M. Eng. Velyan Petkov The European Center for Risk Prevention, Sofia, Bulgaria

ATHENS 12th of April, 2013 1

M. Eng. Velyan PetkovM. Eng. Velyan Petkov

The European Center for Risk Prevention, Sofia, The European Center for Risk Prevention, Sofia, BulgariaBulgaria

Bulgarian seismic design codes and Bulgarian seismic design codes and civil construction practice. Infilled RC civil construction practice. Infilled RC frames - Application and assessment of frames - Application and assessment of

their resistance to lateral forcestheir resistance to lateral forces

THE EUROPEAN CENTER FOR RISK PREVENTION, SOFIA, BULGARIA

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1.INTRODUCTION1.INTRODUCTION•Eurocode 8 and Eurocode 8 Part 3 in BulgariaEurocode 8 and Eurocode 8 Part 3 in Bulgaria

•Assessmen and retrofit programs in BulgariaAssessmen and retrofit programs in Bulgaria

•Type of existing buildings structures.Type of existing buildings structures.

- large panel systems- large panel systems

- masonry and masonry infilled structures- masonry and masonry infilled structures

ATHENS 12th of April, 2013

M.Eng. Velyan PetkovM.Eng. Velyan Petkov

The European Center for Risk Prevention, Sofia, BulgariaThe European Center for Risk Prevention, Sofia, Bulgaria

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2.Bulgarian Seismic Codes and Eurocode 8 (Eurocode 8-3)2.Bulgarian Seismic Codes and Eurocode 8 (Eurocode 8-3)• Pre 1947 – No Seismic Design Code

•1947 – First Seismic Design Requirements

•1957 – First Seismic Design Code – Low Level Seismic Design Codes

•1964 – New Seismic Design Code – Medium Level Seismic Design Codes - Introduction of Dynamic factors

•1987 – Modern Bulgarian Seismic Design Code and preceding events (1977 Vrancea Earthquake and 1987 Strazhica Earthquake)

•2007, 2009 and 2012 Editions

•2012 - Introduction of Eurocode 8

•2012 – 2014 – Joint Application of Codes

•Post 2014 – Planned retirement of Bulgarian Seismic Code




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Type of Buildings, classified by the reference Seismic Design Code at the time of Type of Buildings, classified by the reference Seismic Design Code at the time of design and constructiondesign and construction

• Pre 1947 Construction

• 1947-1957 Construction

• 1957-1964 Construction

• 1964-1987 Construction – Medium and high rise residential buildings. Large Panel Systems, Lift Slab method, prestressed reinforced concrete buildings, climbing shuttering RC system, and others. Mass construction period.

• 1987 Seismic Design Code

- 1990s period

- 2000-present

Comparison between the codesComparison between the codes

Seismic Assessment of buildings designed according to Bulgarian codesSeismic Assessment of buildings designed according to Bulgarian codes




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3.Pernik Earthquake 20123.Pernik Earthquake 2012




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3.Pernik Earthquake 2012 – Pernik Region – Undamaged buildings3.Pernik Earthquake 2012 – Pernik Region – Undamaged buildings




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3.Pernik Earthquake 2012 – Pernik Region –Damaged buildings3.Pernik Earthquake 2012 – Pernik Region –Damaged buildings




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3.Pernik Earthquake 2012 – Sofia region – Elastic Spectrum soil type “C”3.Pernik Earthquake 2012 – Sofia region – Elastic Spectrum soil type “C”




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3.Pernik Earthquake 2012 – Sofia region – Damages3.Pernik Earthquake 2012 – Sofia region – Damages




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3.Pernik Earthquake 2012 – Sofia region – Damages3.Pernik Earthquake 2012 – Sofia region – Damages




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4.Assessment of masonry-infilled reinforced concrete frames and buildings• Masonry together with reinforced concrete is the most widely used material in Bulgarian cuvil practice.• In Bulgaria masonry is used as:

• Structural material• Non-structural materialUntil now the effects of the infill on the behavior of the structures were usually

neglected in seismic analysis of RC/masonry buildings.

4.1 Masonry-infills in Bulgarian civil construction practice•Use of masonry as primary (structural) material in combination with wood, steel etc.•Use of masonry as primary (structural) material in combination with RC.

• 50s 60s – application in low rise buildings with up to 4-5stories (usually 3-4)

• 60s – present – use in low rise residential houses up to 2(max 3) stories high•Use of masonry as nonstructural material

• 60s – present – application in low, middle and high rise buildings with RC as primary structural material (usually RC shear walls are used)

•Positives of the early designs:• Usually regular buldings• Use of bricks with no openings




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• Quality control of the masonry• Conservative design• Usually low height• High safety against premature out of plane failure of the masonry

• Deficiencies of the early designs:• Designed according to low-level Design codes and usually lower seismic

design acceleration• Poor detailing – lack of enough transverse rebar• Use of smooth rebar steel rods.• Low-strength concrete• Unknown quality of the mortar• Possible degradation of strength due to removed internal walls.• Brittle failure is expected

• Positives of later designs• Designed according to better Design codes• Better detailing• Use of textured rebar steel rods (especially in the last decades)• High-strength concrete.




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• Neglecting the strength of infill is often on the safe side• Better performance might be expected (especially in buildings designed

according to 1987 Seismic Design Code)

• Deficiency of late designs • Use of bricks with high percentage of openings• Less conservative designs• Neglecting the stiffness of infill might be dangerous• Low quality control of the masonry construction due to the understanding

that it is not important for the overall behavior of the structure.• Design and construction of irregular structures (especially after 1990)• 1990s period when overall control was not strict enough• Possible premature collapse of internal masonry walls (due to lack of

connection between the frame and the masonry)




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4.2 Assessment of the behavior of in-filled RC frames •Importance•Lack of modeling techniques, prescribed in Seismic codes (either Bulgarian or Eurocode 8)•Types of infill models:

• Macromodels• Bare frame• Single-strut• Double Strut• Multi-Strut• EQUIVALENT WIDTH OF THE STRUT




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• Micromodels




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• Comparison of Analysis Results




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• Nonlinear Strut Models• Importance and field of application• Data input requirements




SEISMOSTRUCT Software Package nonlinear model

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4.3 Application of infill modeling in assessment and retrofit•Linear Strut Models




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4.3 Application of infill modeling in assessment and retrofit•Nonlinear Strut Models




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4.3 Application of infill modeling in assessment and retrofit•Nonlinear Strut Models – Capacity Curves – Base Shear vs. Top displacement




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4.3 Application of infill modeling in assessment and retrofit•Nonlinear Strut Models – Strengthening of existing structures




• Nonlinear Micromodels

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CONCLUSIONS1.BDS EN 1998-3:2005 Assessment and Retrofitting of Buidings is the first complete structural code of this type in Bulgaria.

2.Contradictions between the old and the new code exist- Completely different principles- Different definition of “seismically resistant structure”

3.2012 Pernik Earthquake conclusions:- The event in Pernik may be classified as a strong one (ref. 1987 code)- The event in Pernik may be classified as moderate or even low- Almost all buildings properly designed and constructed withstood the

earthquake without severe damages.- Many of the Buildings that were constructed without proper designs and

in contradiction to “good construction practices” were heavily damaged.- Some deficiencies of old local codes designs were detected.- Danger of falling non-structural elements of buildings during an

earthquake exists.

4.Masonry-infills influence the behavior of structures.




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CONCLUSIONS5. Modeling of masonry requires a lot of information and knowledge. 6. Introduction of national application handbooks is required.

7. Many of the buildings in Bulgaria may be rated as not seismically resistant if checked in accordance to BDS EN 1998-3:2005

8. But the last doesn’t mean that they really aren’t.




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TNANK YOU FOR YOUR ATTENTIONTNANK YOU FOR YOUR ATTENTION

M. Eng. Velyan Petkov The European Center for Risk Prevention, Sofia, Bulgaria

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