THE ALGERIAN SEISMIC CODE AND PERSPECTIVES OF THE AFRICAN CODE

N. Bourahla1, S. Tafraout1 and F. Bouriche2 1 Civil Engineering Department, University of Blida, Algeria 2 National Earthquake Engineering Centre, Algiers, Algeria

ABSTRACT Studies of the seismicity of Africa recognise that several regions are seismically very active, especially the northern and the east southern parts of the continent. The recent earthquakes which occurred in these areas, caused high death toll and incurred intensive damage to all types of constructions, which demonstrated how vulnerable most of buildings are. The post seismic investigations indicate that inappropriate design and construction of concrete structures may lead to premature ruin under earthquake ground motion. In order to mitigate the seismic hazard, some countries developed their own seismic codes which provide design provisions to enhance the performance of structures to withstand seismic loadings. In this paper, a brief account on the seismicity of the continent is first outlined showing the seismically hazardous locations. Northern Algeria is one of these areas which are highly exposed to severe earthquakes. The experience of the development of the Algerian seismic code is summarised through a historical background which highlights the major revisions. The latest version of the code (RPA99v2003) is then schematically outlined. Finally, the specific measures related to concrete design are extracted from the Algerian seismic code and a proposal is suggested for extending their application at a continent level within the framework of the African Concrete Code.

INTRODUCTION

Different parts of Africa are recognised by a high seismic activity. Inappropriately designed and detailed reinforced concrete structures to withstand seismic loading can be disastrous [1]. The rate of severe damage and collapses of such structures during recent earthquakes is very alarming [4]. Within the framework of earthquake hazard mitigation, seismic codes provide provisions to enhance the seismic resistance of the buildings and consequently protect human lives and minimise property lost. Seismic design practice in different regions of the African continent can be improved by experience exchange and cooperation. This paper presents the Algerian experience on earthquake engineering through the development of its seismic code and the performance of buildings especially reinforced concrete structures during the last earthquake that hit the eastern part of Algiers. The overall structure of the code is critically

presented with a particular emphasis on the design and detailing of reinforced concrete which represent the final outcome of a seismic design. The minimum seismic requirement provided by the RPA99, in terms of material characteristics, resisting element dimensions, reinforcement ratios, as well as some safety verifications are outlined. An adaptation of these provisions would represent a minimum seismic protection in the absence of a proper seismic code.

SEISMIC CONTEXT OF THE AFRICAN CONTINENT

Studies of seismicity of different regions in Africa indicate that those areas close to the borders of the African plate are seismically active. As can be seen on Figure 1, the yellow lines represent plate boundaries: African lithosphere junctions with Eurasian (EU), Arabian (AR) and Antarctic (AN) plates. North Africa, east Africa along the Gulf of Aden and the Arabian and Red Seas to the east African rift system as well as the southern part of Africa, are the regions with high level of seismic activities.

Figure 1 Seismicity of Africa 1990 – 2000 (USGS, National Earthquake Information Center) In terms of Acceleration ground motion, Figure 2 represents the seismic hazard map adapted from the Global Seismic Hazard Assessment program (GSHAP). The map depicts the seismic hazard as a peak ground acceleration (PGA) with a 10% chance of exceedance in 50 years, corresponding to an earthquake with 475 years return period. Different colours on the map indicate the areas with different seismic hazard levels, corresponding to PGA values expressed in m/sec2. For example, low hazard areas are characterized with PGA less than 0.08g, moderate hazard areas with PGA less than 0.25g, high hazard areas with PGA less than 0.4 g and very high hazard areas with PGA over 0.4g.

Figure 2 Peak Ground Acceleration (m/s2) with 10% probability of exceedance in 50 years

(GSHP)

EARTHQUAKE DAMAGE TO CONCRETE BUILDINGS

Recent major earthquakes that occurred in many locations in Africa, incurred severe damage to reinforced concrete buildings. The May 21, 2003 earthquake of a magnitude 6.8 which hit the northern-centre part of Algeria was particularly damaging to reinforced concrete frame buildings with hollow brick masonry infill walls. The performance of these buildings was poor with many total collapses, in contrast to structures with reinforced concrete shear walls which endure slight damage (Figure 3). Among the main causes related directly or indirectly to concrete construction practice we can mention the following:

− Large spacing of shear reinforcements in the columns near joint zones − Lack of shear reinforcements in nodal zones − Inappropriately designed short columns − Poor concrete strength and lack of confinement

This fact implies that a tremendous effort is required to improve the seismic performance of reinforced concrete buildings in those parts of the continent.

Figure 3 Damaged reinforced concrete residential buildings

ALGERIAN SEISMIC CODE Historical background After the devastating earthquake which stroke Algiers in 1716 and caused death to 20000 victims and severe damage to the majority of existing buildings, specific recommendations for seismic resistant buildings was issued for reconstruction by the Dey (Governor of Algiers) for the first time [1]. Later, following the 1954 Orléansville (Chlef) earthquake (Ms 6.8) disaster, the French government edited the seismic recommendations for construction called (AS 55) which became after revisions the French seismic code (PS69). In 1976 a study was launched with the collaboration of Stanford University to investigate the seismic risk in Algeria, which was achieved two years later. The results of this investigation were used as a basis for the development of the first version of the actual code which was issued just after the destructive 1980 El-Asnam (Chlef) earthquake (Ms 7.3). The RPA81 (Règles Parasismiques Algériennes) has been revised in 1988 (RPA88) and was also revised in 1999 (RPA99).

The last revision of the seismic code was made just after the May 21, 2003 Boumerdes earthquake which amended the following clauses:

− Subdivision of moderate seismicity zone into two sub-zones IIa and IIb − The seismic zoning map is revised to include the recently affected area in zone III. − The maximum value of the seismic zoning factor, A, is increased to 0.40 from 0.35. − Restriction on the number of storeys for buildings with reinforced concrete frames and

recommends the use of concrete shear walls. − Restrictions on open space at the ground floor level to avoid the soft story problem − Strength of the cast-in-place concrete − The size of structural elements, especially the columns.

Scope, Objectives and field of application The RPA99 aims at giving an acceptable protection for human lives and constructions against the adverse effects of seismic actions through an appropriate design and detailing. Its purpose is to ensure that:

− Non-structural damage is limited against frequent moderate earthquakes − Structural damage is limited (no collapse) against rare severe earthquakes − Vital constructions for civil protection remain operational after major earthquakes

The RPA99 applies to the design and construction of buildings and some minor civil engineering works in seismic regions. Civil engineering works like nuclear power plants, LNG facilities, dams, marine works, bridges, tunnels and buried networks are not covered by the RPA99 provisions. The structure of the seismic code As can be seen on the chart represented on figure 4, the seismic code RPA99 is structured in a simple manner where we can distinguish five main parts, namely: Seismic conceptual rules, classification criteria, loading specifications with dynamic analysis methods, safety verifications and finally the design and detailing for reinforced concrete, steel, masonry and foundation. General rules for conception are given to guide the engineer choosing the site of the project in order to avoid seismically hazardous locations. It recommends the type of soil investigation to be performed for specific cases and the foundation conception. Basic principles for seismic resisting constructions are then outlined in terms of configuration regularity, seismic joints between adjacent blocks, materials specifications and construction technology. Finally directives for the arrangement of the lateral load resisting systems are suggested together with notions on element and global ductility enhancement. Some recommendations on modelling and analysis terminate the first section of the seismic code. The second section deals with the classification criteria of the seismic zones which determine the acceleration factor according to the construction importance. It should be noted that the latter is a classification which sets minimum protection thresholds that can be modified by a building owner only by over classifying the building for a higher protection level taking into account its nature and destination with regard to the aimed objectives. The soil conditions are classified into four categories according to the geotechnical properties of the site which determine essentially the frequency range of the maximum level of the dynamic amplification factor for the equivalent static force or the design response spectra for the dynamic analysis. The classification of the lateral load resisting systems is made according to their reliability and their capacity of energy dissipation which depends on the constitutive materials, the type of construction, the aptitude of load redistribution in the structure and the global ductility. Each category is affected a numerical value of the behaviour coefficient. Although, the

behaviour factors for a specific type of lateral load resisting system give the impression that they should be comparable in most seismic codes, however, there are significant differences in some cases, probably due to local design and construction conditions and the seismic loading characteristics. The last paragraph of the classification chapter stipulates criteria for plan and height configuration regularity and irregularity. Like most seismic codes, the RPA99 specify the seismic loading as a global design base shear for the equivalent static force method and design response spectra for modal spectrum analysis method. The RPA99 recommends the use of the time history dynamic analysis method for structures that do not comply with the conditions of application of the code but it does not give specific procedure for determining the design ground acceleration time histories.

Figure 4 Chart of the content of the seismic code RPA99

RPA 99 R2003

General Seismic conception

rules

Classification criteria

Seismic action and method of

analysis

Safety verification

Design & Detailing

Scope and objectives

Field of application

Site selection

Soil investigation and ground conditions

Foundation

Structure

Modeling and analysis

Seismic zones

Importance of construction

Equivalent static force

Design response spectrum

Combinations of actions

Resistance condition

Reinforced concrete structures

Steel structures

Non-structural elements

Classes of the ground

Structural configuration (regularity)

Lateral load resisting systems

Seismic joint condition

Resistance and stability of foundation

Resistance of horizontal

diaphragms

Overturning and sliding condition

Ductility condition

P-Delta effect

Lateral displacement

condition

Masonry

Fountion and retaining walls

The results of the seismic analysis are then used to check the resistance and stability of the building using the safety verifications. First the combinations of the seismic load case with the different load cases are given followed by a set of requirements on deformation, storey drift, resistance condition, equilibrium condition, resistance of horizontal diaphragms and resistance of foundation. Four chapters are dedicated to design and detailing of reinforced concrete, steel structure, masonry and foundation with retaining walls. Finally, the last chapter provides provisions for non-structural elements. For the purpose of the African Concrete Code, only the design and detailing of the reinforced concrete will be presented.

SEISMIC DESIGN AND DETAILING OF REINFORCED CONCRETE IN SEISMIC ZONES

The specific rules for concrete buildings of the RPA99 are first summarised. The methodology to adapt them for the African concrete code will be then proposed. Scope The design and detailing of reinforced concrete section of RPA99 applies to buildings in seismic regions. Only monolithically cast-in-situ concrete buildings are addressed. Material requirements Concrete of strength class 20MPa ≤ fck ≤ 45MPa should be used in primary elements. With the exceptions of closed stirrups and cross-ties, only high strength steel bars with an elastic strength limit less than 500 MPa and a minimum strain equal to 5% under maximum loading shall be used in primary elements. Design and safety conditions Structural types and behaviour factors The values of the behaviour factor for the different types of lateral load systems are given in the table below.

Structural type R value Frame system without infill rigid masonry 5 Frame system with infill rigid masonry 3.5 Shear wall system 3.5 Shear wall core 3.5 Dual system 5 Wall equivalent dual system 4 Cantilever system with uniformly distributed mass 2 Inverted pendulum system 2 Safety verifications For ultimate limit state verifications the partial factors for material properties are:

Steel: γs=1 Concrete: γc = 1,15

The design strength of concrete is fcd =0.85c

ckf

γ

Requirements for columns Minimum dimensions of columns are as follows: - Min (b1 , h1) ≥ 25 cm for zones I and IIa

- Min (b1 , h1) ≥ 30 cm for zones IIb and III - Min (b1 , h1) ≥ he/20 all zones - 1 / 4 < b1/h1 < 4 all zones For circular cross sections, the diameter D should satisfy the following conditions: - D ≥ 25 cm zone I - D ≥ 30 cm zone IIa - D ≥ 35 cm zones IIb and III - D ≥ he / 15. all zones

Figure 5 Column dimensions and critical (nodal) zone Longitudinal reinforcement Minimum longitudinal reinforcement: - 0,7% zone I - 0,8% zone IIa - 0,9% zone IIb and Zone III Maximum longitudinal reinforcement: - 4% ordinary zone - 6% lap zone Minimum diameter of longitudinal reinforcement steel bars is 12mm Minimum lap length: - 40 φ zone I and IIa - 50 φ zone IIb and III Distance between vertical bars along a face of a column should not be more than: - 25 cm zone I et II - 20 cm zone IIb and III

Lap zones should be if possible out of the critical zones.

Transverse reinforcement Transverse reinforcement of the columns is calculated using the following formula:

s

ut

fh

V

s

A

1

ρ=

A minimum transverse reinforcement lies between 0.3% and 0.8% of the gross cross-section of the column depending on the slenderness ratio of the latter. At is the reinforcement cross-section, Vu is the shear force, h1 is the height of the cross-section, fs is the steel yield stress and ρ is a correction coefficient equal to 3.75, or 5 for slenderness ratio of the column greater than 5. s is the spacing between the hoops and shall not exceed:

s ≤ 15 ∅l zone I and IIa s ≤ Min (b1/2, h1/2, 10 ∅1) zone IIb and III

Critical zone :

s ≤ Min (10∅l, 15cm) zone I and IIa s ≤ 10 cm. zone IIb and III

∅1 is the minimal diameter of the longitudinal reinforcement of the column

Resistances Flexural and shear resistance shall be computed in accordance with the Algerian Concrete Code CBA, using the value of the axial, bending, shear forces from the analysis in the seismic design situation.

Axial force shall verify the condition: 30.0≤=ckc

d

fB

Nυ

Nd is the axial force, Bc is the gross cross-section of the column. The design shear stress is limited to: ckdcu fρτ =

ρd is equal to 0.04 or 0.075 for slenderness ratio greater than 5. Special considerations are prescribed for shear resistance of short columns. Requirements for beams Dimensions requirements are:

- b ≥ 20cm - h ≥ 30cm - h/b ≤ 4.0 - bmax ≤ 1,5h + b1

Figure 5 Dimensions requirements for beams

)2/,2/( 11 hbMax≤

)2/,2/( 11 hbMax≤

Column

Minimum longitudinal reinforcement is 0.5%, maximum reinforcement is 4% and 6% in lap zone. The minimum lap length is 40φ in seismic zone I and IIa, and 50φ in seismic zone IIb and III.

Transverse reinforcement: At = 0.003 s.b

Where s is the spacing, shall not exceed h/2 out of the critical zone or min(h/4,12φ) in critical zone (φ is the smallest longitudinal diameter). Figure 6 synthesises some details concerning the longitudinal and transverse reinforcement for the column, the beam and the joint.

Figure 6 Reinforcement details of the beam, the column and the joint

Requirements for beam-column joints In order to ensure that plastic hinges will occur first in the beams, the following conditions need to be satisfied except for the last upper two storeys of buildings (optional):

|Mn| + |Ms| ≥ 1.25 ( |Mw| + |Me| )

|M′n| + |M′s| ≥ 1.25 ( |M′w| + |M′e| )

≥50 φ

S S’ h

l’

h’

t’

t

b

h1

he

b1

cmt

cmMint

10

)15,10(

≤≤ φ

²)3;4/;4/(' ''

1 cmAAMaxA l≥

)10;2/;2/('

15

11 φφ

hbMinS

S

≤≤

=

=

cmhbh

Maxh

hl

e 60;;;6

'

2'

11

²)3;4/;2/( '11 cmAAMaxA ≥ lA

2A '1A

Figure 7 Beam-column joint requirements Requirements for shear walls The thickness of the wall (in metre) should satisfy the following expression: a ≥ max(0.15, he/20) and a minimum length of 4a. Rules for stiffness and resistance calculation of composite wall sections consisting of connected or intersecting rectangular segments (L-, T-, U-, I- or similar sections) are given RPA99. In addition to the CBA verification of shear stresses, the latter are limited by:

ckcc f2.0=≤ ττ

Lintel wall Reinforcement of lintel is considered for two cases:

Case 1: ττττc ≤≤≤≤ 0.06fck: The lintel is calculated as a flexural element which yields the following reinforcements:

Longitudinal reinforcement

sfz

MA

.1 ≥

Where z = h-2d' ,h is the total height of the lintel and d' is the concrete cover. Transverse reinforcement:

a) Slender lintel:

V

zfAs st≤

s is spacing, At is the transverse reinforcement and V is the shear force of the considered

section.

b) Short lintel:

st

st

fAV

lfAs

+≤

V = min (V1,V2) V2= 2Vu calculated

Mw Me

Ms

Mn

M’ w M’ e

M’ s

M’ n

ij

cjci

l

MMV

+=1

With Mci and Mcj are the ultimate resisting moments of the end cross-sections of the lintel of length lij and calculated by: Mc = Al fsz

Case 2: ττττc >0.06fck:

In this case, the minimum longitudinal and transverse reinforcement, as specified by the code, will be used. The resistance to seismic actions should be provided by reinforcement arranged along both diagonals (Figure 7) in accordance with the following expression:

αsin2 sD f

VA =

Where V is the design shear force, AD is the total area of steel bars in each diagonal direction and α is the angle between the diagonal bars and the axis of the beam.

Figure 8 Diagonal reinforcement of lintel Minimum reinforcement

a) Longitudinal reinforcement : (A l, A'l) ≥ 0,0015bh (0,15%)

b) Transverse reinforcement : - for τb ≤ 0,025 fck : At ≥ 0,0015bs ( 0,15% ) - for τb > 0,025 fck : At > 0,0025bs (0,25%)

c) Out of boundary elements reinforcement Total reinforcement for two layers in this region shall not be less than 0.20% Spandrel wall (free edge wall end) Flexural and shear resistances shall be computed exclusively in plane direction in accordance with the Algerian Concrete Code (CBA), provided that the above mentioned minimum dimensions and geometric configuration are satisfied. If the latter is not verified, the

10

Fc

Ft

10

A

A

α

4/hS≤

φ504/ +≥ h

h

b

A’ l

At

Ac

calculation in both directions including out of plane direction should be performed in accordance with the DTR-BC 2.4.2 (Regulation for design of concrete walls). In this case, calculation should be performed using vertical strips of width d≤ min (he/2, 2l'/3). l' is the length of the compression zone. he is the clear height of the wall.

Vertical reinforcement

When a tension region in the wall is induced under combined vertical and horizontal forces, then the tension force shall be resisted entirely by reinforcement. The vertical reinforcement ratio in the tension zone should not be less than 0.20%.

Vertical bars in boundary elements should be tied by horizontal hoops with spacing less than the thickness of the wall. In case of high compression at the ends of the cross-section, the vertical reinforcement should comply with the provisions of columns. Vertical bars at the top level should terminate with hooks. Spacing of vertical bars at the ends of the wall cross-section should be reduced to half or 15 cm whichever is less over a length of 1/10 of the wall length (figure 9).

Horizontal reinforcement Horizontal bars should be provided with 135° hook having 10ø long or an appropriate lap length.

Provisions for vertical and horizontal reinforcement:

• The minimum reinforcement ratio is 0.15% over all cross-section of the wall and 0.10% away from the boundary elements.

• The spacing of the vertical bars should be less than 1.5a or 30cm whichever is less. • The reinforcement layers in both sides should be engaged by at least 4 cross-ties /m2.

Except the cross-section ends, the diameter of the vertical and horizontal bars should not be more than 1/10 of the wall thickness.

• The lap lengths are 40ø in tension zones and 20ø in only compression zones. • Against sliding shear at horizontal construction joints should be resisted by anchorage

length of clamping bars crossing the interface using the formula:

svj f

VA 1.1=

This reinforcement quantity is to be added to the reinforcement resisting the tension force.

Figure 9 Reinforcement of free-edge wall end

S/2 S

L/10 L/10

L

104HA≥

a

Slabs and diaphragms RPA99 emphasises only on horizontal ties of diaphragms with lateral load resisting elements. It states that concrete diaphragms should have a peripheral tie-beam with reinforcement cross-section not less than 3cm². Tie-beam with a minimum reinforcement of 1,5cm² should be provided at intersections of lateral loading resisting element with the diaphragm.

Concrete foundation elements The design of concrete foundation elements such as tie beams, peripheral foundation walls and retaining walls are given in a separate chapter.

PERSPECTIVES OF THE AFRICAN CODE

Considering that seismic code for the entire African continent is a major task that requires a tremendous effort to determine a harmonized seismic risk in all seismic regions, it is worthwhile, however, as a first step, adopting in an appendix to the concrete code the relevant seismic design and detailing issues in order to ensure a minimum seismic protection. Many practical provisions which are not explicitly dependant on specific seismic site conditions can be easily adopted and could enhance considerably the seismic behaviour of the elements and the structures. Although, most of recommendations and regulations as outlined above may generally be accepted for common seismic zones, it is indispensable for specific points to be thoroughly studied. The following points need to be considered in order to generalise the use of the seismic design and detailing provisions:

1. Standardization of seismic zones with uniform nominal peak ground acceleration. 2. Adjustment of behaviour factors for the lateral load resisting systems. For instance, the

last earthquake which stroke the region of Algiers confirmed that shear wall systems had superior overall seismic resistance than the framed structures.

3. Adjustment of minimum requirements in terms of dimensions, reinforcement ratios, transverse reinforcement spacing.

To help realise the above motioned points, it is recommended that only two seismic levels to be used and the minimum seismic requirements will be adjusted for these two levels. Adapt a set of behaviour factors for the common lateral load resisting systems to be refined on the basis of local experience feedback. These factors have minor effect on the design and detailing issues.

CONCLUSION

This paper highlights the seismicity of the African continent to demonstrate the need for seismic provisions to be adopted for the African Concrete Code. The Algerian experience and specifically the latest revision of the Algerian seismic code RPA99 is critically presented. The seismic design detailing of reinforced concrete has been specifically exposed in details in order to demonstrate the possibility of extending the application at a continent level within the framework of the African Concrete Code to ensure a minimum seismic protection. Many aspects in terms of minimum design detailing requirements might be readily adopted in a first step, provided that some adjustments to be made.

REFERENCES

1. Bourahla N. And Tafraout S., “Analysis of collapsed buildings from Algiers Earthquake of 21

Mai, 2003”, Proceedings of the Seventh International Conference on Computational Structures Technology, B. H. V. Topping and C.A Mota Soares, (editor), Lisbon, Portugal, Civil-Comp Press, paper 153, 2004.

2. Eurocode 8: Design of structures for earthquake resistance, European Committee for Standardization, December 2003.

3. Mc Guire R. K. (editor), The practice of earthquake hazard assessment, European Seismological Commission, 1993

4. Oussalem H. and Bechtoula H., Report on the damage investigation and post-seismic campaign of the 2003 Zemmouri earthquake in Algeria, August 2003, University of Tokyo and Kyoto.

5. RPA-99 (2004). Règles Parasismiques Algériennes 1999. Centre National de Recherche Apliquée en Génie Parasismique, Alger.

6. Salhi A., Presentation des Regles Parasismiques Algeriennes RPA81, Proceedings of the 1er Seminaire Inter-Arabe sur le seisme et ses consequences, Algiers 1982.

7. Wium J. A. and Zijl G P A G, ‘The South African loading code revision of provisions for seismic loading’, Proceedings of the African Concrete Symposium 2005, Tripoli Libya.