Report: 69

Building Type: Confined Masonry Building : Clay brick masonry, with concrete tie-columns and beams

Country: Serbia

Author(s): Nikola Muravljov

Radovan Dimitrijevic

Last Updated:

Regions Where Found: Buildings of this construction type can be found in urban and rural areas of Yugoslavia.

Summary: This type of construction has been used for single-family and medium-rise residential buildings throughout urban and rural Yugoslavia during the past 30 years. The structure consists of load-bearing masonry (brick, stone, concrete block) walls confined with reinforced-concrete posts and tie-beams. The walls are typically made of hollow clay tiles. Floor slabs are composed of prefabricated joists infilled with brick elements and topped with a reinforced-concrete slab in-situ.

Length of time practiced: 25-60 years

Still Practiced: Yes

In practice as of:

Building Occupancy: Residential, 10-19 unitsResidential, 50+ units

Typical number of stories: 2-4

Terrain-Flat: Typically

Terrain-Sloped: Typically

Comments: Buildings of this type are single family houses frequent, but there are a lot of multiple housing units and mixed too.

Plan Shape: Other

Additional comments on plan shape: Usually regular shape.

Typical plan length (meters): 10

Typical plan width (meters): 10

Typical story height (meters): 2.8,3.0

Type of Structural System: Masonry: Confined Masonry: Clay brick masonry with concrete posts/tie columns and beams

Additional comments on structural system: Masonry walls transfer all gravity loads from the roof and floor slabs to the foundations. Minimum thickness for bearing masonry walls is 190 mm (as prescribed by the code). The Yugoslav National Building Code classifies masonry buildings into three categories, depending on the wall layout: - buildings with walls in transverse direction; - buildings with walls in longitudinal direction, and - buildings with walls in both directions. The main lateral load-resisting system for this housing type is a wall structure in with brick walls laid in both directions (transverse and longitudinal) carry lateral seismic forces and transfer them to the foundations. Reinforced concrete posts and tie-beams are effective in increasing the stiffness and ductility in this construction type and providing an improved level of seismic safety for this type of construction. Details of concrete posts and tie-beams are shown in Figure 9. A possible failure mechanism for confined masonry walls is illustrated in Figure 13.

Gravity load-bearing & lateral load-resisting systems:

Typical wall densities in direction 1: 5-10%

Typical wall densities in direction 2: 10-15%

Additional comments on typical wall densities: The typical structural wall density varies from 6% to 12%.

Wall Openings: According to the Yugoslav National Building Code, size of the openings should not exceed 2.5 to 3.5 meters (depending on the seismic zone). The size can be increased up to 30% if the openings are confined with reinforced concrete posts and tie beams.

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

Modifications of buildings: The most common pattern of modification in residential houses is complete removal or displacement of interior walls and columns. House owners usually perform modifications without seeking an advice of a competent technician (engineer/architect).

Type of Foundation: Shallow Foundation: Reinforced concrete strip footing

Additional comments on foundation: Foundation details are illustrated in Figure 10.

Type of Floor System: Other floor system

Additional comments on floor system: Structural Concrete: waffle slabs (cast-in-place) According to the National Building code, floor/roof must act as rigid diaphragm.

Type of Roof System: Roof system, other

Additional comments on roof system: Structural Concrete: waffle slabs (cast-in-place) According to the National Building code, floor/roof must act as rigid diaphragm.

Additional comments section 2: When separated from adjacent buildings, the typical distance from a neighboring building is 5 meters. Typical Plan Dimensions: Plan dimensions (i.e. length and width) of this building type should not exceed 40 to 50 meters. Length to width ratio is usually on the order of 3-4. In case the plan dimensions exceed these values, the walls need to be divided into sections by means of control joints. Typical Story Height: Typical floor height for residential buildings is 2.8 m, and 3.0 m for public buildings. Total height for buildings of this type should not exceed 20 meters (according to the code). Typical Span: Typical span (between the adjacent concrete posts) ranges from 3 to 6 m. The National Building Code prescribes the maximum span of 5 meters.

Who is involved with the design process? EngineerArchitect

Roles of those involved in the design process: Architects and engineers design buildings of this type. If certified engineers are not involved in the building design (i.e. non-engineered construction), National Building Code allows the construction of a building up to max. two-storey high. Engineers and architects work jointly on developing project for buildings of this construction type.

Expertise of those involved in the design process:

Who typically builds this construction type? Builder

Roles of those involved in the building process: Typically, builders (developers) build the housing of this type. In some cases, builders live in the houses of this type, too.

Expertise of those involved in building process:

Construction process and phasing: In this construction system, brick elements must be built and tied together in a specific way. Thickness of reinforced concrete vertical posts must be equal to the wall thickness. Minimum reinforcement to be provided in vertical posts consists of 4 -14 mm diameter steel bars tied with 6 mm diameter stirrups spaced at 250 mm on centre. Tie beams are constructed after the bricklaying is completed. Minimum reinforcement required in the tie beams consists of 4 - 12 mm diameter bars tied with 6 mm stirrups at 250 mm on centre spacing. In seismic areas lime/cement mortar has to be used. The construction of this type of housing takes place incrementally over time. Typically, the building is originally designed for its final constructed size. In some cases there are changes and differences between the designed and the constructed building. In such case, National Building Code prescribes the development of technical documentation describing the “as constructed” condition. Modifications (especially vertical expansion) for the buildings of this type are common, especially in case of single-family houses.

Construction issues:

Is this construction type address by codes/standards? Yes

Applicable codes or standards: The year the first code/standard addressing this type of construction issued was 1987. Technical regulations for construction in seismically prone areas, Title of the code or standard: Technical regulations for masonry construction. For all newly constructed buildings, building permits confirming that the construction has been done in conformance with the National Building Code must be issued; the code also prescribes the seismic zone the buildings are located in. Yugoslavia (and Serbia) are the part of the Balkan Peninsula, which is known to be one of the most seismically prone regions of Europe. However, until the catastrophic 1963 Skopje (Macedonia) earthquake, there were no seismic codes or regulations in the country. In 1964, the Preliminary National Building Code (including the seismic provisions) was issued. The latest edition of the National Building Code was issued in 1987. In addition to the National Code, Euro Codes have been used in the country as well.

Process for building code enforcement: For all newly constructed buildings, building permits confirming that the construction has been done in conformance with the National Building Code must be issued; the code also prescribes the seismic zone the buildings are located in. Yugoslavia (and Serbia) are the part of the Balkan Peninsula, which is known to be one of the most seismically prone regions of Europe. However, until the catastrophic 1963 Skopje (Macedonia) earthquake, there were no seismic codes or regulations in the country. In 1964, the Preliminary National Building Code (including the seismic provisions) was issued. The latest edition of the National Building Code was issued in 1987. In addition to the National Code, Euro Codes have been used in the country as well.

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:

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

Additional comments on maintenance and building condition: In the urban areas, public companies take care of maintenance for the housing stock.

Unit construction cost: Construction cost is about 100 to 150 $US/m# of built-up area (structure only), whereas the price of the finished building is on the order of 200 to 300 $US/m# of built-up area.

Labor requirements: Construction of a typical building of this type (built-up area of 120-200 m#) takes approximately 6 months, depending on the finances. It should be also noted that this type of construction requires a limited number of trained labor and technical personnel. Majority of the other construction labor involved in this type of construction are generally unskilled.

Additional comments section 3:

Patterns of occupancy: Usually one family occupies one housing unit. Each building typically has 10-20 housing unit(s). It varies from one unit per building (single-family house) to 50 units in condominiums.

Number of inhabitants in a typical building of this construction type during the day: 5-10

Number of inhabitants in a typical building of this construction type during the evening/night: 10-20

Additional comments on number of inhabitants:

Economic level of inhabitants: Low-income class (poor)Middle-income class

Additional comments on economic level of inhabitants: In the last 10 years, the economic situation in Yugoslavia has been very bad. The average net salary is less than 50 $ US per month. Economic Level: For Poor Class the ratio of the Housing Price Unit to their Annual Income is 50:1. For Middle Class the ratio of the Housing Price Unit to their Annual Income is 30:1.

Typical Source of Financing: Owner financedCommercial banks/mortgagesGovernment-owned housing

Additional comments on financing:

Type of Ownership: RentOwn outrightOwn with debt (mortgage or other)Units owned individually (condominium)

Additional comments on ownership:

Is earthquake insurance for this construction type typically available?: Yes

What does earthquake insurance typically cover/cost: The annual insurance rate is 0.45% of the building value increased by 15% to account for earthquake risk.

Are premium discounts or higher coverages available for seismically strengthened buildings or new buildings built to incorporate seismically resistant features?: No

Additional comments on premium discounts:

Additional comments section 4:

Damage patterns observed in past earthquakes for this construction type: Damage to confined masonry buildings in these earthquakes was not extensive. Figure 14 shows damage to masonry buildings in the 1998 Mionica earthquake (magnitude 5.7). A number of older unreinforced masonry buildings were damaged in the earthquake however confined masonry buildings performed well and did not suffer any significant damage, as illustrated in the figure.

Additional comments on earthquake damage patterns: Typical Earthquake Damage Patterns (Wall): Diagonal tension cracks

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:

Vertical irregularities typically found in this construction type: Other

Horizontal irregularities typically found in this construction type: Other

Seismic deficiency in walls: Low shear strength and diagonal tension cracking as a result of brittle seismic response of unreinforced masonry walls subjected to seismic shear forces and gravity loads; brittle behavior.

Earthquake-resilient features in walls: Typical brick strength is more than 20 MPa.

Seismic deficiency in frames:

Earthquake-resilient features in frame:

Seismic deficiency in roof and floors: In some cases inadequate rigidity of roof and floor slabs

Earthquake resilient features in roof and floors:

Seismic deficiency in foundation:

Earthquake-resilient features in foundation:

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

Additional comments section 5:

Additional comments on seismic strengthening provisions:

Has seismic strengthening described in the above table been performed? Yes, all strengthening methods were used in design practice. Structural engineers provide design specifications for the seismic strengthening design.

Was the work done as a mitigation effort on an undamaged building or as a repair following earthquake damages? In most of the cases, seismic strengthening has been performed as a part of post-earthquake rehabilitation (e.g. after the 1979 Montenegro earthquake or 1998 Mionica earthquake).

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? Usually it was owner/user who performed the construction, with competent participation of an architect and engineer.

What has been the performance of retrofitted buildings of this type in subsequent earthquakes? The buildings that were seismically upgraded were generally not subjected to another major earthquake. However, it is expected that the strengthened buildings would show improved seismic performance if subjected to an earthquake.

Additional comments section 6:

• Collection of Yugoslav Codes and Standards : Lateral Forces - Masonry Belgrade, Yugoslavia 1995

• Selected Chapters of Earthquake-Resistant Construction Petrovic,B. Structural Book, Belgrade, Yugodslavia 1985

• The Earthquake Influence on Buildings in the Region of Mionica Acimovic,B. Proceedings, Institute for Testing Materials IMS, Belgrade, Yugoslavia 1998

• Masonry and Wooden Construction Muravljov,M. and Stevanovic,B. Faculty for Structural Engineering, Belgrade, Yugoslavia 1999

• Effects of Concrete Posts in Block and Brick Walls Muravljov,M., Muravljov,N. and Denic,D. Congress SIG, Vrnjacka Banja 1999