Report #: 88

Report Date:

Country: SLOVENIA

Housing Type:

Housing Sub-Type:

Author(s): Marjana Lutman , Miha Tomazevic

Last Updated:

Regions Where Found: Buildings of this construction type can be found in most parts of Slovenia and it is estimated that it accounts for 40 % of the entire housing stock in Slovenia. This type of housing construction is commonly found in both rural and urban areas.

Summary: This is a very common single-family residential construction practice found throughout Slovenia, both in urban and rural areas. It is estimated that this construction accounts for approximately 40% of the entire housing stock in the country. Confined masonry has been practiced since the wide use of perforated clay blocks has started in the 1970s. The walls areconstructed using perforated clay blocks in lime/cement or cement mortar. The main confining elements include horizontal reinforced concrete bond beams constructed atop the structural walls at each floor level, and vertical reinforced concrete tie-columns at the wall intersections. Floors are either of composite construction, consisting of concrete joists and hollow masonry tiles, or cast in-situ reinforced concrete slabs. Timber roofs are typically used in this type of construction. Since the first national seismic code was issued in 1964, the use of vertical reinforced concrete tie-columns is typically prescribed by the structural design. However, many existing houses were constructed without these critical structural elements. An additional deficiency characteristic for this construction practice is the absence of the top bond-beams along the gable walls (crown beams). This construction is expected to show good seismic performance. Buildings of this type were generally not affected by the past earthquakes in Slovenia.

Length of time practiced: 25-60 years

Still Practiced: Yes

In practice as of:

Building Occupancy: Single dwellingResidential, 2 units

Typical number of stories: 1-2

Terrain-Flat: Typically

Terrain-Sloped: Typically

Comments:

Single and Multiple Housing Units. By and large, houses of this construction type have maximum two housing units.

Plan Shape: Rectangular, solid

Additional comments on plan shape:

Typical plan length (meters): 10-14

Typical plan width (meters): 8-12

Typical story height (meters): 2.75

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

Additional comments on structural system: The vertical load-resisting system is un-reinforced masonry walls. The gravity-load bearing structure consists of roof and floor structures and structural walls. Typically, this construction is characterized with pitched roofs made out of wood. In case of the older, pre-1975 houses, the floor structures are one-way bearing r.c. floor structures, made of hollow clay tile blocks and reinforced concrete joists. In the case of newer houses, the floor structures are simple cast in-situ r.c. slabs. In both cases, the floor structures are cast in-situ, together with r.c. bond-beams atop all structural walls.

The lateral load-resisting system is un-reinforced masonry walls. The lateral load-resisting system consists of exterior and interior block masonry walls, uniformly distributed both in the transverse and longitudinal direction. The wall thickness varies from 190 mm (interior walls) to 290 mm (exterior and some interior walls). The walls are constructed using perforated clay blocks in lime/cement or cement mortar. According to Tomazevic (1999), a perforated block has vertical perforations accounting for 25-50% of the total block volume. In order to improve structural integrity, masonry walls are confined with vertical and horizontal confining elements. Horizontal bond beams are the main horizontal confining elements. These beams, mainly of reinforced concrete construction, have been cast atop all structural walls at the floor level together with floor structures. The reinforcement of horizontal bond beams consists of minimum 4 steel bars of 12 mm diameter. In addition to this, the walls are confined vertically with tie columns located at all wall corners and intersections. The tie columns are constructed either as reinforced concrete columns, cast in-situ using regular shuttering after the masonry has been constructed , or by using special blocks with vertical holes, in which the vertical reinforcement is placed and filled with infill or grout. The tie columns are reinforced with minimum 4 steel bars of 14 mm diameter. These columns should be also constructed at all free ends of the walls and around larger openings, however these elements are often omitted. The walls are typically supported by reinforced concrete strip foundations.

Gravity load-bearing & lateral load-resisting systems:

Typical wall densities in direction 1: 5-10%

Typical wall densities in direction 2: 5-10%

Additional comments on typical wall densities: The wall density at the first floor level ranges from 5 to 8 % in each direction.

Wall Openings: The average area of a window opening in the front exterior wall is 1.4 m.sq. The door opening area in exterior and interior bearing walls is on the order of 1.8 m.sq. The overall opening area in the exterior walls, expressed as a fraction of the overall wall surface area, is approximately equal to 25% on the sunny side of the house and 10% on the shaded side of the house.

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

Modifications of buildings: Since this construction type has been used for the last 30 years, no significant structural modifications have been observed. In some cases, the extensions to the original houses have been made, often without the adequate structural connections between the original and the extended part.

Type of Foundation: Shallow Foundation: Reinforced concrete strip footing

Additional comments on foundation:

Type of Floor System: Other floor system

Additional comments on floor system: Structural concrete: Solid slabs (cast-in-place), Solid slabs (precast)

Type of Roof System: Roof system, other

Additional comments on roof system: The choice of the roof covering material depends on the roof slope and the architectural design style typical for a particular area. Typically, the roof is covered with concrete or clay tiles, or metal, asbestos-cement or plastic corrugated sheets.

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 and structural engineers design houses of this type. It is a very common type of residential construction.

Expertise of those involved in the design process: If there are no significant irregularities in structural design, its design and construction expertise is usually good. The supervision has to be carried out by a structural engineer. Architects are in charge of the architectural design, and structural engineers are in charge of the structural design, construction process and supervision.

Who typically builds this construction type?: OwnerOther

Roles of those involved in the building process: Houses of this type are generally built by construction companies. Smaller houses (plan area less than 250 m.sq.) may be alternatively built by the owner himself/herself with the help of semi-skilled workers.

Expertise of those involved in building process:

Construction process and phasing: The construction process begins with the excavation, forming and stabilization of the ground. Subsequently, the strip foundations are constructed, by placing the shuttering and reinforcement and pouring the concrete. The basement walls are usually built using perforated concrete blocks. However, the walls at the upper stories are built using perforated clay blocks. The construction of r.c. tie columns consists of reinforcement placed in vertical holes, enclosed either by shuttering or special clay blocks. The concrete of r.c. tie columns for each story is poured once the wall construction is complete in order to achieve good bond between the walls and the tie-columns. Subsequently, the shuttering and the reinforcement for the floor structure, bond beams and lintel beams is placed and the concrete is poured. Mortar for masonry construction is prepared at the site by using the ready dry mix; water is added at the site, and the mortar is mixed using machine mixers. Concrete for r.c. elements is plant-mixed concrete, taken to the site by the truck mixer and poured by mobile concrete pump. 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: National Seismic Code for Buildings (1981). The year the first code/standard addressing this type of construction issued was 1964. The first code including design vertical load, wind and seismic load was the Preliminary National Building Code (1948). After the catastrophic 1963 earthquake in Skopje, Macedonia (in the former Yugoslavia), the first Seismic Code addressing this type of construction was issued (1964). In addition to the National Seismic Code for Buildings, Eurocode 8 is being used at the present time. The most recent code/standard addressing this construction type issued was 1981.

Process for building code enforcement: In order to get the building permit, the design of a building has to be approved by the state authorities. Structural engineers have to and usually do consider the requirements of National Seismic Code for Buildings, but for these individual residential houses it is usually not verified. The supervision during the construction also has to be provided. However, the technical inspection after the construction is completed is usually not carried out (although this should be performed in order to get the building use and occupancy permit).

Are building permits required?: Yes

Is this typically informal construction?: Yes

Is this construction typically authorized as per development control rules?: No

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:

Unit construction cost: The unit construction cost (per m.sq.) for this housing type is approx. 500-750 $US/m.sq., whereas the market unit cost for the completed house is approx. 750-1200 $US/m.sq. (including the price of the lot and complete infrastructure).

Labor requirements: The design of a house takes about 3 months. These houses are usually built individually. The construction requires approx. 5 skilled workers and it takes up to 1 year.

Additional comments section 3:

epicenter in Slovenia in the 20th century occurred on April 12, 1998. The epicenter was located approx. 6.3 km South-East from the town of Bovec, and the focal depth was between 15 and 18 km. No building collapses were reported, however out of 952 inspected buildings, 337 were found to be unsafe (out of which 123 were beyond repair). The effectiveness of strengthening methods applied after the 1976 earthquake was studied. The majority of damaged buildings were of rubble-stone masonry construction. Buildings of confined masonry construction either remained undamaged or experienced a minor damage (a few cracks developed). Most of the observed damage was due to the mistakes made by the building owners, who have built their houses by themselves. In some cases, masonry units and/or mortar of poor quality were used in the construction. Also, the spacing between the cross walls was too large in some cases; the floor slabs were found to be too slender in some cases. Often, the houses were built without vertical tie-columns at the wall corners and intersections, and without the bond beams along the gable walls (crown beams), see Fig.15.

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: Generally, it can be found that the width of door and window openings in one out of 10-15 walls exceeds # of the wall length (i.e. the distance between the two adjacent cross walls).

Vertical irregularities typically found in this construction type: Other

Horizontal irregularities typically found in this construction type: Other

Seismic deficiency in walls: This structural type, usually used for 2-story houses, does not have major structural deficiencies, however some common construction faults that could influence the seismic performance are as follow s: i) masonry units should be soaked in water before the construction in order to prevent burning of the mortar; in some cases, this rule is not respected and consequently the units absorb the moisture from mortar; ii) head joints (vertical joints) should be completely filled with mortar in order to provide adequate seismic resistance of masonry; in construction practice, head joints are sometimes filled only partially on both wall faces.

In the case of an earthquake with high intensity, shear (“X”) cracks might develop in the walls.

Seismic deficiency in roof and floors: In case of one-way roof and floor structures (i.e. carry load in one direction only), the walls in the longitudinal and transverse direction are not equally loaded.

In the case of one-way systems horizontal cracks might develop along the wall-floor joints in the walls that do not carry gravity load.

Earthquake resilient features in roof and floors: If floor structures are r.c. floor slabs equally reinforced in two directions, a good distribution of gravity load onto the bearing walls is achieved; consequently, similar seismic resistance is developed in both orthogonal directions.

Other seismic deficiencies: Building and structural layout: The distance between the adjacent structural walls is sometimes too large. Vertical tie-columns that should be placed at all vertical wall intersections and at both sides of larger openings are often missing. In addition to the above, gable walls are typically not confined by means of top bond beams (crown beams).

Other earthquake-resilient features: The building layout is usually regular, wall densities for both orthogonal directions are very similar, and bearing walls are distributed uniformly in case r.c. floor slabs are used.

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

Has seismic strengthening described in the above table been performed?: Since this construction practice has been followed only in the last 30 years and no significant earthquake damage has been reported so far, only a few repair or retrofit interventions have been carried out so far.

Was the work done as a mitigation effort on an undamaged building or as a repair following earthquake damages?: The work was done as a repair following the 1998 earthquake, however a very few houses of this type that have been damaged in recent earthquakes have been repaired and strengthened.

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

Who performed the construction: a contractor or owner/user? Was an architect or engineer involved?: An architect and a structural engineer is involved in the retrofit design. The construction is carried out by a contractor. After the 1998 Bovec earthquake, all contractors who were performing repair and strengthening were additionally trained.

What has been the performance of retrofitted buildings of this type in subsequent earthquakes?: Information is not available, as damaging earthquakes were not reported since 1998.

Guidelines and Procedures Used to Eliminate the Impact of the Earthquake in the Soca Valley Ladava Proceedings, Social and Economic Aspects of Earthquakes, Jones, B.G., and Tomazevic, M. (Editors), Ljubljana-Ithaca, Institute for Testing and Research in Materials and Structures, Ljubljana - Cornell University, USA, pp. 413-423 1982

Report on Mitigating and Consequences of the Earthquake of Bovec of April 12, 1998, Ljubljana ACP Administration for Civil Protection and Disaster Relief, Government of Slovenia, Ljubljana, Slovenia (in Slovene) 1998

The Seismic Resistance of Historical Urban Buildings and the Interventions in their Floor Systems: an Experimental Study Tomazevic,M., and Lutman,M., and Weiss,P. The Masonry Society Journal, 12 (1), pp. 77-86 1993

Earthquake-Resistant Design of Masonry Buildings Tomazevic,M. Series on Innovation in Structures and Construction, Vol.1, Imperial College Press, London, UK 1999