Seismic assessment of existing RC buildings based …Seismic assessment of existing RC buildings based on different hazard maps Boll. Geof.Teor.Appl.,52, 000-000 the municipalities

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Seismic assessment of existing RC buildings based on different hazard maps

V. MANFREDI and A. MASI

Dip. Strutture, Geotecnica, Geologia Applicata, Università della Basilicata, Potenza, Italy

(Received: June 15, 2010; accepted: October 25, 2010)

ABSTRACT The current Italian structural code (NTC2008) includes the results of recent studies onItalian seismic hazard. Seismic actions are referred to a continuous scale, thusenabling a higher level of accuracy of the intensity values with respect to the pastseismic classification. Such a difference can determine remarkable effects on thecurrent seismic deficit of the existing building stock designed only to vertical loads,thus on the amount of possible retrofitting interventions. In this paper, the Italianprovincial capitals and the municipalities of Basilicata region are referred to for thesignificant differences between design seismic actions provided by the NTC2008 codeand the past seismic classification (OPCM3274) and are discussed first. Moreover, byadopting both classifications, the performances of typical RC buildings designed onlyto vertical loads have been assessed. Results have been compared in terms of buildingnumber and volume, as well as people residing in the Basilicata region.

Key words: RC buildings, design code, hazard maps, seismic assessment, non-linear analysis.

1. Introduction

Seismic classification is a fundamental tool for an appropriate mitigation strategy againstearthquake risk. In Italy, the first seismic classification was proposed after the 1908 ReggioCalabria-Messina earthquake. Subsequently, after other seismic events, about 25% of the Italianterritory was classified as seismic. In more recent years, in particular after the 1980 Campania-Basilicata earthquake and the 2002 Molise earthquake, about 70% of the Italian territory wasincluded in the seismic classification. Moreover, after the OPCM3274 (2003) law was passed, alow intensity of seismic action was assigned also to areas where a negligible seismic hazard wasevaluated.

The Basilicata region was classified mostly after the 1980 earthquake. Before 1980, only 6 ofBasilicata’s 131 municipalities were classified and, thus, most of the existing buildings, bothprivate and public, are not protected against earthquake actions. After OPCM3274 (2003) all theregional territory was classified and, in particular, 33% of the municipalities of Basilicata wereclassified as high seismicity zones (Seismic Zone 1, SZ1), while 52% and 15% of themunicipalities were classified as medium (SZ2) and low seismicity zones (SZ3), respectively.

Over the years the Italian seismic classification has been carried out by taking some ranges ofexpected seismic intensity values, each one relevant to each adopted SZ, and assigning the upperlimit value of the range to the whole territory of each municipality. This procedure unavoidablyoverestimates seismic actions with respect to the actual local hazard and, in particular, can cause

Bollettino di Geofisica Teorica ed Applicata Vol. 52, n. 2, pp. x-xx; June 2011

DOI 10.4430/bgta0010

© 2011 – OGS

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Boll. Geof. Teor. Appl., 52, 000-000 Manfredi and Masi

too much conservative and expensive retrofitting interventions on existing buildings. Seismicretrofitting of existing Reinforced Concrete (RC) buildings designed only to vertical loadslocated in medium-low seismicity areas can, however, be reduced, or made unnecessary, byadopting an accurate evaluation of the seismic actions. In fact, existing buildings generally havea surplus of capacity with respect to the design of vertical loads and this can give an effectiveeven though partial contribution for the protection of the structures against earthquakes (Masi etal., 2008).

The current Italian seismic code (NTC2008, 2008) includes the results of recent studies onItalian seismic hazard (OPCM3519, 2006). Seismic actions relevant to different return periodsare defined over the whole Italian territory with reference to a continuous scale of shaking values,as a function of the geographical coordinates of the construction site.

In this paper, some significant differences between design seismic actions based on theNTC2008 code and those based on the previous OPCM3274 classification are discussed.Moreover, with reference to the building stock of the Basilicata region in southern Italy, analysesof the performances of typical RC building structures designed only to vertical loads have beenperformed. Specifically, performances resulting from different values of seismic actions arecompared on structural types designed after 1971. It is worth noting that a large number ofprivate RC buildings (42%) now existing in Basilicata were built before the 1980 earthquake,namely before the seismic classification, thus neglecting seismic actions. Among these buildings,9,500 were built after 1971 with a global volume equal to about occupied by about 100,000inhabitants, which is 17% of the regional population.

2. Design seismic actions: OPCM3274 vs NTC2008

Italian seismic classification according to OPCM3274 was performed by subdividing thenational territory into four Seismic Zones (SZ1 to SZ4): the hazard within each zone wasassumed to be constant and it is described in terms of Peak Ground Acceleration (PGA) evaluatedon rigid soil (type A) relevant to return periods of 475 years (or equivalently a probability ofexceedance of 10% in 50 years). Different spectrum shapes were considered with different soiltypes, the shape was assumed constant when varying the return period TR. The considered valuesof PGA were 0.35 g, 0.25 g, 0.15 g and 0.05 g for SZ1, SZ2, SZ3 and SZ4, respectively. Now,the new Italian seismic code (NTC2008) includes recent hazard studies (Gruppo di Lavoro MPS,2004; OPCM3519, 2006) carried out by the National Institute of Geophysics and Vulcanology(INGV). It provides the expected seismic intensities, still in terms of PGA values, according to aspatial continuous distribution and thus it is able to go beyond the previous situation wherebyadministrative municipal boundaries functioned as the limits of different seismic zones.Moreover, the spectrum shape depends on the geographical coordinates of the construction siteand varies both with the return period TR and the soil type. Figs. 1 and 2 show the expected valuesof PGA derived from the OPCM3274 classification map (Fig. 1) and from the NTC2008 hazardmap (Fig. 2), both considering soil type A (rock or other rock-like geological formation) anda TR = 475 years.

It is worth noting that for both the OPCM3274 and NTC2008 codes, the greater seismicitylevels in Italy are expected to be located along the Apennine mountain chain and in the Friuli

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Seismic assessment of existing RC buildings based on different hazard maps Boll. Geof. Teor. Appl., 52, 000-000

region, but with rather different values of PGA. In accordance with NTC2008, in those areasPGA values are generally in the range of 0.25-0.275 g and no area has PGA values greater than0.30 g, whereas the maximum value of PGA in OPCM3274 is 0.35 g (SZ1). On the contrary,according to NTC2008, the medium-low seismicity zones located in the central-southern part ofSicily are larger if compared with OPCM3274. Also the values of PGA along the boundariesshared by the Piemonte and Valle d’Aosta regions are currently higher than in the previousclassification.

Fig. 3 shows the frequency distribution of the expected seismic values in the Italian territoryfor both NTC2008 and OPCM3274 codes. The first column of the frequency distribution reportsthe percentage of regular grid points utilized by the NTC2008 hazard model and the second onethe number of Italian municipalities (OPCM3274), with respect to four ranges of PGA, namely0-0.05 g, 0.05-0.15 g, 0.15-0.25 g, 0.25-0.35 g. According to NTC2008 most of the Italianterritory (about 50% of the points on the grid) is in the medium-low seismic intensity range (0.05-0.15 g), whereas the OPCM3274 percentages prevail for the other three ranges. In particular, 9%

Fig. 1 - Italian seismic classification map inaccordance with OPCM3274 (soil type A,TR=475 years).

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of municipalities (716 of 8101) are in SZ1 (0.25-0.35 g), and 42% (3427 of 8101) in the seismicrange with lowest values (0-0.05 g).

Figs. 4 and 5 show a comparison between NTC2008 and OPCM3274 seismic intensity valuesrelevant to the 106 Italian provincial capital cities. The comparison has been performed both interms of PGA value (Fig. 4) and in terms of elastic acceleration spectrum values Se computed inthe period T=1.0 s (Fig. 5). Seismic intensity values relevant to NTC2008 have been computed atthe construction site of the municipality town hall. Many cities, classified in very low seismicityzones (SZ4) in OPCM3274, after NTC2008 have PGA values greater than 0.05 g, including, forexample, Cuneo, Sondrio, and Trieste. On the contrary, in other ranges, the NTC2008 PGAvalues are generally lower than the OPCM3274 ones. For example, the city of Rome wasclassified SZ3 (PGA = 0.15 g) in OPCM3274 whereas its PGA value is 0.107 g (-29%)considering NTC2008 and also the city of Naples shows a remarkable reduction in its PGA value(-33%). It is worth noting that only a few cities, such as L’Aquila (+4%), have an NTC2008 PGAvalue greater than the OPCM3274 one. Greater differences can be found when the comparison is

Fig. 2 - Italian seismic hazard map inaccordance with NTC2008 (soil type A,TR=475 years).

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carried out in terms of spectral values Se(T=1.0 s): in fact for all the Italian provincial capitalsclassified in SZ1, SZ2, SZ3 the NTC2008 values are lower than the OPCM3274 values. Forexample, considering the city of Rome, the difference between OPCM3274 and NTC2008Se(T=1.0 s) is equal to -41% (it was -29% in terms of PGA), and it is equal to -15% for the cityof L’Aquila (it was +4% in terms of PGA).

In Figs. 6 and 7, the comparison between NTC2008 and OPCM3274 is performed withreference to the 131 municipalities of the Basilicata region, both in terms of PGA and elasticacceleration response spectrum values Se(T=1.0 s). The values relevant to the NTC2008 arecomputed at the site of construction of the municipality town hall. In this case, the NTC2008PGA values are generally lower than the OPCM3274 ones. In particular, in 94% of the

Fig. 3 - Frequency distributions ofthe regular grid points utilized bythe NTC2008 hazard model (firstcolumn) and of the municipalitiesas classified in the OPCM3274(second column) with respect tofour PGA range values.

Fig. 4 - Comparison between NTC2008 and OPCM3274 PGA values (return period TR=475 years) relevant to theItalian provincial capitals.

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municipalities NTC2008 PGA values show decreases ranging from 2% to 185% with respect tothe OPCM3274 ones. Only in 6% of the municipalities the NTC2008 PGA values are greater thanthe OPCM3274 ones, moreover with very small increases. The greater differences are found for

Fig. 6 - Comparison between NTC2008 and OPCM3274 PGA values relevant to the 131 municipalities of Basilicataregion (return period TR=475 years).

Fig. 5 - Comparison in terms of elastic acceleration response spectrum Se at period T=1 s (return period TR=475 years)between NTC2008 and OPCM3274 relevant to the Italian provincial capitals.

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the municipalities located in the area of medium-high seismicity, while almost negligibledifferences are found in the areas with the lowest hazard values. For example, the current PGAvalue (TR=475 years) evaluated for the town of Potenza is equal to 0.202 g whereas it wasclassified in SZ1 (PGA=0.35 g) according to OPCM3274, with a decrease of the PGA valueequal to 43%, while for the town of Matera (classified SZ3 in OPCM3274) a decrease equal to8% is found. Greater differences can be found when the comparison between OPCM3274 andNTC2008 is performed in terms of spectral acceleration Se(T=1.0 s): in fact, for all themunicipalities of the Basilicata region the NTC2008 values are lower than the OPCM3274 ones.As an example, for the town of Matera the decrease of Se(T=1.0 s) value is equal to -20%, and itgoes up to -49% for the town of Potenza.

3. Methodology

In order to understand the effects of the variation of seismic intensities returned by differenthazard maps on the seismic assessment of typical existing buildings, nine types of RC existingframed structures designed only to vertical loads have been studied. They can be consideredrepresentative of structures widely present in the post-1970 Italian building stock as well as oftypical European building structures (Masi, 2003; Masi and Vona, 2004).

The selected frame structures have 2, 4 and 8 stories (Fig. 8a) representative of low-, mid- andhigh-rise buildings, respectively, with an inter-storey height of 3 m. The buildings have arectangular plan shape of 10x15 m and a bay length of 5 m in both directions. In the exteriorframes, the beams are 30x50 cm (Rigid Beam, RB), while in the interior frames there are nobeams along the transverse direction and the columns are connected through a RC slab strip of

Fig. 7 - Comparison in terms of elastic acceleration response spectrum Se at period T=1 s between NTC2008 andOPCM3274 relevant to the 131 municipalities of Basilicata region (return period TR=475 years).

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22x20 cm (No Beams, NB). Moreover, taking into account the typical characteristics of framebuilding structures, the presence and the position of infill masonry walls along the exteriorframes have also been considered, thus obtaining the BF (Bare Frame), IF (Infilled Frame), andPF (Pilotis Frame) types (Fig. 8b).

3.1. Simulated design and modelling of selected structural types

The simulated design of the selected structural types has been performed taking into accountonly gravity loads, referring to the codes in force, the available handbooks and the typical currentpractice of the period (Masi, 2003). According to the standards of the 1970s, simulated designhas been performed by adopting the allowable stress method in the safety verifications, andconsidering mechanical properties of materials typically adopted in the period, that is mediumquality concrete C20/25 and deformed steel with a grade close to the S400 type. The columnshave been designed for an only axial load and by adopting the minimum requirements providedin the Italian code of the period. The beams have been designed on the basis of the simplifiedmodel of a continuous beam resting on simple supports.

To assess the resistance of the selected types to seismic loads Non-Linear Static Analyses havebeen performed on 3D models using the finite element computer program (SAP2000, 2006). The

Fig. 8 - Selected structural types in terms of number of stories (a): 2 stories (left), 4 stories (centre), 8 stories (right);selected structural types with regard to the presence and position of masonry infills (b): Bare Frame (left), InfilledFrame (centre), Pilotis Frame (right).

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mechanical properties of the constituent materials have been assumed considering the expectedvalues for residential building structures realized in the period under examination. Specifically,on the basis of the results of some in-situ experimental campaigns, a lower bound value has beenconsidered for the concrete compressive strength, fcm=18 MPa, while for steel, a yielding strengthfsm=400 MPa has been assumed.

In the calculation of the structural capacity, a normal knowledge level KL2 (CEN, 2004) hasbeen assumed, therefore, mean strength values have been divided by the confidence factorCF=1.2. A lumped plasticity modeling for the RC structural members has been used. At both endsof each structural member (beams and columns) a bending moment – rotation relation has beendefined through a bi-linear curve described by the values of the yielding moment (My), chordrotation at yielding (θy), and ultimate chord rotation (θum). Moment values have been computedaccording to NTC2008 considering a parabola–rectangle diagram for concrete undercompression, characterized by an ultimate strength equal to fcm/CF, no tensile strength, strain atpeak stress εco=0.002, and unconfined ultimate strain εcu=0.0035. An elastic, perfectly plastic,steel stress–strain diagram is considered, characterized by a maximum strength equal to fsm/CFand ultimate strain εsu=0.01.

θy and θum values have been computed using the following equations given in the NTC2008Commentary (CIR617, 2009):

(1)

(2)

where: γel is equal to 1.5 for primary seismic elements; h is the depth of the cross-section; LV=M/Vis the ratio moment M/shear V at the end section; ν=N/bhfc ,with b the width of compression zone,N axial force positive for compression; ω and ω’ are the mechanical reinforcement ratios of thetension and compression, respectively, longitudinal reinforcement; fc and fyw are the concretecompressive strength (MPa) and the transverse steel yield strength (MPa), respectively;ρsx=Asx/bwsh ratio of transverse steel parallel to the direction of loading (=stirrup spacing); ρd=0is the steel ratio of diagonal reinforcement in each diagonal direction; a is the confinementeffectiveness factor, calculated by using the following expression:

(3)

where: bo and ho are the dimensions of confined core to the centerline of the hoop; bi is thecenterline spacing of longitudinal bars (indexed by i) laterally restrained by a stirrup corner or across-tie along the perimeter of the cross-section.

α = −⎛⎝⎜

⎞⎠⎟

−⎛⎝⎜

⎞⎠⎟

−⎛

⎝⎜

⎞∑1

21

21

6

s

b

s

h

b

h bh

o

h

o

i2

o o ⎠⎠⎟

θγ

ωω

νum

el

= ⋅10 016 0 3

0 01

0 01, ( , )

max( , ; ')

max( , ; )ff

L

h

f

fc

Vsx

yw

c⎡

⎣⎢

⎤

⎦⎥

⎛⎝⎜

⎞⎠⎟

⎛

⎝⎜

⎞

⎠

0 225 0 35

25

. , αρ ⎟⎟( , )1 25100 ρd

θ φ φy yV

Vy

b y

c

L h

L

d f

f= + +

⎛⎝⎜

⎞⎠⎟

+3

0 0013 1 1 5 0 13, , ,

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As regards the infills, in existing RC buildings they are usually made up of two layers ofhollow brick masonry with a total thickness equal to about 200 mm and low mechanicalcharacteristics. The total resistance and stiffness capacity of each masonry panel has been definedby considering an equivalent diagonal strut, whose area has been determined by multiplying thepanel thickness tw (equal to 200 mm) by an equivalent width bw (Masi, 2003). The expressionelaborated by Mainstone (1974), relevant to rectangular masonry panels, has been used tocompute the equivalent width bw of the strut:

(4)

where hw = panel height, dw = diagonal length, θ = angle of the diagonal with the horizontal,Ew=2,000 N/mm2 is the modulus of elasticity of the masonry, Ec = modulus of elasticity ofconcrete and Ip = moment of inertia of the columns. The characteristics of the equivalent strut arethe area As = bw⋅tw, the stiffness Ks = (Ew As/dw) and the ultimate resistance Fus = As⋅fw. A typicalvalue has been assumed for the compressive strength of masonry fw, equal to 2.0 N/mm2. Thenon-linear and degrading behaviour of the diagonal strut has been modelled using a plastic hingelocated at the ends of the strut defined only for compressive strength. Similarly to the modellingadopted in (Masi, 2003) the force-displacement relationship F-d (Fig. 9) is defined as follows:

(5)

3.2. Seismic capacity evaluation of selected structural types

The structural performances of the selected types have been evaluated through Non-LinearStatic (pushover) Analyses (NLSA) according to NTC2008 and, in particular, adopting thedetailed provisions reported in the relevant Commentary (CIR617, 2009). The pushover curves,that represent the relation between the base shear force and the control node displacement, havebeen achieved under conditions of constant gravity loads and monotonically increasing horizontalloads according to uniform and modal pattern distributions for each of the two orthogonaldirections in the plan. A bi-linear curve relevant to an idealized equivalent single degree offreedom (SDOF) system has been computed on the basis of the following assumptions:

i) maximum displacement dmax corresponding to 0.85 Fmax, ii) for a base shear force value F = 0.6 Fmax the pushover curve and the bi-linear one have the

same displacement value,

0 1≤ ≤ → = ×d d F k de , where kF

d

d d de

e e

1

2

=

< ≤ × →

max ;

;

(

max

max

F F

d d d F F k de e

=

× < ≤ × → = − × −2 7 22 ×× =××

> ×

d kF

d

d d

ee

e

),.

;maxwhere 2

0 9

5

7 . max→ = ×F F0 1

b

d

E t h

E Iw

w

w w w

c p

= ⋅ ( )⋅⋅ ⋅ ⋅ ( )

⋅⎛

⎝⎜0 20 2

23

. sinsin

θθ ⎞⎞

⎠⎟

−0 1.

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iii) the yield force Fy of the SDOF system is determined in such a way that the areas underthe push-over curve and the bi-linear one are equal (Fig. 10).

The period T of the idealised SDOF system is given by:

(6)

where m is the mass, dy is the yield displacement, Fy is the yield force. For each structural type under study, the capacity of the SDOF system has been defined as theminimum value of the displacement, dLS, corresponding to the Limit State of Life Safety. It hasbeen achieved by checking that the demand does not exceed the corresponding capacity in termsof deformations for ductile elements and in terms of strengths for brittle elements. Table 1summarises the values of T, dLS, and Se,LS for the selected structural types, where the elasticacceleration spectral values corresponding to the Limit State of Life Safety, Se,LS, have been

Tm d

Fy

y

=⋅

2π

Fig. 9 - Force-displacementrelationship of the equivalentstrut modelling the non-linearbehaviour of masonry infills.

Fig. 10 - Typical pushover curveand determination of the idealizedelasto-perfectly plastic force –displacement curve.

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computed using the following relationship:

(7)

As for the role of the masonry infills, higher values of capacity dLS are generally shown by IFtypes with the exception of the 2s type, while the lowest values are always shown by PF types,coherently with the results achieved in Masi (2003) and Masi et al. (2010) through Non-LinearDynamic Analyses. As an example, considering the 4s type, dLS is equal to 0.024 m for IF type,whereas it is equal to 0.020 m and 0.014 m for BF and PF types, respectively. With regard to thenumber of stories, the 4s types have the lowest capacity values and, in particular, far lower values thanthe relevant 2s types. This result can be explained taking into account the minimum requirements onmember sections and reinforcing steel areas prescribed by the design code which providedremarkable overstrengths to the 2s structures with respect to the vertical load design effects.Moreover, the different failure mechanism suffered by the structural elements has been predicted. Inparticular, for the 4s and the 8s types a brittle failure (verification in terms of strength) has beenfound, whereas for the 2s type a ductile failure (verification in terms of deformations) has beenpredicted, therefore, leading to remarkably higher displacement capacities.

In the following, safety verifications on each structural type under study have been carried outby comparing the Se,LS values corresponding to the Limit State of Life Safety (capacity C) withthe spectral values Se,475 provided by both NTC2008 and OPCM3274 at the fundamental periodT and considering a return period of 475 years (demand D). Safety verifications are satisfiedwhen Se,LS ≥ Se,475 is found.

4. Safety verification results: application to RC buildings in the Basilicata region

In this section, some results of analyses on the Basilicata region building stock with reference

S dTe = ⎡

⎣⎢⎤⎦⎥

22π

Table 1 - Displacements dLS and elastic acceleration spectral values Se,LS evaluated at the Limit State of Life Safety forthe selected structural types. T is the fundamental period of the idealised system.

type storey number T [s] dLS [m] Se,LS [g]

BF

2s 0.52 0.045 0.669

4s 1.15 0.020 0.061

8s 1.73 0.038 0.051

IF

2s 0.37 0.027 0.792

4s 0.76 0.024 0.167

8s 1.19 0.048 0.136

PF

2s 0.43 0.025 0.543

4s 0.94 0.014 0.064

8s 1.35 0.025 0.055

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to RC buildings realized after 1971 and designed only to vertical loads, are reported. At the time of the last census of the Italian population and residential buildings (ISTAT, 2001),

33,000 out of the 150,000 private buildings were RC buildings. In Fig. 11, the frequencydistribution of the building number in terms of age (Fig. 11a) and the frequency distribution ofthe volume relevant to the buildings realized after 1971 without seismic design in terms ofnumber of stories (Fig. 11b), are shown. More than half of the RC private buildings (58%) wasbuilt after the 1980 earthquake considering seismic forces, even though no adequate seismicdesign criteria were at the time enforced, while about 42% of buildings already existing at thetime of the classification was designed taking into account only vertical loads. Most of thevolume is made up of low-rise buildings with 1-3 stories (69%), while only 4% of the globalvolume is relevant to high-rise buildings with a number of stories higher than 7. It is worth notingthat also the public building stock of the Basilicata region suffers from the same problem, asabout 2,000 out of 3,000 existing public buildings were designed only to vertical loads.Therefore, a more accurate definition of seismic actions is strongly necessary, as it could allow areduction in the number as well as the time required for retrofit interventions.

The safety verifications have been carried out with reference to all the municipalities of theBasilicata region with the exception of 6 of Basilicata’s 131 municipalities, which were classifiedafter the 1930 earthquake (SC in the following), where no RC buildings designed only to verticalloads can be found. For the sake of simplicity, in the safety verifications a constant value of theseismic action, evaluated at the site of the municipality town hall, has been assumed throughoutthe whole municipality area. It has to be underlined that most of the RC buildings are located inthe urban areas, thus taking into account the small variations of intensity values inside eachmunicipality territory (typically within the range ±0.025 g), such assumption, moreoverunavoidable on large scale evaluations, has reduced effects on the results. Globally, the resultsshow that 29% of the buildings under study are not verified when OPCM3274 seismic intensitiesare considered, while a remarkably lower percentage (11%) can be found adopting NTC2008seismic intensities.

To display the geographic distribution of the safety verification results among themunicipalities of the Basilicata region, in Fig. 12 the results specifically achieved on the IF types,for both NTC2008 and OPC3274 Se,475 values, are shown. When the OPCM3274 Se,475 values are

Fig. 11 - Frequency distribution of the RC building number of Basilicata region in terms of age (a) and frequencydistribution of the RC building volume (built after 1971 without seismic design) in terms of number of stories (b).

a b

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a) 8-storey IF structural types (8s_IF).

b) 4-storey IF structural types (4s_IF).

c) 2-storey IF structural types (2s_IF).

Fig. 12 - Results of safety verifications on the IF types considering both OPCM3274 Se,475 (left) and NTC2008 ones(right) in the municipalities of Basilicata region.

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a) 8-storey IF structural types (8s_IF).

b) 4-storey IF structural types (4s_IF).

c) 2-storey IF structural types (2s_IF).

Fig. 13 - Values of the capacity-demand ratio α=Se,LS/Se,475 calculated for the IF types considering OPCM3274 Se,475

(left) and NTC2008 ones (right).

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considered, the safety verification of 8s_IF and 4s_IF types is not satisfied in all themunicipalities of Basilicata, while it is not satisfied in 58% and 89% of the municipalities,respectively for 8s and 4s type, when NTC2008 Se,475 are considered. For the 2s type the safetyverification is always satisfied when NTC2008 Se,475 is considered, while it is not verified in 31%of the municipalities adopting OPCM3274 Se,475.

A greater understanding of the variation of the seismic deficit on the structural types understudy can be obtained by considering the capacity-to-demand ratio values, that is the ratio α =Se,LS /Se,475. Table 2 shows the percentages of buildings having α values belonging to four ranges,computed considering OPCM3274 and NTC2008 seismic intensities. The results show that 11%of the buildings of the selected types has a remarkable deficit of seismic resistance, with α valuesin the range 0.25-0.5, when the demand is evaluated according to the OPCM3274 seismicclassification, while this percentage decreases to 2% considering NTC2008 seismic intensities.

To display the geographic distribution of the capacity-to-demand values among themunicipalities of the Basilicata region, in Fig. 13 some maps of the region are displayed showingthe α values relevant to the IF type. In all the towns of Basilicata the value of the α ratiocomputed for the 2s_IF type is greater than 1 when NTC2008 Se,475 is considered, whereas inseveral municipalities (31%) the α values are in the range of 0.75-1 considering OPCM3274seismic intensities. The worst performances are shown by the 4s_IF type: in fact, consideringOPCM3274 in 93% of the Basilicata towns the α value is in the range 0.25-0.5, whereas nomunicipality has a α value lower than 0.5 when NTC2008 seismic intensities are considered.

The effect of the safety verifications has been analysed also in terms of number of buildings,built volume and residents. For this purpose, data derived from the last National Institute ofStatistics census (ISTAT, 2001), available for all the 131 municipalities in the Basilicata region ina uniform and complete form, has been considered. However, the ISTAT (2001) database containspoor information about the typological characteristics of buildings and, in particular, whenconsidering global dimensions, it provides only the number of stories for each building.

Fig. 14 - Number of buildings (percentage) with not satisfied safety verification evaluated for each municipality ofBasilicata region considering OPCM3274 (left) and NTC2008 (right) seismic actions.

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Therefore, another database, available for 63 out of the 131 municipalities located in theBasilicata region and containing more complete and reliable data from field surveys, has beenconsidered. Correlating data from the two databases (Samela et al., 2009), building volume andresidents in the structural types under study have been estimated for all the municipalities of theBasilicata region. Then, comparing Se,475(T) and Se,LS, the relevant percentages with no satisfiedsafety verification have been found. The results are shown in Figs. 14 to 16 for both OPCM3274and NTC2008 seismic actions.

Fig. 15 - Volume of buildings (percentage) with not satisfied safety verification evaluated for each municipality ofBasilicata region considering OPCM3274 (left) and NTC2008 (right) seismic actions.

Fig. 16 - Residents (percentage) in building for which a not satisfied safety verification has been evaluated withreference to each municipality of Basilicata region considering OPCM3274 (left) and NTC2008 (right) seismicactions.

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When OPCM3274 seismic intensity values are considered, most of the selected building types(range 80%-100%) existing in 33% of the municipalities of the Basilicata region are not verified.These municipalities are located close to the Apennine mountain chain, where high seismicintensities are expected. On the contrary, in most of the municipalities, the percentage of notverified buildings is lower than 20% when seismic intensity values provided by the NTC2008 areutilized, while only in a few municipalities are the values in the range of 40%-60%. Withreference to the whole of the Basilicata territory, the percentage of buildings not verified adoptingthe OPCM3274 seismic intensity values is 41%, while a strong reduction (11%) can be obtainedby using the NTC2008 values. In terms of volume, a value greater than six million cubic meters(about 42% on the total) should be retrofitted when the OPCM3274 seismic intensity values areconsidered; a strong reduction (about 21% of volume of the buildings) can be calculated whenNTC2008 seismic intensity values are adopted. Concerning the possible effects on human beings,the results in Fig. 16 shows that 63% of people live in buildings with not satisfied safetyverification when using OPCM3274 seismic intensity values, whereas this percentage goes downto 33% when NTC2008 seismic intensity values are considered.

5. Conclusions

In the recent NTC2008 Italian structural code, a revised seismic hazard map for the whole ofthe national territory was adopted. Seismic actions for both the design of new buildings and theassessment of existing ones are referred to a continuous scale of values and are evaluated as afunction of the geographical coordinates at the construction site. This hazard map enables thedefinition of the design seismic actions with a higher level of accuracy with respect to theprevious seismic classification (OPCM3274, 2003), based on seismic zones providing a constantvalue over the whole municipality area. Such a difference, in terms of seismic demand candetermine remarkable effects on the current seismic deficit of the existing building stockdesigned only for vertical loads, thus on the amount of possible retrofitting interventions. For thispurpose, in the paper comparisons between seismic action values relevant to the Italian provincialcapitals and the municipalities of the Basilicata region, by referring to both the NTC2008 and theOPCM3274 seismic classification, have been performed. Values of NTC2008 PGA475 aregenerally lower than those provided by OPCM3274, showing great differences, in particular,where the highest seismic intensities are expected. Even greater differences can be foundcomparing the spectral values Se,475.

To understand the consequences of the modified seismic demand, if any, safety verificationson a set of structural types representative of existing RC buildings designed only to vertical loadshave been carried out. Buildings designed after 1971 and located in the Basilicata region have

Seismic actions 0.25≤α≤0.5 0.5<α≤0.75 0.75<α=<1.0 α≥1.0

OPCM3274 11% 9% 9% 71%

NTC2008 2% 6% 3% 89%

Table 2 - Values of the capacity-to-demand · ratio considering both OPCM3274 and NTC2008 seismic intensities

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been evaluated considering both NTC2008 and OPCM3274 seismic actions. Results show thatfor some building types safety verification is not satisfied when the OPCM3274 actions areconsidered, contrary to what can be achieved by adopting the NTC2008 seismic intensity. Inparticular, using OPCM3274 seismic intensity a volume greater than 6⋅106 m3 (about 42% on thetotal) should be retrofitted, whereas this share decreases to 21% considering NTC2008.

The results show that a more accurate definition of seismic intensity, based on local values,opposite to constant values assigned to the whole municipality territory, could decreaseremarkably both the number of buildings requiring retrofitting interventions and the amount ofintervention required. Thus, a reduction in both costs and time necessary to carry out a moreeffective policy of seismic risk reduction can be obtained. Further, the geographical distributionof the demand-to-capacity ratios, highlights the Basilicata region municipalities where the deficitof seismic resistance is higher, thus providing a tool to effectively allocate resources in theseismic mitigation policy.

As far as the developments of this study are concerned, the following points need to bepursued:

• site effects have been neglected, therefore results achieved in the present study provide alower bound estimate of the global possible effects on the building stock under study. As amatter of fact, to know where possible site effects can occur over large areas such as thewhole territory of the Basilicata region is a very expensive task. Proposals to estimate siteeffects on the basis of surface geological data drawn from large scale geological maps arenot very satisfactory. A broad study aimed at evaluating, in a simplified as well assufficiently accurate way, the presence and number of site effects in the whole Basilicataterritory is currently in progress;

• the evaluations carried out are relevant only to post-1971 RC buildings, therefore furtheranalyses are in progress which also consider ante-1971 RC building types designed only tovertical loads.

REFERENCES

CEN; 2004: Eurocode 8 - Design of structures for earthquake resistance - Part 3: assessment and retrofitting ofbuildings. UNI-EN 1998-3, June 2004, Brussels.

CIR617 - Circolare 2 febbraio 2009, n. 617; 2009: Istruzioni per l’applicazione delle “Nuove norme tecniche per lecostruzioni”. G. U. n. 47, 26.02.2009, Supplemento Ordinario n. 27 (in Italian).

Gruppo di Lavoro MPS; 2004: Redazione della mappa di pericolosità sismica prevista dall’Ordinanza PCM 3274 del20 marzo 2003. Rapporto Conclusivo per il Dipartimento della Protezione Civile, INGV, Milano-Roma, aprile2004, 65 pp. + 5 app. (in Italian).

ISTAT; 2001: 14° Censimento generale della popolazione e delle abitazioni.<http://dawinci.istat.it/MD/http://dawinci.istat.it/MD/> (in Italian).

Mainstone R.J.; 1974: Supplementary note on the stiffness and strength of infilled frames. Current Paper CP13/74,Building Research Establishment, London.

Masi A.; 2003: Seismic vulnerability assessment of gravity load designed R/C frames. Bulletin of EarthquakeEngineering, 1, 371-395.

Masi A. and Vona M.; 2004: Vulnerabilità sismica di edifici in c.a. realizzati negli anni ’70. In: Atti del XI CongressoNazionale L’Ingegneria Sismica in Italia, Genova (in Italian).

Masi A., Vona M. and Manfredi V.; 2008: A parametric study on rc existing buildings to compare different analysismethods considered in the european seismic code (EC8-3). In: The 14th World Conference on Earthquake

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Engineering, Beijing, China.

Masi A., Vona M. and Mucciarelli M.; 2010: Selection of natural and synthetic accelerograms for seismic vulnerabilitystudies on RC frames. Journal of Structural Engineering, 137, 367-378, doi: 10.1061/(ASCE)ST.1943-541X.209.

NTC2008 - Decreto Ministeriale D.M. 14.1.2008; 2008: Nuove Norme Tecniche per le Costruzioni NTC2008. G. U.n. 29, 04.02.2008 – Supplemento Ordinario n. 30 (in Italian).

OPCM3274 - Ordinanza del Presidente del Consiglio dei Ministri n.3274/2003; 2003: Primi elementi in materia dicriteri generali per la classificazione sismica del territorio nazionale e di normative tecniche per le costruzioniin zona sismica. Supplemento ordinario G. U. n. 105, 8 maggio 2003 (in Italian).

OPCM3519 - Ordinanza del Presidente del Consiglio dei Ministri n.3519/2006; 2006: Criteri generali perl’individuazione delle zone sismiche e per la formazione e l’aggiornamento degli elenchi delle medesime zone. G.U. n. 108, 11 maggio 2006 (in Italian).

Samela C., Masi A., Chiauzzi L., Tosco L. and Vona M.; 2009: Analisi delle caratteristiche tipologiche e valutazionedella vulnerabilità sismica del patrimonio edilizio privato della regione Basilicata. In: Atti del XIII CongressoNazionale L’Ingegneria Sismica in Italia, Bologna (in Italian).

SAP2000 Advanced I 10.0.7; 2006: Structural Analysis Program, 2006. Computer and Structures Inc., University Ave.,Berkeley CA, U.S.A.

Corresponding author: Vincenzo ManfrediUniversità degli Studi della BasilicataVia dell’Ateneo Lucano, 85100 Potenza, ItalyPhone: +39 0971 205095; fax: +39 0971 205070; e-mail: [email protected]

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Seismic assessment of existing RC buildings based …Seismic assessment of existing RC buildings based on different hazard maps Boll. Geof.Teor.Appl.,52, 000-000 the municipalities