SURVEY UPON THE SEISMIC RESISTANCE

OF RC STRUCTURES

Calin MIRCEA1,2

ABSTRACT

The paper presents results and conclusions of more than a decade of investigations

concerning the safety against earthquakes of buildings with RC structures in

Romania. Investigations were done according to the evaluation procedure given

by the Romanian code of practice for seismic evaluation. After a general

introduction about the earthquakes threat for buildings and human life, the paper

continues with the seismic activity background in Romania.

Next sections reveal general information about the evaluation procedures and

methods used in buildings assessment. Results are presented in a synthetic manner

for the different structural solutions considered, based on the nominal grade of

safety against earthquakes and other factors considered.

The paper ends with conclusions and commentaries about seismic behavior and

safety against earthquakes induced progressive collapse of common structural

types.

1. INTRODUCTION

The paper presents a brief synthesis of the information gathered within an

investigation performed in more than ten years. The investigation consists in the

assessment of the seismic behavior and safety of RC structures (of basic structural

types) built in Romania after the Second World War.

Earthquakes are caused by the sudden crushing slippage between adjacent crusts of

the earth. Consequently, the released energy is consumed through ground motions

that propagate in the surrounding ground in all directions. A significant earthquake

takes place with the release of an enormous quantity of energy (e.g., 1019-10

26 ergs)

and lasts tenths of seconds. Ground motion shakes buildings and acts upon them

through the dynamic alternant inertia forces that occur due to the different

vibration properties of the structure, foundation system and surrounding soil.

Under these inertial forces, the structure is deformed mostly by strong horizontal

displacement and drifts, which generate important shearing forces on its members

and joints. Thus, collapse of buildings or building components fall can occur, life

being threatened. Failure mechanism is a shear-based, usually preceded by

distortional effects. _____________________________________________________________________________

1 Technical University of Cluj-Napoca, 400020 Cluj-Napoca, 15 C. Daicoviciu Street, Romania;

E-mail: calin.mircea@bmt.utcluj.ro 2 National Building Research Institute (INCERC) Cluj-Napoca Branch, 400524 Cluj-Napoca,

117 Calea Floresti, Romania; E-mail: calin.mircea@incerc-cluj.ro

Table 1: Earthquakes Richter magnitude in equivalent TNT explosive

Richter

magnitude

TNT equivalent energy yield

[tons]

Richter

magnitude

TNT equivalent energy yield

[tons]

4.0 103

7.0 32×106

4.5 5.1×103 7.5 160×10

6

5.0 32×103 8.0 10

9

5.5 80×103 8.5 5×10

9

6.0 106

9.0 32×109

6.5 5×106 10.0 10

12

Table 2: Effects considering modified Mercalli intensity scale

Intensity Effects

1 People do not feel any earth movement.

2 A few people might notice movement if they are at rest and/or on the upper floors

of tall buildings.

3 Many people indoors feel movement. Hanging objects swing back and forth.

People outdoors might not realize that an earthquake is occurring.

4

Most people indoors feel movement. Hanging objects swing. Dishes, windows,

and doors rattle. The earthquake feels like a heavy truck hitting the walls. A few

people outdoors may feel movement. Parked cars rock.

5

Almost everyone feels movement. Sleeping people are awakened. Doors swing

open or close. Dishes are broken. Pictures on the wall move. Small objects move

or are turned over. Trees might shake. Liquids might spill out of open containers.

6

Everyone feels movement. People have trouble walking. Objects fall from shelves.

Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and

bushes shake. Damage is slight in poorly built buildings. No structural damage.

7

People have difficulty standing. Drivers feel their cars shaking. Some furniture

breaks. Loose bricks fall from buildings. Damage is slight to moderate in well-

built buildings; considerable in poorly built buildings.

8

Drivers have trouble steering. Houses that are not bolted down might shift on their

foundations. Tall structures such as towers and chimneys might twist and fall.

Well-built buildings suffer slight damage. Poorly built structures suffer severe

damage. Tree branches break. Hillsides might crack if the ground is wet. Water

levels in wells might change.

9

Well-built buildings suffer considerable damage. Houses that are not bolted down

move off their foundations. Some underground pipes are broken. The ground

cracks. Reservoirs suffer serious damage.

10

Most buildings and their foundations are destroyed. Some bridges are destroyed.

Dams are seriously damaged. Large landslides occur. Water is thrown on the

banks of canals, rivers, lakes. The ground cracks in large areas. Railroad tracks are

bent slightly.

11 Most buildings collapse. Some bridges are destroyed. Large cracks appear in the

ground. Underground pipelines are destroyed. Railroad tracks are badly bent.

12 Almost everything is destroyed. Objects are thrown into the air. The ground

moves in waves or ripples. Large amounts of rock may move.

The magnitude Richter scale is used to express the earthquake released energy by

each earthquake through the relationship:

MES 5.18.11log += (1)

where ES [ergs] is the amount of energy radiated from the earthquake as seismic

waves, and M is the magnitude of the earthquake. Table 1 shows the seismic wave

energy for several Richter magnitudes of significant earthquakes, related to the

quantities of the explosive TNT (one kilogram of TNT exploded below ground

yields 310 million ergs of seismic wave energy).

Each earthquake has a unique magnitude, but its effects will vary greatly according

to distance, ground conditions, construction standards, and other factors.

Therefore, the intensity scale differs from the Richter magnitude scale in that the

effects of any one earthquake vary greatly from place to place, so there may be

many intensity values measured from one earthquake. Thus, the Mercalli intensity

scale expresses better the variable effects of an earthquake. According to Federal

Emergency Management Agency [FEMA Report 226, 1992], effects on the

modified Mercalli intensity scale are given in Table 2.

2. BACKGROUND OF ROMANIAN SEISMIC ACTIVITY

Romania has a significant seismic history. Vrancea land is far the most active

epicenter location in Romania and one of the most actives in Europe. Two

earthquake types are specific to this area: crustal low depth (i.e., less than 60 km)

earthquakes that leads to relatively small magnitudes (maximum 5.2 magnitude)

and sub-crustal deep (i.e., depths between 60 km and 220 km) earthquakes,

reaching magnitudes of 7.8-8.0. Table 3 presents the most significant earthquakes

of this epicenter region occurred within the last 200 years. Other epicenter regions

for crustal earthquakes are Banat (i.e., the most severe location after Vrancea),

Făgăraş-Câmpulung, Crişana, Maramureş, Transylvania and Dobrogea, where rare

superficial earthquakes (i.e., epicenter depths between 5 km and 35 km) occur.

These are characterized by lower magnitudes and are less extended, having a local

character. As shown previously, Romania presents a severe seismic activity.

Depending by the composition of the geological layers, each zone from Romania

reacts differently to an earthquake. Figure 1 shows the general map of the seismic

intensity on the Romanian territory. However, larger intensities may be found in

micro-regions, where the ground stratification varies significant on small areas.

Table 3: Important earthquakes in Vrancea epicenter

Date Magnitude

26 of October 1802 7.9

26 of November 1829 7.3

23 of January 1838 7.5

31 of August 1894 7.1

6 of October 1908 7.1

10 of November 1940 7.6

4 of March 1977 7.4

31 of August 1986 7.1

Figure 1: Seismic map of Romania on the modified Mercalli intensity scale

3. GENERAL PRINCIPLES USED IN SEISMIC ASSESMENT

According to the Romanian actual seismic design code [P-100/92, 1992], exposure

of buildings in respect with earthquakes is assessed for classes of seismic risk, as

shown in Table 4.

Table 4: Seismic risk classes

Seismic risk Description

RsI Constructions with high risk failure to earthquakes with design intensities

RsII Constructions presenting a lower failure probability, but major structural

degradations are expected under design earthquake

RsIII

Constructions with no significant degradations in respect with safety

against design earthquake, but important damages of non-structural

elements are expected

RsIV Constructions with a seismic response similar to the one expected

according to the actual design earthquake

Anti-seismic structural engineering is not an exact discipline. There are a lot of

uncertain factors that can argue with the design predicted behaviour to a design

earthquake. Therefore, many conservative assumptions are considered. For the

buildings under investigation, the criteria considered when determining the seismic

risk classes are detailed below.

Type of the structural system and anti-seismic conformation. The paper refers

forward to the following categories: planar and spatial frame structures, RC

bearing walls structures, RC hybrid structures (i.e., bearing frames and shear

walls). Structural systems and buildings conformation were assessed in respect

with the requirements of performance-based design mentioned by the European

code [Eurocode 8, 1998]:

� Structural simplicity presumes structural continuity and an uninterrupted flow

of the inertia forces into the foundation system. These are gathered by the floor

systems through the horizontal diaphragm effect (i.e., in plane action) and

distributed to the vertical members of the structure.

� Structural redundancy provides safety against progressive collapse through the

efficient redistribution of the internal forces and ensures an energy dissipation

mechanism by developing sufficient plastic hinges.

� Structural robustness ensures separation of collapsing areas by mobilizing the

ultimate capacity of the structural members placed within the proximity of critical

elements in respect with progressive collapse initiation.

� Adequate geometrical configuration in horizontal plan provides twisting

stiffness and structural strength, while a relatively monotonic vertical development

ensures a favourable distribution of the masses with a small eccentricity between

the rigidity and mass centres.

� Strength and rigidity on the principal directions of the structure guarantees the

sustaining of the arbitrary earthquake induced forces and limited displacements

(global and relative). This latter limitation is very important to avoid the weak

floor mechanism, to control the interaction of the structure with non-structural

elements and to avoid cascade failures caused by the impact between adjacent

structures.

Nominal grade of safety against earthquakes – R, and presence of weak zones in

relation with the resistance capacity. The value of this grade provided the most

important criterion in assessing the buildings vulnerability to earthquakes. Values

were calculated with the following relation:

d

R

S

SR = (2)

where SR is the resistant base shear force of the building or structural member, and

Sd is the design value of the base shear force of the building or structural member,

calculated according to the actual seismic design code [P-100/92, 1992]. The

minimum values recommended by the code are given in Table 5, in relation with

the class of the buildings importance. Values related to the structural members

were calculated for the identification of the weak collapsing areas.

Table 5: Minimum values recommended for the nominal grade of

safety against earthquakes - Rmin

Importance class

of the building* I II III IV

Rmin 0.70 0.60 0.50 0.50

*Note: - class I: vital buildings that have to remain in full service during and

after earthquake;

- class II: essential buildings with severe limits for damages;

- class III: ordinary buildings;

- class IV: buildings of low importance.

Probable nature of failure of the structural members vital for the stability of

structure. Critical structural members and their associated failure mode were

identified through aspect ratios and the values of the dimensionless parameters

corresponding to the internal forces:

cdc

Ed

cdc

Ed

ctdc

Ed

dfA

M, m

fA

N, n

fA

Vv === (3)

with

VEd - design value of the applied shear force at the ultimate limit state;

Ac - gross cross-sectional area of the structural member;

fctd - design value of concrete tensile strength;

NEd - design value of the applied axial force;

fcd - design value of concrete cylinder compressive strength;

MEd - design value of the applied internal bending moment;

d - effective depth of a cross-section.

Age of the building and function history. This information was very useful

especially when evaluating old buildings. Due to the lack of design drawings, the

data provided indicators concerning the building technology and eventual

associated hidden defects, quality of materials, existing and/or repaired damages

and defects etc. Also, depending by the age of the building and its service

conditions (e.g., some old buildings were declared as patrimony monuments), more

or less conservative assumptions were taken into account when calculating the

nominal grade of safety against earthquakes.

Ground conditions. Buildings on rock survive earthquakes much better than those

with foundations on soft or unstable soil. Soft ground shaken by the rock below

vibrates like jelly, amplifying the seismic motion in the rock, and resulting in

greater distortions and forces in the building. Soft ground may also be unstable,

and can liquefy (like quicksand) or slide during an earthquake, resulting in large

ground distortions and severe damage to the building. Where poor ground

conditions were identified, a lower risk seismic class was adopted even if

theoretical values of the nominal grade of safety against earthquakes were

acceptable.

Time history of the structural system and the actual technical state. Information

gathered in relation with the previous earthquakes (less or more severe) sustained

by the structure and the eventual damages/repairs that affected the structures

during service, completed with the information collected by visual inspections

concerning the actual technical state, provided the background for the choice of the

investigation methods and represented an indicator of the structural behavior under

earthquakes.

Height regime and mass of the building. Vertical development and distribution of

masses is very important for the structural integrity. In this manner were

emphasized weak areas (e.g., slender floors, uneven vertical development, heavy

equipment masses etc.) that usually engage higher vibration modes, not considered

in simplified structural analyses performed in current practice. These can lead easy

to the initiation of the progressive collapse and obviously increases the

vulnerability to earthquakes.

4. SCREENING BUILDINGS FOR SEISMIC EVALUATION

Three methods of investigation were used in the assessment of the buildings and

their seismic risk.

Rapid method: The method is based on a visual inspection of each building and its

examination of its drawings. A score (similar to the fast assessments used in bridge

inspections [Mircea at all, 2005]) was given for each building based on the

following seismic risk factors:

� Seismicity;

� Ground conditions;

� Type and age of construction (both of which influence integrity, strength and

ductility);

� Building irregularities on horizontal and vertical directions;

� Use (e.g., hospital, office, apartment building or industrial building) and service

conditions are important factors for the evolution of the global and local technical

state;

� Presence of heavy or dangerous nonstructural building components, which may

fall, or building services lines and equipment, which may fail;

� Environmental conditions (see Figure 2), global and elementary technical state,

which affect, together with the service conditions, the durability and consequently

the evolution of the seismic risk.

Ukraine

Hungary

Moldovia

Bulgaria

Black

Sea

Serbia

Strong agresivity Medium agresivity

Figure 2: Map of environment agresivity in Romania

Testing method: Where necessary, non-destructive testing of concrete through the

combined method (i.e., Schmidt hammer method and ultrasound method) and

semi-destructive method (i.e., laboratory testing of compressive strength on

cylinder concrete samples). Rebar locators were used to locate the both

longitudinal and transversal reinforcement, to identify its orientation, to find out

the depth of the cover layer and rebar size.

Analytical method: Computer aided structural analyses were performed, on the

ground of more or less simplified assumptions (e.g., elastic structural analyses with

or without redistribution of the internal forces, post-elastic structural analyses), in

order to analyze structural behavior and calculate the nominal safety grade against

earthquakes.

5. RESULTS

This section presents results obtained on 43 buildings with various functionalities

and structural type (see Figure 3). Most of all were apartment buildings (19

buildings with height regime from for to ten floors), 9 industrial halls and 15 with

other functions (e.g., administrative, hospital, school etc.). Buildings were given in

current service between 1967 and 1981.

a. bearing walls structures

x

y

yG=10.49

yCR=10.38

x G=9.75

xCR=9.5

b. frame structure

D

B

G

H

J

E

A

C

F

G

I

J

2 3 5 7

2 3 4 6 8

x

y

c. hybrid structrues

Figure 3: Typical investigated structures

Nominal grades of safety against earthquakes resulted from structural analyses, the

corrected ones due to external factors or doubts, are summarized in Table 6

together with seismic risk classes and complementary data.

Table 6: Synthesis of seismic assessment

R

analysis effective

No. of

buildings

Structural

type*

Importance

class

Richter

magnitude

Mercalli

intensity

Seismic

risk

0.97-1.12 0.97-1.12 8 A III 6.5-7 6-7 RsIV

0.67 0.50 1 A III 7 7 RsI

1.38-2.07 1.35-2.07 9 B III 6.5-7 6-7 RsIV

1.32-1.74 1.32-1.74 6 B III 7.5 8 RsIV

1.03-1.12 1.03-1.12 3 B III 6-6.5 6-7 RsIV

0.87 0.60 1 B III 7 8 RsII

1.84-2.45 1.84-2.45 3 B II 6 6-7 RsIV

1.88-2.87 1.88-2.87 7 C II 6-6.5 6-7 RsIV

1.43-1.62 1.43-1.62 3 C III 6-6.5 6-7 RsIV

1.74-2.62 1.74-2.62 2 C II 7.5 8 RsIV *Note: A - frame structure;

B - bearing and stiffening walls structure;

C - hybrid structure.

As shown in table above, in two cases the theoretical nominal grades of safety

against earthquakes were adjusted to lower values. The first case is about an

industrial hall with a RC frame structure, presenting many corrosion visible signs

and a chloride service environment. Even if structural analysis was performed after

in situ and laboratory testing, it was considered that results are informative and

corrosion of steel reinforcement and concrete depreciation might be in a more

advanced state than the one identified through tests. In the second case, an

apartment building suffered several unathorized wall removals at the ground floor,

which could affect the buckling behavior of the members in the proximity areas,

aspect that was not considered in the structural analysis.

As results reveal, frame structures have the lower nominal grades of safety against

earthquakes, while bearing wall structures and hybrid structures have large strength

reserves. A mention should be done for the case of prefabricated wall-panels

buildings, which have the higher risk to present hidden defects due to the

pretentious horizontal and vertical shear joints. Seven such buildings were

investigated, and considering that no signs occurred after previous earthquakes, the

seismic risk was considered the one established with the results of the structural

analyses.

6. CONCLUDING REMARKS

Frame structures are flexible and not appropriate for tall buildings unless

supplementary stiffening is provided. Buildings with planar bearing frames

disposed transversally or longitudinally, stiffened by normal frames undertaking

only horizontal actions, have collapse vulnerability of the planar bearing frames,

but failure may be limited to the adjacent bays. This solution appears suitable for

buildings with a limited number of floors, the structural damage being restricted

and consequently progressive collapse too. Spatial frame structures have more

redundancy and robustness potential than planar frames, but the lack of a column

increases spectacular bending and shear actions acting on the two beam segments

adjacent to the missing column in each direction. Seismic detailing (see Figure 4.a)

of the members and connections can reduce significant the damages [Corley at all,

2002].

a. earthquake resistant design b. design for gravity load

Figure 4: Missing member behavior of dual frames

and ordinary moment frames

Frame structures require supplementary lateral rigidity at tall buildings, which

usually is provided by shear walls and/or vertical bracings. However, a more

redundant hybrid structure has to consider distribution of the reinforcement both at

the top and at bottom of the frame beams, as shown in Figure 4. Federal

Emergency Management Agency [FEMA Report 277, 1996] shows that the

supplementary costs of earthquake resistant design instead of traditional gravity

resistant design are just 1-2 % of the entire cost of the building. Nevertheless, as

shown by the author [Mircea, 2006], redundancy and robustness can be also

ensured by incorporating redundant facilities, components and paths into the

structural system (e.g., the implementation of tie action as revealed in Figure 5) in

various configurations.

a. cables disposed inside columns section b. outside disposed cables

Figure 5: Tie action of cables activated by vertical members removal

Often after 1965, functional requirements lead to the free floor architectural

concept. In the case of RC frame structures (see Figure 6), this concept easily

results in a weak floor mechanism and lack of structural integrity during

earthquakes. Thus, an initial collapse story can propagate to the scale of the entire

structure.

Figure 6: Free ground floor of a RC frame structure

Since the middle of the years 1970, this solution was replaced with much more

stable ones. A continuous rigid girder on V shaped columns (see Figure 7.a and

7.b) ensures a more natural flow of the loads and provides sufficient lateral

stiffness. Even in the case of a missing member, the bearing capacity of the girder

is sufficient if the remaining columns have enough strength potential and buckling

resistance. The spatial extent of the solution is even more redundant and robust.

Another approach consists in the compensation of the lateral stiffness using truss

systems, as shown in Figure 7.c. Even if very rigid, this solution is less redundant

than the previous ones. The loss of a compressed strut or tensioned tie generates a

local mechanism that can easily lead to local instability and failure, progressive

collapse being more likely to initiate.

a. pine ended slender V columns b. rigid V columns c. truss system

Figure 7: Redundant and robust free ground floor concepts

REFERENCES

[1] FEMA Report 226, Mitigation Strategies and Techniques Infrastructure Design

Building Design Risk Assessment, Feburary 1992, 110 pp.

[2] P100-92, Seismic Design Code for Buildings (in Romanian), Bucharest, 1992,

152 pp.

[3] Eurocode 8 – EN 1998, Design of structures for earthquake resistance.

[4] Mircea C., Filip M., Ciocoi E. and Cionca S., Condition Survey of Romanian

Prestressed Concrete Bridges, Proceedings of International Conference held at

the University of Dundee, vol. “Repair and Renovation of Concrete

Structures”, Dundee – UK, 5-6 July 2005, pp. 25-30.

[5] Corley W.G., Applicability of Seismic Design in Mitigating Progressive

Collapse, NIST WORKSHOP – Jul 10-11, 2002, 13 pp.

[6] FEMA Report 277, The Oklahoma City Bombing: Improving Building

Performance Through Multi-Hazard Mitigation, Federal Emergency

Management Agency, Aug. 1996, 98 pp.

[7] Mircea C., Risk factors in the Redundancy and Robustness of RC Structures

Subjected to Blasts and Earthquakes, Proceedings of Concrete Solutions –

Second International Conference on Concrete Repair, St. Malo-France, 27-29

June 2006, pp. 782-792.