1- Introduction

Over the last three decades, natural disasters, particularly earthquakes, have become increasingly destructive as they affect large concentrations of population and property. The cost of replacing and repairing earthquake-damaged buildings is a significant drain on the economies of earthquake-prone countries. Therefore, it is imperative to achieve methods of reducing earthquake damage to an economically supportable level.

Estimates of seismic risk can be of considerable use to physical and economic planners concerned with investment and settlement in seismic areas. Also to those concerned with insurance against such losses and to civil defense officials whose task is to prepare plans for rescue, relief and rehabilitation after a possible future earthquake disaster.

Although there have been many studies of the assessment of seismic hazard, there have been relatively few attempts to assess the probability of damage or destruction of existing buildings. This is probably due to the lack of precise information on the vulnerability of existing buildings, i.e., on the degree of damage or proportional loss of value that they are likely to suffer when subjected to ground motion of various intensities.

 

2- Basic Methodology

It is hard to find two identical buildings. In order to be able to derive information of prognostic value from the inspection of earthquake damage, buildings must; therefore, be classified into a limited number of structural types. The definition of each type must not be only narrow enough to ensure some degree of uniformity in seismic response of all buildings within the type, but wide enough to ensure that the number of buildings in each type is large enough for statistical analysis. Therefore, it is necessary to define the relation between building types (in addition to the architectural and structural building configurations) and vulnerability classes, as well as between vulnerability classes and damage grades. The studies of vulnerability of buildings in the West Bank were done according to the EMS-98 (European Macro seismic Scale).

3- Building Types

The buildings in the West Bank were divided into categories according to the construction system:

-         Stone masonry.

-         Brick masonry.

-         Reinforced concrete beam-column –slab system (Joist System).

-         Reinforced concrete braced (infield) frame structures.

-         Reinforced concrete shear wall structures.

-         Bearing wall structures.

 

4- Factors Affecting the Seismic Vulnerability of Buildings

There are different factors affecting the overall vulnerability of a structure besides its construction type. These factors are generally applicable to all types of structures.

According to the site and building investigations, the following factors have been considered: -

-         Site conditions.

-         Regularity and configuration of structural and architectural elements.

-         Position: severe damage can occur to two adjacent buildings if they do not have enough spacing (seismic joint).

-         Strengthening

-         Earthquake resistant design (ERD).

-          Importance: the importance of a building is determined by the number of occupants or visitors, the use of buildings or the danger for public and environment in case of building failure.

 

5-Assigning Vulnerability Classes

Assessing the vulnerability classes of building structures was done according to the EMS-98.

For reinforced concrete (RC) frame buildings without earthquake resistant design ERD, vulnerability classes B to C are probable with C being most likely. For RC frame buildings with serious defects (such as soft storeys, weak columns, lack of stiffening elements, long or very long cantilevers with heavy loads at the end…etc), vulnerability class B or even A maybe appropriate. For regular RC buildings without ERD, but incorporating a certain level of lateral resistance (due to wind load design or stability verification), vulnerability class D might be representative for exceptional cases. For RC wall structures without ERD, vulnerability classes C to D are probable with C being the most likely one. For RC walls with serious defects, vulnerability class B can be regarded as the exceptional case. One should notice that defects will not lead to such a drastic decrease of vulnerability which can be observed in case of RC frame structures.

To emphasize the necessary data required for assigning the vulnerability classes of Palestinian buildings, most major Palestinian cities were investigated by collecting information based depending on the following parameters:

-         Site slope.

-         Existence of basements.

-         Soil type.

-         Number of storeys .

-         Slenderness ratio.

-         Building age.

-         State of concrete.

-         Construction over an existing old building.

-         Buildings within the vicinity.

-         Building type.

-         Structural system.

-         Existence of cantilever structural system.

-         Soft storey.

-         Symmetry of stiffness in horizontal plan.

-         Horizontal and vertical stiffness Variations.

-         Staircase location.

-         Staircase structural system.

-         Building entrances and exists.

-         Symmetry in horizontal configuration.

-         Symmetry in vertical configuration.

-         Horizontal symmetry in mass.

-         Formation of short columns.

The results of the pre-mentioned investigations were arranged in standard tables for all cities included in the study.

For each city, a representative zone or more were selected for investigations. The number of buildings in each zone varied between 80 and 700.

In addition to the above mentioned, the parameters were documented and tabulated to be considered as standard forms of Palestinian Building Vulnerability.

(Representative photos for selected Buildings in Palestinian Cities are shown in appendix a)

 

 

 

 

According to the survey of the buildings in major Palestinian cities, the results showed that the major seismic deficiencies in configuration and structural systems are distributed, but not limited to, as follows (Table 1)

 

Table1: Distribution of Seismic Deficiencies

Soft storey	15-25%
Existence of cantilever structural system	16-35%
Lack of seismic joints (for adjacent buildings and where necessary)	90-100%
Buildings within the vicinity	16-23%
Slenderness ratio (H/B) > 4(5) (with lack of symmetry in vertical and horizontal configurations)	11-19%
*Building entrances and exists

 

 

 Based on collected data and analysis done according to EMS 98, the following vulnerability classes were obtained. (Table 2)

 

Table2: Vulnerability Classes

City	Vulnerability Classes				No. of Buildings
	A	B	C	D
Nablus	35.5 %	42%	18%	4.5%	700
Ramallah & Abu-dis	32%	39%	22%	7%	120
Hebron	43%	31%	26%	0%	120
Jenin	45%	43%	12%	0%	100
Qalqilia	34%	45%	21%	0%	100
Tulkarm	41%	37%	19%	3%	80
Bethlehem	42%	39%	19%	0%	100

 

 

6- Damage Grades

Combining the influence of intensity with the obtained vulnerability, the expected grades of damage according to EMS-98 can be summarized as shown in table 3.

 

Table3: Classification of Damage

City	MM=VII			MM=VIII			MM=IX
	Damage grades
	5	4	3	5	4	3	5	4	3
Nablus	-	-	-	5.3%	21%	19%	14.2%	22%	14%
Ramallah & Abu-dis	-	-	-	4.8%	19%	19%	-	-	-
Hebron	-	-	-	6.5%	22%	17%	-	-	-
Jenin	-	-	-	6.75%	24%	19%	-	-	-
Qalqilia	-	5.1%	20%	5.1%	20%	21%	-	-	-
Tulkarm	-	6.15%	22%	6.15%	22%	18%	-	-	-
Bethlehem	-	-	-	6.3%	23%	19%	-	-	-

 

7- Comments and Conclusions

 

The main concluding remarks of the study are:

-         The buildings included in the investigations were relatively new.

-         Absence of a national seismic code is a major problem.  However, other codes are currently used on a limited scale.  These codes do not represent the real seismic performance of our building.

-         Codes are generally applied depending on engineer’s personal judgements, which in many cases produce a wide variation of results. For instance, codes in Palestine are usually applied for buildings having soft storeys, although it is quite clear that applying codes using equivalent static method for such buildings is not valid.

-         Considering the results obtained above, as well those obtained by the static analysis method (Dabbeek 2001), it is essential to make dynamic analysis for different types of Palestinian buildings. This is important to identify the variation in results obtained from the static method and to find relevant values for the critical building characteristics, such as: natural period of structural systems of buildings, properties of building materials, end conditions of structural elements and spectral analysis.