Report #: 15

Report Date:

Country: GREECE

Housing Type:

Housing Sub-Type:

Author(s): T. P. Tassios, Kostas Syrmakezis

Last Updated:

Regions Where Found: Buildings of this construction type can be found in the main cities of the country, at an estimated percentage of 30% on the entire housing stock. This type of housing construction is commonly found in urban areas.

Summary: These buildings represent a typical multi-family residential construction, mainly found in the suburbs of Greek cities. This housing type is very common and it constitutes approximately 30% on the entire housing stock in Greece.Buildings are generally medium-rise, typically 4 to 5 stories high. The main lateral load-resisting structure is a dual system, consisting of reinforced concrete columns and shear walls. A relatively small-size reinforced concrete core usually exists, serving as an elevator shaft. The roof and floor structures consist of rigid concrete slabs supported by the beams. Seismic performance of these buildings is generally good, provided that the seismic design takes into account the soft ground floor effects e.g. by installing strong RC shear walls. Failure of the soft ground floor is the most common type of damage for this type of structure. Some buildings of this type were damaged in the 1999 Athens earthquake.

Length of time practiced: Less than 25 years

Still Practiced: Yes

In practice as of:

Building Occupancy: Residential, 10-19 units

Typical number of stories: 4-6

Terrain-Flat: Typically

Terrain-Sloped: Typically

Comments:

Plan Shape: Rectangular, solid

Additional comments on plan shape:

Typical plan length (meters): 10

Typical plan width (meters): 15

Typical story height (meters): 3

Type of Structural System: Structural Concrete: Moment Resisting Frame: Dual system Frame with shear wall

Additional comments on structural system: The gravity load-bearing structure consists of RC solid slabs, transferring the gravity loads to the beams and columns and finally to the footings.

The main lateral load-resisting system consists of reinforced concrete shear walls. The stiffness of brick infill walls is generally not considered in the design, however self-weight of brick walls is taken into account. The lateral drift of the structure is governed by the stiffness of its columns and walls. The 3-D response of the frame under earthquake actions is strongly affected by the column and wall layout. The walls located at the perimeter of the building in both directions contribute to minimizing the torsional effects. Floor slabs behave as diaphragms during a seismic event.

Gravity load-bearing & lateral load-resisting systems:

Typical wall densities in direction 1: 4-5%

Typical wall densities in direction 2: 4-5%

Additional comments on typical wall densities: The typical structural wall density is up to 5 %.

Total wall area/plan area (for each floor) 3-4%.

Wall Openings: Such a building has 12-15 openings per floor, of an average size of 3.0 m.sq. Estimated percentage of opening area to the total wall surface is 25%. Infill walls are generally not considered in the design.

Is it typical for buildings of this type to have common walls with adjacent buildings?: No

Modifications of buildings: Usually demolition of interior infill walls.

Type of Foundation: Shallow Foundation: Reinforced concrete isolated footing

Additional comments on foundation:

Type of Floor System: Other floor system

Additional comments on floor system: Structural concrete: cast-in-place and precast solid slabs

Type of Roof System: Roof system, other

Additional comments on roof system: Structural concrete: cast-in-place and precast solid slabs

Additional comments section 2: When separated from adjacent buildings, the typical distance from a neighboring building is 10 meters.

Who is involved with the design process?: EngineerArchitect

Roles of those involved in the design process: Architects are responsible for architectural drawings and civil engineers for the structural design.

Expertise of those involved in the design process: Structural Engineer (five years University studies and minimum 5 years experience).

Who typically builds this construction type?: Other

Roles of those involved in the building process: These buildings are usually built by developers.

Expertise of those involved in building process: Experienced professionals for the construction. Occasional low quality construction is observed.

Construction process and phasing: Developers are usually builders of this type of construction. Ready-mixed concrete is usually used. Concrete pumps and concrete vibrators are used in situ. The construction of this type of housing takes place in a single phase. Typically, the building is originally designed for its final constructed size.

Construction issues

Is this construction type address by codes/standards?: Yes

Applicable codes or standards: Greek Code for Earthquake Resistant Design (NEAK) Greek Code for Earthquake Resistant Design (NEAK), Athens 1995. Greek Code for Reinforced Concrete Design (NKOS), Athens 1995.

Process for building code enforcement: Building design must follow the National Building Code and EuroCodes.

Are building permits required?: Yes

Is this typically informal construction?: No

Is this construction typically authorized as per development control rules?: Yes

Additional comments on building permits and development control rules:

Typical problems associated with this type of construction: Special attention is due to the construction of joints and reinforcement detailing. Uniform distribution of over strength throughout the building elevation is not always achieved.

Who typically maintains buildings of this type?: Owner(s)Renter(s)

Additional comments on maintenance and building condition:

Unit construction cost: 250000 DRA/m.sq. (600 US$/m.sq.)

Labor requirements: 1 month per floor 50 man-months per floor

Additional comments section 3:

Damage patterns observed in past earthquakes for this construction type: On September 7, 1999, at 14:56 local time, a strong earthquake occurred 18 km northwest of the center of Athens. The earthquake was magnitude .s =5.9 and the coordinates of the epicentre were located at 38.12.-23.64., in the area of Parnitha mountain. This earthquake came as a surprise, since no seismic activity was recorded in this region for the last 200 years. According to strong-motion recordings, the range of significant frequencies is approximately 1.5-10 Hz, while the range of the horizontal peak ground accelerations were between 0.04 to 0.36g. The most heavily damaged areas lie within a 15 km radius from the epicentre. The consequences of the earthquake were significant: 143 people died and more than 700 were injured. The structural damage was also significant, since 2,700 buildings were destroyed or were damaged beyond the repair and another 35,000 buildings experienced repairable damage.

According to the EERI Reconnaissance Report, a number of RC buildings sustained severe structural damage and some of them collapsed, totally or partially. Most of the severely damaged structures were designed according to older seismic codes, with significantly lower seismic forces than those experienced during the earthquake. The overall behavior of RC structures was satisfactory.Some of the recorded ground accelerations show elastic spectral accelerations on the order of 0.6 to 0.8 g for structures with periods in the range of 0.15 to 0.3 sec, corresponding to two- to five-story buildings in Athens. Most of these buildings were designed according to the old code, with about ten times lower seismic forces. This factor is expected to be significantly higher in the epicentral area, where the effective ground acceleration should have exceeded the value of 0.5 g. The majority of the RC structures in the broader area of Athens suffered only minor structural damage because they had strength reserves such as infill walls, over-strength and redundancy.

Additional comments on earthquake damage patterns: Cracking in shear walls of the elevator shaft (1999 Athens earthquake), see Figure 9.

Joint failure in poorly constructed structures. Damage to column-beam joints due to bad concrete quality and insufficient reinforcement was observed in the 1999 Athens earthquake (EERI). In many cases, stirrup reinforcement was almost nonexistent (see Figures 7 and 8).

Soft ground floor (where there is an absence of infill walls at the ground floor) may cause damage, leading to the development of collapse mechanisms. In the 1999 Athens earthquake, the damage occurred mainly to the joints, which were totally destroyed in a number of cases. As a result, the structural system became a mechanism, and large permanent horizontal displacements were observed. In some cases, collapse of the soft story was occasioned by P-d effect, combined with high vertical accelerations. (EERI)

The main reference publication used in developing the statements used in this table is FEMA 310 Handbook for the Seismic Evaluation of Buildings-A Pre-standard, Federal Emergency Management Agency, Washington, D.C., 1998.

The total width of door and window openings in a wall is: For brick masonry construction in cement mortar : less than ½ of the distance between the adjacent cross walls; For adobe masonry, stone masonry and brick masonry in mud mortar: less than 1/3 of the distance between the adjacent cross walls; For precast concrete wall structures: less than 3/4 of the length of a perimeter wall.

Additional comments on structural and architectural features for seismic resistance: Building configuration - buildings of this type are considered to be regular in elevation due to the uniform column and wall sections thoughout the building height. According to the Code, it is not acceptable to have stiffeness variation of over 30%.

Vertical irregularities typically found in this construction type: Other

Horizontal irregularities typically found in this construction type: Other

Seismic deficiency in walls: Clay brick infill with low tensile strength. Nonuniform wall distribution (in elevation or in plan) may create problems related to seismic performance.

Earthquake-resilient features in walls: The presence of minimum RC shear walls (a Code requirement) led to an improved structural performance

Seismic deficiency in frames: #NAME?

Earthquake-resilient features in frame: -Capacity design of beam-column joints ensures ductile behavior of the structure

-Good seismic performance on condition of careful detailing during design and construction after the application of the 1985 Code.

Earthquake resilient features in roof and floors: Rigid diaphragms (insignificant relative in-plane displacements).

Other seismic deficiencies: Soft story at the ground floor level. Buildings with a soft ground floor are a common practice in Greece. Significantly less rigidity in this floor, compared to the rest of the building, leads to large deformations of the soft story. (EERI)

Other earthquake-resilient features: In the 1999 Athens earthquake, the soft-story effect was more pronounced in buildings without shear walls (EERI).

For information about how seismic vulnerability ratings were selected see the Seismic Vulnerability Guidelines

Additional comments on seismic strengthening provisions: Strengthening of damaged concrete columns using the reinforced concrete jackets was used in Greece after the 1981 Athens earthquake. More details on this technique can be found in UNIDO (1983).

Has seismic strengthening described in the above table been performed?: Yes, to a great extent.

Was the work done as a mitigation effort on an undamaged building or as a repair following earthquake damages?: Repair following the earthquake damage.

Was the construction inspected in the same manner as new construction?: Yes

Who performed the construction: a contractor or owner/user? Was an architect or engineer involved?: The construction was peformed by a contractor, with the involvement - supervision of an architect and a civil engineer.

What has been the performance of retrofitted buildings of this type in subsequent earthquakes?: The performance was satisfactory.

ITSAK. Report on the 1999 Athens Earthquake, Institute of Engineering Seismology and Earthquake Engineering, Thessaloniki, Greece (www.itsak.gr)

EERI. Special Earthquake Report: The Athens, Greece Earthquake of September 7, 1999 (www.eeri.org/earthquakes/Reconn/Greece1099/Greece1099.html)

EQE. September 7, 1999 Athens, Greece Earthquake (www.eqe.com/revamp/greece1.htm) UNIDO. Repair and Strengthening of Reinforced Concrete, Stone and Brick Masonry Buildings. Volume 5,

Building Construction Under Seismic Conditions in the Balkan Region, UNDP/UNIDO Project RER/79/015, United Nations Industrial Development Organization, Vienna, Austria, 1983.