e still u s e d in social life. H e n c e , a p p r a i s a l of t h e s e s t r u c t u r e s c o n s i d e r i n g future existence a n d u s e is i m p o r t a n t at the national level. But this is often difficult o w i n g to the lack of defined m e t h o d s . This p a p e r describes h o w health m o n i t o r i n g can be d o n e u s i n g analytical a n d e x p e r i m e n t a l considerations. In this context, three cases studies; n a m e l y , a n old building, a n old reservoir a n d a n old bridge, in relation to load carrying capacity a n d displacements are u s e d as illustrative e x a m p l e s . Keywords: E x p e r i m e n t a l m e t h o d s , Analytical m e t h o d s , H e a l t h m o n i t o r i n g , O l d structures

1. Introduction

m a n y t y p e s of structures, the process of health m o n i t o r i n g is different d e p e n d i n g o n the t y p e of s t r u c t u r e . H e n c e , h e a l t h m o n i t o r i n g as a first job, m a y b e c o m e rather difficult to e v e n a well qualified engineer.

In t o d a y ' s r a p i d l y c h a n g i n g w o r l d , the quality of h u m a n life a n d e c o n o m i c p r o g r e s s of a country d e p e n d on the quantity, quality and e f f i c i e n c y of its i n f r a s t r u c t u r e s s y s t e m [1]. E v e r y w h e r e a r o u n d the w o r l d , there is a n e e d for s i g n i f i c a n t i n v e s t m e n t t o r e p a i r c i v i l infrastructures, w h i c h are d e t e r i o r a t i n g u n d e r h e a v y use, severe e x p o s u r e conditions, a n d age [1,2]. Economic d e p r e s s i o n s that h a p p e n e d in m i d 1990's in t h e w o r l d h a v e in fact c a u s e d m o r e research e x p e n d i t u r e o n i n n o v a t i v e health m o n i t o r i n g t e c h n i q u e s that will enable o p t i m u m m a i n t e n a n c e of structures.

T h e first d e c i s i o n t h a t a n i n v e s t i g a t o r h a s to m a k e is w h a t to investigate a n d h o w to d o it in o r d e r to a c h i e v e t h e p u r p o s e s . T h e k e y for s o u n d a p p r a i s a l is the correct identification of the cause or causes of structural d a m a g e . Most traditional b u i l d i n g s h a v e a large factor of safety, w h i c h shields t h e m from c a t a s t r o p h e or progressive collapse, so that there is time for the i n v e s t i g a t i o n to b e t h o r o u g h , the w o r k b e i n g u n d e r n o c o m m e r c i a l p r e s s u r e s of t i m e a n d money.

Most of civil e n g i n e e r i n g structures h a v e three p h a s e s in their service life; design, construction a n d m a i n t e n a n c e . W i t h o u r k n o w l e d g e in structural e n g i n e e r i n g a n d p r e s e n t d e v e l o p m e n t of t e c h n o l o g y , d e s i g n a n d c o n s t r u c t i o n of structures are well defined tasks at present. It is a m a t t e r of following well-described codes a n d guidelines and execution with desirable c o n s t r u c t i o n t e c h n i q u e s . In contrast, the m a i n t e n a n c e s t a g e of a s t r u c t u r e is t h e least a t t e n d e d s t a g e m o s t of t h e t i m e . D u r i n g t h e m a i n t e n a n c e stage, c o n t i n u o s health m o n i t o r i n g of s t r u c t u r e s is a n e s s e n t i a l p a r t . T h a t w i l l r e d u c e t h e life cycle cost of t h e s t r u c t u r e a n d e n h a n c e the p e r f o r m a n c e of the structure.

The i m p o r t a n t t h i n g w i t h health m o n i t o r i n g of old structures is that it could determine w h e t h e r any s t r e n g t h e n i n g of the old b u i l d i n g is really n e e d e d . This h e a l t h m o n i t o r i n g is particularly important in cities like K a n d y , which w a s n a m e d as a w o r l d h e r i t a g e city b y U N E S C O . In s u c h cities, d e s i g n e r s a n d p l a n e r s a r e faced w i t h problems of preserving historical buildings w i t h the p h a s e of development. In such circumstances, h e a l t h m o n i t o r i n g of o l d s t r u c t u r e s is of p a r a m o u n t importance.

T h e r e are n u m b e r of different t y p e s of h e a l t h m o n i t o r i n g used for v a r y i n g p u r p o s e s . S o m e are c o m p r e h e n s i v e a n d some have only an incidental structural interest. It is often involved w i t h t h e a v a i l a b l e r e s o u r c e s . Since t h e r e a r e

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Eng. (Dr.) P.B.R. Dissanayake, B.Sc. Eng. (Hons) (Peradeniya), C. Eng., MIE(Sri Lanka), M.Eng. (Ehime), Dr. Eng. (Ehime), Senior Lecturer in Civil Engineering, Department of Civil Engineering, University of Peradeniya.

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2. Theoretical background of the health monitoring of structures

critical situations occurring can only be acquired t h r o u g h scientific k n o w l e d g e . In t h e c a s e of a n c i e n t s t r u c t u r e s , t h e l a c k of s c i e n t i f i c k n o w l e d g e w a s usually c o m p e n s a t e d by practical experience a n d b y construction of the structural c o m p o n e n t s w i t h m o r e t h a n a d e q u a t e dimensions. When innovative structural c o n c e p t s a r o s e , s u c h a s in t h e case of G o t h i c cathedrals, numerous collapses and much d a m a g e h a d to o c c u r before the b u i l d e r s a c q u i r e d the necessary experience a n d a r r i v e d at a successful structural solution.

It is i m p o r t a n t to b e familiar w i t h different t e r m s used in h e a l t h m o n i t o r i n g . F u r t h e r m o r e , there s h o u l d b e a c l e a r u n d e r s t a n d i n g of w a y s i n w h i c h condition of structures deteriorate. 2.1 D a m a g e a n d decay The t e r m d a m a g e is u s e d to describe a situation in w h i c h a s t r u c t u r e h a s lost s o m e or all of its b e a r i n g capacity, a c o n d i t i o n t h a t c a n lead to failure a n d collapse. D a m a g e is b r e a k i n g a w a y of e l e m e n t s , p e r m a n e n t d e f o r m a t i o n , o u t of p l u m b n e s s a n d etc.

2.2.3

Every construction is d e s i g n e d w i t h a m o r e or less consciously p r e d e t e r m i n e d life expectancy. This c o n c e p t is implicit in t h e f o r m u l a t i o n of m o d e r n c o d e s , in w h i c h "safety coefficients" a n d the c o r r e s p o n d i n g probability that critical s i t u a t i o n will a r i s e , a r e fixed in r e l a t i o n to a certain life e x p e c t a n c y for t h e s t r u c t u r e . This m a y v a r y from d e c a d e s to centuries, d e p e n d i n g o n t h e m o r e or less r a p i d e v o l u t i o n of factors such as functionality, convenience, m a i n t e n a n c e costs, etc.

D e c a y or d e t e r i o r a t i o n is a n a l t e r a t i o n of t h e m a t e r i a l t h a t u s u a l l y l e a d s to a r e d u c t i o n in resistance, increased brittleness, porosity a n d a loss of m a t e r i a l t h a t u s u a l l y b e g i n s from t h e o u t s i d e a n d w o r k i n w a r d . It is m a i n l y related to p h y s i c a l o r c h e m i c a l a c t i o n s . C o r r o s i o n of reinforcement is a g o o d e x a m p l e of decay. 2.2 T h e o r i g i n s of d a m a g e a n d d e c a y The origins of d a m a g e a n d deterioration can be attributed to o n e or m o r e of the following factors. 2.2.1

Ancient b u i l d i n g s h a v e often s u r v i v e d well b e y o n d their life expectancies, so that d a m a g e , a n d especially w e a t h e r i n g a n d d e c a y , c a n b e considered n o r m a l p h e n o m e n a .

Lack of due care in the original design

T h e safety of a c o n s t r u c t i o n c a n n e v e r b e a n a b s o l u t e c e r t a i n t y , s i n c e it is affected b y t h e u n c e r t a i n t y of e v a l u a t i o n of t h e v a r i o u s p h e n o m e n a a n d characteristics and therefore d e p e n d s o n probabilities. Its s t r e n g t h a n d the action to w h i c h it m a y b e subjected m a y h a v e m o r e u n f a v o u r a b l e v a l u e s t h a n those envisaged in its design. Safety coefficients b y architects a n d e n g i n e e r s are d e s i g n e d to meet these uncertainties.

2.2.4

Errors and imperfections in the original design

P r o g r e s s in t h e field of c o n s t r u c t i o n h a s b e e n achieved m o r e by intuition a n d experience t h a n b y scientific k n o w l e d g e a n d therefore safety w a s e n s u r e d by r e p e a t i n g similar shapes and d i m e n s i o n s . The d e v e l o p m e n t of forms, s h a p e s a n d p r o p o r t i o n s w a s b a s e d o n earlier m o d e l s a n d experiences that h a d p r o v e d less p r o n e to d a m a g e a n d collapse, b u t e v e n t h a t r o u t e d i d not totally avoid all risk.

T h u s , t h e p r o b a b i l i t y t h a t s o m e e v e n t , as for example an earthquake, may have a more serious effect o n a s t r u c t u r e w i t h r e g a r d to say d a m a g e or collapse t h a n those envisaged in the design d e p e n d s o n the d e g r e e of c a u t i o n u s e d w h e n the safety levels w e r e established at the d e s i g n stage. H o w e v e r , safety h a s a cost a n d even m o d e r n codes accept a certain probability. 2.2.2

Use of construction beyond their life expectancy

2.2.5

Introduction of new and u n k n o w n factors

C o n s t r u c t i o n s m a y h a v e b e e n d e s i g n e d for conditions different from those that effectively occur. E n v i r o n m e n t a l c h a n g e s (speeding u p the process of deterioration of materials), variations in the u s e of s t r u c t u r e s (increased l o a d s , etc.), structural alterations (extensions, partial d e m o l i t i o n , etc.), e a r t h w o r k s , ( e x c a v a t i o n s ,

Lack of scientific knowledge

A p r o p e r u n d e r s t a n d i n g of t h e m a i n p h e n o m e n a t h a t i n c r e a s e t h e p r o b a b i l i t y of

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variations in w a t e r levels, e m b a n k m e n t s , Fillings, galleries) a n d e a r t h q u a k e s in areas not originally considered seismic are a m o n g the principal causes of d a m a g e a n d decay.

s i t u a t i o n of e q u i l i b r i u m c a n s p r e a d , u n t i l , in s o m e cases, especially w h e n c o m p r e s s e d areas are involved, a m e c h a n i s m l e a d i n g to collapse can be i n d u c e d . T h e z o n e s in w h i c h t h e c r a c k s first a p p e a r d e p e n d o n the t y p e of structure a n d materials, t h e p r e s e n c e of p o s s i b l e w e a k e r or s t r o n g e r zones, the distribution of the forces a n d finally the stresses.

2.3 Kinds of damage 2.3.1

The visible signs

D e p e n d i n g o n t h e t y p e of m a t e r i a l a n d c o n s t r u c t i o n , v i s i b l e s i g n s of d a m a g e can b e d i v i d e d into three m a i n categories: 2.3.1.1

To identify the direction a l o n g w h i c h the cracks will d e v e l o p , the isostatic lines, b e i n g the lines a l o n g w h i c h the m a i n stresses flow, can be considered. Cracks initially will be p e r p e n d i c u l a r to t h e t e n s i o n i s o s t a t i c l i n e s , w h e r e the stresses exceed the strength; with i n c r e a s i n g stress, t h e s e lines m a y p r o g r e s s in s l i g h t l y different d i r e c t i o n s s i n c e t h e c r a c k s themselves c h a n g e the original isostatic lines.

Crack in materials not resistant to tensile stresses

This is m o s t frequent in m a s o n r y , w h i c h h a s a v e r y l o w r e s i s t a n c e t o t e n s i o n . In c o n c r e t e structures, cracks are n o r m a l l y associated w i t h insufficient r e i n f o r c e m e n t . But this is n o t t h e case in every time. It can be attributed to several other r e a s o n s as m e n t i o n e d in section 2.1 a n d 2.2. 2.3.1.2

A l t h o u g h the evolution of the crack p a t t e r n s can only be fully identified u s i n g a step b y step n o n ­ linear p r o c e d u r e , b e c a u s e of t h e p r o g r e s s i v e variations in the stiffness of the structure. It is usually possible to m a k e a qualitative e v a l u a t i o n of h o w t h e i s o s t a t i c l i n e s w i l l b e c h a n g e d from the initial cracks.

Permanent deformations:

T h e s e are p a r t i c u l a r l y l i n k e d to t h e effects of bending induced by off-centre loads and h o r i z o n t a l t h r u s t s ( a r c h e s a n d etc). A n o t h e r i m p o r t a n t f a c t o r m a y b e r e l a t e d t o soil deformation. 2.3.2

The e x a m i n a t i o n of the p a t t e r n of cracking also m a k e s it possible to identify the k i n d of actions that p r o d u c e d t h e m a n d consequently the cause of the actual d a m a g e .

Cracks and cracks patterns

Cracks are the m o s t frequent sings of d a m a g e . The p r o b l e m is t h u s to establish the relationship b e t w e e n crack p a t t e r n s a n d the distribution of forces, possible soil deformations, e a r t h q u a k e s , etc.

2.3.3

Crushing occurs w h e n compressive stresses r e a c h t h e s t r e n g t h of m a t e r i a l s a n d is m o r e complex than p h e n o m e n a caused by tensile stresses.

O n actual b u i l d i n g s the p r o b l e m can b e m a d e m o r e d i f f i c u l t b y t h e p r e s e n c e of l o c a l weakening (cavities, cracks, etc) or s t r e n g t h e n i n g ( r e i n f o r c e m e n t in c o n c r e t e structures and chains), which are not always visible.

As c o m p r e s s i o n stresses increase, m i n u t e cracks a p p e a r m a i n l y parallel to the d i r e c t i o n of t h e s t r e s s . If t h e r e is n o l a t e r a l c o n s t r a i n t , t h e situation can deteriorate until transversal swelling occurs, flakes b e c o m e d e t a c h e d a n d the internal n u c l e u s of the c o m p r e s s e d e l e m e n t m a y s u d d e n l y crumble.

I n m a t e r i a l s u c h a s m a s o n r y , w h i c h is n o t resistant to tension a n d is conceived essentially to w i t h s t a n d only c o m p r e s s i o n , the cracks in the tensile z o n e s are the m o s t visible signs. Their presence, h o w e v e r , is not u s u a l l y an indication of d a n g e r o u s situations.

C r u s h i n g p h e n o m e n a are extremely d a n g e r o u s o n account of the fact that they often give v e r y little w a r n i n g a n d can h a v e possible catastrophic consequences for a w h o l e b u i l d i n g if adjacent structural e l e m e n t s or areas are not able to c o m p e n s a t e in s o m e w a y .

By i n c r e a s i n g f o r c e s , o r a l t e r i n g t h e i r distribution, cracks that initially represent merely the a d j u s t m e n t of the material to a n e w

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Crushing

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The visible signs of c r u s h i n g v a r y according to the materials i n v o l v e d a n d m a y be barely detectable, or h i d d e n a n d d u e to the small axial deformation involved can usually only be seen on the affected elements while n o signs will be visible on other p a r t s of the construction. 2.3.4

I

Diagnosis

The above sections briefly described s o m e of the most frequent causes of d a m a g e a n d failure a n d the c o r r e s p o n d i n g signs visible o n s t r u c t u r e s . The d e s i g n e r , h o w e v e r , is in q u i t e a different s i t u a t i o n . S t a r t i n g f r o m h i s o b s e r v a t i o n of a b n o r m a l situations, h e m u s t identify the cause or m o r e frequently, the u n d e r l y i n g causes.

Figure 1: CDP 50 Displacement Transducer

This connection b e t w e e n a n effect a n d its cause is c o m p l i c a t e d by the i n t e r a c t i o n of different e v e n t s o c c u r r i n g at v a r i o u s t i m e s . All t h e s e factors can alter a n d influence the d a m a g e a n d the resulting crack p a t t e r n s , m a k i n g the designer's task m u c h m o r e difficult.

Figure 2: Electrical Resistance Strain Gauge

m e a s u r e m e n t s , w h i l e it can be u s e d for b o t h static a n d d y n a m i c m e a s u r e m e n t s .

The process of identifying the causes of d a m a g e in structures a n d decay in materials involve the use of different p r o c e d u r e s or criteria, based on the observation of the building.

Electrical resistance strain g a u g e s as s h o w n in Figure 2 are devices used to m e a s u r e strains in b o t h static a n d d y n a m i c load c o n d i t i o n s . T h e data is acquired t h r o u g h TDS-303 d a t a logger. It is a sensitive a n d accurate e q u i p m e n t used for recording displacement, strain, a n d acceleration m e a s u r e m e n t s , a n d it a l l o w s a u t o m a t i c r e c o r d i n g of data. M e a s u r e d d a t a are p r i n t e d and recorded in the internal m e m o r y of the data logger or in a disk, w h i c h can be u p l o a d e d to the c o m p u t e r , via either GP-IB or RS 232C interface.

To i m p r o v e the k n o w l e d g e of the structural a n d material characteristics a n d obtain m o r e reliable data it is usually necessary to carry o u t specific investigations, sometimes using a monitoring s y s t e m t o r e c o r d t h e c h a n g e s in v a r i o u s p h e n o m e n a , so as to b e t t e r identify p o s s i b l e causes. As in m e d i c i n e , a c o r r e c t a n d c o m p l e t e d i a g n o s i s can only b e a c h i e v e d if all the d a t a a n d information are c o m b i n e d w i t h intuition, experience a n d i n d i v i d u a l ability.

4. Case Study I: Maligakanda Old Reservoir Wall The M a l i g a k a n d a service reservoir, built a b o u t 120 years ago, has the internal d i m e n s i o n s of 192 ft in length a n d w i d t h while the height is 45 ft. Later, all its four sides h a v e been inter connected w i t h a grid of prestressed concrete b e a m s , 16 in e a c h d i r e c t i o n a s s h o w n in t h e F i g u r e 3 . H o w e v e r s l i d i n g m o v e m e n t s in t h e c o n c r e t e w a l l s of the r e s e r v o i r w e r e s u s p e c t e d , a n d in o r d e r to assess the structural integrity of walls, a n experimental a n d a n u m e r i c a l analysis of a wall w a s carried out. The objective of this s t u d y w a s to find o u t w h e t h e r t h e r e a r e significant m o v e m e n t s in the wall d u e to w a t e r l o a d i n g [3].

3. Experimental measurements In m o s t t i m e in h e a l t h m o n i t o r i n g , s t r a i n s , d i s p l a c e m e n t s a n d acceleration are m e a s u r e d and electrical resistance strain g a u g e s and d i s p l a c e m e n t t r a n s d u c e r s a r e u s e d for s t r a i n and displacement m e a s u r e m e n t s . Where the d y n a m i c n a t u r e of s t r u c t u r e s is to be m e a s u r e d , acceleration g a u g e s are used. The CDP-50 d i s p l a c e m e n t transducer, s h o w n in Figure 1, is a c o m p a c t d i s p l a c e m e n t t r a n s d u c e r that is very easy to operate. Its large o u t p u t a n d excellent consistency e n s u r e high-precision

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M

M o d e l (2) : L a t e r a l t r a n s l a t i o n a l d e g r e e of freedom is free (allow to move) a n d o t h e r t w o translational d e g r e e s of freedoms are restrained at the base of the wall a n d subjected to the water pressure at water height of 6m from the bottom. (Base is laterally free and water height is 6m)

4.1 Experimental set u p It h a s b e e n o b s e r v e d t h a t m o v e m e n t s of t h e reservoir walls occur w h e n the reservoir is filled w i t h water, a n d this usually h a p p e n s from 6 P M to 6 A M of the next date. A s d i s p l a c e m e n t w a s the m a i n quantity to be checked, d i s p l a c e m e n t t r a n s d u c e r s w e r e attached to the reservoir wall a n d d i s p l a c e m e n t s w e r e m e a s u r e d as the reservoir w a s being filled a n d also w h e n it w a s emptied.

Model (3) : All T h r e e t r a n s l a t i o n a l d e g r e e s of freedoms are restrained at the base of the wall a n d subjected to the water p r e s s u r e at w a t e r h e i g h t of 8.5m from the b o t t o m . (Base is laterally fixed a n d water height is 8.5m)

4.2 Finite element analysis To identify the best structural m o d e l that suits t h e b e h a v i o u r of t h e w a l l a f i n i t e e l e m e n t analysis w a s carried o u t e m p l o y i n g the generalp u r p o s e p a c k a g e SAP2000.?Four p l a n e s t r a i n finite e l e m e n t m o d e l s w i t h v a r i o u s t y p e s of restrains w e r e d e v e l o p e d as m e n t i o n e d below, a n d the results from these finite element m o d e l s w e r e c o m p a r e d w i t h field m e a s u r e m e n t s . A four m e t e r w i d e strip of the wall w a s u s e d in the analysis. P r e s t r e s s i n g force a p p l i e d by the t e n d o n o n the wall w a s replaced b y equivalent forces of 2900 k N at relevant n o d e s of the m o d e l . The prestressed concrete b e a m s w e r e replaced by s p r i n g s u p p o r t s w i t h a stiffness v a l u e of 293 k N / m m , w h i c h w a s calculated b y c o n s i d e r i n g b o t h effects of concrete a n d steel t e n d o n s . T h e w a t e r p r e s s u r e w a s a p p l i e d to the inner surface of the wall t a k i n g the u n i t w e i g h t of w a t e r as 10 kN/m .

Model (4) : L a t e r a l t r a n s l a t i o n a l d e g r e e of freedom is free (allow to move) a n d other t w o translational d e g r e e s of freedoms are restrained at the base of the wall a n d subjected to the water p r e s s u r e at w a t e r h e i g h t of 8.5m from the b o t t o m . (Base is laterally free a n d water height is 8.5m). 4.3 Validation of the Finite Element M o d e l Validation of m o d e l w a s carried b y c o m p a r i n g t h e results t a k e n from the analysis w i t h those from field tests. Pressure loading, as the w a t e r h e i g h t rises from 6.0 m t o 8.5 m h e i g h t w a s t a k e n as t h e l o a d a c t i o n o n t h e w a l l . For t h e c o m p a r i s o n of results c h a n g e of d i s p l a c e m e n t w e r e considered separately in M o d e l s (1) & (3)(Base is laterally fixed w h o s e lateral stiffness is infinity) a n d M o d e l (2) & (4)-(Base is laterally free w h o s e lateral stiffness is zero). Then, these r e s u l t s w e r e c o m p a r e d w i t h a c t u a l field measurements.

3

These results suggest that the b e h a v i o u r of the Wall is in between laterally base fixed a n d base free conditions. This m e a n s that there is a sliding at the base a n d this can be modelled u s i n g s p r i n g supports. Interpolating linearly the extreme cases the stiffness v a l u e w a s d e t e r m i n e d as 1 9 5 k N / m m per meter length of the Wall.

BUHS CSOS3 SECTION MAUCAIOKDA OLD RESEftvlon

Figure 3: A view of the Maligakanda dam

4.4 Results and conclusions

M o d e l (1) : All T h r e e translational d e g r e e s of f r e e d o m s w e r e r e s t r a i n e d at t h e base of the wall a n d subjected to the w a t e r p r e s s u r e at w a t e r h e i g h t of 6 m f r o m t h e b o t t o m . (Base is laterally fixed a n d w a t e r height is 6m)

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The results s h o w that there is g o o d a g r e e m e n t a m o n g analytical results from the FE M o d e l a n d m e a s u r e m e n t s . H e n c e , it m a y b e c o n c l u d e d that the FE m o d e l is v a l i d a t e d a n d it can b e u s e d in o t h e r a n a l y s e s i n v o l v i n g different l o a d i n g o n the wall.

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F r o m t h e r e s u l t s o b t a i n e d , it w a s f o u n d t h a t n o r m a l l y the p r e s t r e s s e d b e a m s t a k e a r o u n d 30% of lateral w a t e r load a n d the rest is t a k e n by t h e soil backfill a n d b a s e of g r a v i t y w a l l in friction. C o n s i d e r i n g r e s e r v o i r e m p t y stage as the initial position, the total sliding of base of the g r a v i t y w a l l is 4.90 m m a n d m a x i m u m displacement at t o p is 5.39 m m .

f

Experimental and analytical investigations s h o w t h a t t h e r e is s l i d i n g at t h e b a s e of t h e gravity wall d u e to w a t e r load, t h o u g h the post tensioned b e a m s are present. The walls will not b e s t a b l e a g a i n s t s l i d i n g if t h e p o s t t e n s i o n b e a m s loose their effect. C o n s i d e r i n g the above factors it could be said that the risk of failure of the reservoir cannot be easily ignored. This reveals that this old reservoir is in a structurally doubtful stage for future services.

5.

Figure 4: An outside view of the building

Case study II: Galle Face Hotel

The selected structure for the health m o n i t o r i n g is a p a r t of the Galle Face Hotel in Colombo. It is a steel f r a m e d s t r u c t u r e h a v i n g four s t o r e y s , constructed in the 1880's [4]. Figure 4 s h o w s a n o u t s i d e v i e w of the b u i l d i n g .

Figure 5: Displacement transducers in measuring displacement

A s t h e b u i l d i n g is o n t h e s e a f r o n t , h e a v y c o r r o s i o n h a s t a k e n p l a c e in s t r u c t u r a l c o m p o n e n t s s u c h as c o l u m n s a n d b e a m s . Recently, t h e b u i l d i n g w a s r e h a b i l i t a t e d w i t h s o m e structural r e p a i r a n d modifications such as w e l d i n g plates at places w h e r e there is loss of material d u e to c o r r o s i o n [5]. T h e objective of the s t u d y w a s to find o u t w h e t h e r the original b u i l d i n g could safely be u s e d with these modifications. T o w a r d s t h i s o b j e c t i v e , l o a d t e s t i n g of t h e building was carried out. Displacement t r a n s d u c e r s w e r e fixed to flange p l a t e s of the several c o l u m n s as s h o w n in Figure 5, a n d strain g a u g e s w e r e also a t t a c h e d to the r e p a i r e d sections as indicated in Figure 6. Displacements a n d strains w e r e m e a s u r e d w i t h i m p o s e d loads o n floors, a n d w e r e i n t e r p r e t e d t h r o u g h t h e Data Logger TDS-303.

Figure 6: Strain Gauges attached to the columns of the wall floor, w e r e a p p l i e d . T h i s will g i v e t h e s a m e effect o n the g r o u n d floor c o l u m n s as the 4.2 k N / m total load. 2

5.1 R e s u l t s a n d c o n c l u s i o n s F r o m t h e o b s e r v e d s t r a i n s at s o m e s e l e c t e d locations a n d c o m p u t e d s t r e s s e s , it w a s s e e n that in s o m e repaired sections of the c o l u m n s , stresses are not evenly distributed. This m a y b e d u e to the u n e v e n c o n n e c t i o n s a n d stiffness v a r i a t i o n s of t h e c o n n e c t i o n s , g i v i n g r i s e to u n s y m m e t r i c a l b e n d i n g of the connected plates

As per the C o d e s [6 & 7], test load to be applied p e r floor is 1.05 k N / m a n d t h e t o t a l l o a d applied for all four floors is 4.2 k N / m . Since the interest w a s o n the c o l u m n s in the g r o u n d floor, a load of 1.5 k N / m each for first a n d second floors, a n d a l o a d of 1.2 k N / m for the t h i r d 2

2

2

2

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of the r e p a i r e d sections. This will give rise to b e n d i n g in o t h e r p a r t s of the c o l u m n as well. Except at one location at the repaired section of a tested column, stresses found under the applied l o a d i n g w e r e low.

t h e o r y , c u r r e n t c o n d i t i o n of t h e b r i d g e w a s e s t i m a t e d . T a b l e 1 s h o w s t h e r e s u l t s of t h e reliability calculations [8]. F r o m these analyses, it w a s found that currently the condition of the bridge is satisfactory even t h o u g h corrosion h a s been on reinforcements.

6. Case Study III: Reinforced concrete deck bridge at Elhera

7. Future of health monitoring T h e future of h e a l t h m o n i t o r i n g of s t r u c t u r e s seems to be rather optimistic as m o r e a n d more innovative techniques and instruments are i n t r o d u c e d . O n e s u c h a r e a is s m a r t s e n s i n g . These are wireless sensors once attached to the structure, it transmits the information to central processing unit. As such, d u r i n g the construction stage, these sensors can be attached to t h e s t r u c t u r e a n d t h e y w i l l t r a n s m i t t h e deterioration process a n d based on information, health m o n i t o r i n g of s t r u c t u r e s can be carried out a n d p r o p e r m a i n t e n a n c e can be effected at the right time.

The bridge (43/2) situated Naula-ElheraK a l u g a g a (B312) r o a d w a s e v a l u a t e d for its c u r r e n t c o n d i t i o n s . It is a reinforced concrete deck bridge with r a n d o m rubble masonry a b u t m e n t s a n d it w a s constructed in 1979. Views of the b r i d g e are as s h o w n in Figures 7 & 8.

Table 1: Summary of the reliability calculation

Figure 7: A View of the case study bridge (bridge Number 43/2) in B312 route

Case

Reliability index/([i)

Failure probability/(P )

Case I: COV

4.14

1.5

3.92

4A x W

3.42

3.1xW

f

5

xW

(A) required = 0

Casell: COV

U n d e r n e a t h the b r i d g e deck, reinforcement has

(A)required

b e e n exposed a n d c o r r o d e d heavily. There is a

Caselll: COV (A)required

n e e d to e s t i m a t e t h e c u r r e n t c o n d i t i o n of t h e

s

= .05 = 0.1

4

•

b r i d g e o v e r t h e n o r m a l traffic flow of t h i s bridge.

8. Conclusions H e a l t h m o n i t o r i n g of old s t r u c t u r e s i n v o l v e s detective w o r k , b u t w i t h the use of m o d e r n tools for experimental a n d analytical investigation, a satisfactory e v a l u a t i o n can be m a d e . T h e r e b y , i m p o r t a n t decisions can be m a d e r e g a r d i n g their r e m a i n i n g service life a n d w h e t h e r they n e e d m o r e m a i n t e n a n c e to assure the safety of their occupants.

Acknowledgement Figure 8: A side view of the case study bridge

T h e a u t h o r likes to e x p r e s s his s i n c e r e a c k n o w l e d g e m e n t to Prof. M. P. R a n a w e e r a , D e p t . of Civil E n g i n e e r i n g of U n i v e r s i t y of Peradeniya for the valuable contributions given in the case s t u d y w o r k s .

T h i s b r i d g e w a s e v a l u a t e d to c h e c k for its current serviceability c o n d i t i o n s . Strain g a u g e s w e r e attached a n d r e s p o n s e of the b r i d g e w a s m e a s u r e d d u e to a c t u a l v e h i c l e p a s s a g e . In addition, with the experimental findings, theoretically w i t h the u s e of structural reliability

gSlliS

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References 1. Atashi, M., Lachemi M. and Kianoush (2007). Numerical modeling of the behavior of overlaid slab panels for reinforced concrete bridge decks, Journal of Engineering Structures, ELSEVIER, 29 (2), pp. 271-281. 2. Kong, J.S. and Frangopol, D.M. (2004). CostReliability interaction in life-cycle cost optimization of deteriorating structures, Journal of Structural Engineering, ASCE, 130 (11), pp. 1704-1712. 3. M.P.Ranaweera and P.B.R.Dissanayake (2003), Determination of the current state of the Maligakanda old reservoir wall of National Water Supply and Drainage Board, Engineering Design Centre, University of Peradeniya, Peradeniya, Sri Lanka. 4. M.P.Ranaweera and P.B.R.Dissanayake (2003), Load Testing of column being repaired at Gall face Hotel Colombo, Engineering Design Centre, University of Peradeniya, Peradeniya, Sri Lanka. 5. P.B.R.Dissanayake and M.P.Ranaweera (2004), Condition Evaluation of Old Structures, Proc. Of Annual session of IESL Central Province Kandy, Sri Lanka. 6. BS 8110-1 Structural use of concrete Part 1: Code of practice for design and construction (1997), British Standard Institute, UK 7. ES 6399 Loading for Buildings - Part 1: Code of Practice for Dead and Imposed Loads (1984), British Standard Institute, UK. 8. P.A.K. Karunananda, Service life prediction of bridges using structural reliability theory, (M. Phil. Thesis 2004), University of Peradeniya, Sri Lanka.

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