|Report #|| 148|
|Report Date|| 24-02-2008|
||Dominik H. Lang, Alvaro Amador, Lisa Holliday, Claudio Romero L, Armando Ugarte
||Andrew W. Charleson
This encyclopedia contains information contributed by various earthquake engineering professionals around the world. All opinions, findings, conclusions & recommendations expressed herein are those of the various participants, and do not necessarily reflect the views of the Earthquake Engineering Research Institute, the International Association for Earthquake Engineering, the Engineering Information Foundation, John A. Martin & Associates, Inc. or the participants' organizations.
1. General Information
The term 'minifalda', translated 'miniskirt' refers to the building's walls which consist of masonry or concrete in the lower part, while the upper part is made of a light wood construction (also 'madera y concreto'). According to a recent population census carried out in 2005 (INEC, 2006), the total percentage of minifalda houses in Nicaragua was around 7% (8% in urban and 5.6% in rural areas). In the year 1998, minifalda represented 9.8% of the total houses in Nicaragua (12.8% in urban and 6.1% in rural areas; according to OPAS, 2001). Comparing the two numbers, it shows that the rate of this construction type on the total building stock in Nicaragua has reduced considerably. The combination of a more stable and consolidated base made of concrete or masonry and a light and flexible upper part of the walls made of wood frame construction, provides these houses with some advantages. However, the heavy roofs, which consist mostly of tiles, increase the vulnerability of the buildings especially during earthquake action.
2. Architectural Aspects
Buildings of this construction type can be found in all parts of the country, but a concentration of this construction technique can be seen in the municipalities of Managua and Masaya (both >10%) as well as in the municipalities of Rivas and Río San Juan (9.3% and 7.9% respectively). Figure 2 illustrates the percentages of minifalda houses in the 15 municipalities (departamentos) and the 2 autonomous communities (comunidades autónomas) of Nicaragua based on the population census of 2005 (INEC, 2006). This type of housing construction is commonly found in both rural and urban areas.
The percentage of minifaldas in urban areas is slightly larger than in rural areas, e.g., according to OPAS (2001) in 1998: 12.8% in urban and 6.1% in rural areas and according to INEC (2006) in 2005: 8% in urban and 5.6% in rural areas. However, these numbers show large variations between the different municipalities.
This construction type has been in practice for less than 50 years.
Currently, this type of construction is not being built. The Minifalda construction type was introduced as an alternative for an earthquake-resistant house after the 1972 Managua earthquake. Its forerunner was a building type, which had foundation walls several centimeters above the ground surface. Because of the high price of lumber, this construction type is not built very often today. A similar technique uses plasterboard walls (plycem) instead of lumber.
Figure 1: Typical minifalda houses in Masaya, Nicaragua. [Click to enlarge figures]
Figure 2: Percentages of minifalda houses in the 17 different municipalities of Nicaragua after the population census in 2005 (INEC, 2006). [Click to enlarge figures]
3. Structural Details
These buildings are typically found in flat, sloped and hilly terrain. They share common walls with adjacent buildings. Minifalda houses often are built side-by-side without any gaps between them. Especially in Managua, minifalda houses often are built continuously
2.2 Building Configuration
The typical building shape is rectangular in plan. However, houses located at non-rectangular street corners are often irregular or asymmetric in plan.
Figures 3 and 4 illustrate the typical plans of residential minifalda buildings in rural areas of Guatemala. Even though single structural details may differ between Guatemala and Nicaragua, the plans are generally representative for Nicaraguan conditions. A common plan dimension for minifalda houses in Nicaragua is 6m x 6m ('modulo basico' = 36 m²; Figure 5). Minifalda houses have few windows, often with very small dimensions (40*40cm; See Figures 1, 8, and 9). The windows are always located in the wooden (upper) part of the walls. The window sill is often formed by the upper edge of the concrete base. Even when the building is used for small retail trade or handicraft business, larger openings for showcases or sales counters do not exist. Compared to the size of the windows, the doors appear to be oversized. At the positions of the doors there are cut-outs in the concrete wall bases, such that the bottom quarter to half of the door frame consists of concrete, while its upper part is framed with wood.
2.3 Functional Planning
The main function of this building typology is single-family house. Minifalda houses often accommodate small shops (retail trade) or handicraft businesses, in addition to their common use as single family dwellings. In a typical building of this type, there are no elevators and no fire-protected exit staircases. Typically, each house has between 2 and 4 doors, which provide means of escape.
2.4 Modification to Building
One common modification is to change the roof material. During renovation, wooden walls are sometimes replaced by plasterboard walls (plycem).
Figure 3: Plan shape and cross-sections of a residential home consisting of a minifalda and an adobe part (location: Zunil, Guatemala; from Marroquin and G
Figure 4: Plan and elevations of a typical minifalda building (location: San Raymundo, Guatemala; from Marroquin and G
Figure 5: Typical plan of a minifalda building in Nicaragua with a living area of 36 m2 (
4. Socio-Economic Aspects
3.1 Structural System
|Material||Type of Load-Bearing Structure||#||Subtypes||Most appropriate type|
|Masonry||Stone Masonry |
|1||Rubble stone (field stone) in mud/lime |
mortar or without mortar (usually with
|2||Dressed stone masonry (in|
|Adobe/ Earthen Walls||3||Mud walls ||☑|
|4||Mud walls with horizontal wood elements||☐|
|5||Adobe block walls||☐|
|6||Rammed earth/Pise construction||☐|
|7||Brick masonry in mud/lime|
|8||Brick masonry in mud/lime|
mortar with vertical posts
|9||Brick masonry in lime/cement|
|10||Concrete block masonry in|
|Confined masonry||11||Clay brick/tile masonry, with|
wooden posts and beams
|12||Clay brick masonry, with|
concrete posts/tie columns
|13||Concrete blocks, tie columns|
|Reinforced masonry||14||Stone masonry in cement|
|15||Clay brick masonry in cement|
|16||Concrete block masonry in|
|Structural concrete||Moment resisting|
|17||Flat slab structure||☐|
|18||Designed for gravity loads|
only, with URM infill walls
|19|| Designed for seismic effects,|
with URM infill walls
|20||Designed for seismic effects,|
with structural infill walls
|21||Dual system – Frame with|
|Structural wall||22||Moment frame with in-situ|
|23||Moment frame with precast|
|Precast concrete||24||Moment frame||☐|
|25||Prestressed moment frame|
with shear walls
|26||Large panel precast walls||☐|
|27||Shear wall structure with|
|28||Shear wall structure with|
precast wall panel structure
|29||With brick masonry partitions||☐|
|30||With cast in-situ concrete|
|31||With lightweight partitions||☐|
|Braced frame||32||Concentric connections in all|
|33||Eccentric connections in a|
|Structural wall||34||Bolted plate||☐|
|37||Walls with bamboo/reed mesh|
and post (Wattle and Daub)
|38||Masonry with horizontal|
beams/planks at intermediate
|39||Post and beam frame (no|
|40||Wood frame (with special|
|41||Stud-wall frame with|
|42||Wooden panel walls||☐|
|Other||Seismic protection systems||43||Building protected with base-isolation systems||☐|
|44||Building protected with|
|Hybrid systems||45||other (described below)||☐|
The structural system is a mix of a wooden frame standing on walls made of clay bricks, adobe masonry or concrete blocks.
3.2 Gravity Load-Resisting System
The vertical load-resisting system is timber frame load-bearing wall system. The gravity loads on the building mostly result from the roof material itself (i.e. heavy clay tiles, corrugated iron, asbestos sheets). They are transferred from the roof by wooden beams or purlins to the walls (Figure 7). The gravity loads are then transferred from the walls to the foundation.
3.3 Lateral Load-Resisting System
The lateral load-resisting system is timber frame load-bearing wall system. Walls comprise the lateral load-resisting system in the building. The walls are made of masonry (clay bricks, concrete blocks or adobe) in the lower portion and a light wooden construction in the upper portion.
Together the two parts of the wall (the lower massive part and the upper wood frame) are able to resist the lateral loads.
However, the important feature in this respect is how both parts are connected, e.g., how the vertical frame elements (wooden posts) are tied to the masonry walls. In some cases, the posts are embedded between lengths of masonry at the base of the wall (Figure 6).
The gabled or mono-pitched roof normally consists of a very light construction which cannot be considered a diaphragm and therefore may not support any lateral loading.
3.4 Building Dimensions
The typical plan dimensions of these buildings are: lengths between 3.5 and 6 meters, and widths between 3.5 and 6 meters. The building is 1 storey high. The typical span of the roofing/flooring system is 3.5-5.0 meters. The common plan size is 6m x 6m ('modulo basico'; Figure 5). The typical storey height in such buildings is 2.2-3.5 meters. The typical structural wall density is up to 10 %. Detailed measurements for typical wall density are not available.
3.5 Floor and Roof System
|Material||Description of floor/roof system||Most appropriate floor||Most appropriate roof|
|Composite system of concrete joists and|
|Structural concrete||Solid slabs (cast-in-place)||☐||☐|
|Waffle slabs (cast-in-place)||☐||☐|
|Flat slabs (cast-in-place)||☐||☐|
|Precast joist system||☐||☐|
|Hollow core slab (precast)||☐||☐|
|Solid slabs (precast)||☐||☐|
|Beams and planks (precast) with concrete|
|Steel||Composite steel deck with concrete slab|
|Timber||Rammed earth with ballast and concrete or|
|Wood planks or beams with ballast and concrete or plaster finishing||☐||☐|
|Thatched roof supported on wood purlins||☐||☐|
|Wood shingle roof||☐||☐|
|Wood planks or beams that support clay tiles||☐|
|Wood planks or beams supporting natural|
|Wood planks or beams that support slate,|
metal, asbestos-cement or plastic corrugated
sheets or tiles
|Wood plank, plywood or manufactured wood|
panels on joists supported by beams or walls
The floor is directly built on the ground. The roof is not considered to act as a rigid diaphragm.
|Type||Description||Most appropriate type|
|Shallow foundation||Wall or column embedded in|
soil, without footing
|Rubble stone, fieldstone|
|Rubble stone, fieldstone strip|
|Deep foundation||Reinforced-concrete bearing|
|Steel bearing piles||☐|
|Steel skin friction piles||☐|
|Cast-in-place concrete piers||☐|
Figure 6: Detailing of the transition zone between the masonry base and upper wooden part of the wall. [Click to enlarge figures]
Figure 7: Roof made of asbestos sheets supported by wooden beams which loosely rest on the wooden walls. [Click to enlarge figures]
5. Seismic Vulnerability
4.1 Number of Housing Units and Inhabitants
Each building typically has 1 housing unit(s). and typically one family occupies the house. The number of inhabitants in a building during the day or business hours is less than 5. According to the recent population census conducted in 2005, 46.2% of all conventional houses in Nicaragua are occupied by less than 5 people, 46.3% by 5 to 9, and 7.5% by 10 or more persons (INEC, 2006). The number of inhabitants during the evening and night is 5-10.
4.2 Patterns of Occupancy
No details are available on this.
4.3 Economic Level of Inhabitants
|Income class||Most appropriate type|
|a) very low-income class (very poor)||☑|
|b) low-income class (poor)||☐|
|c) middle-income class ||☐|
|d) high-income class (rich)||☐|
A typical house of this type costs US $3,770, while a typical annual income for a poor family is US $730.
|Ratio of housing unit price to annual income||Most appropriate type|
|5:1 or worse||☑|
|1:1 or better||☐|
|What is a typical source of|
financing for buildings of this
|Most appropriate type|
|Informal network: friends and|
|Small lending institutions / micro-|
|Combination (explain below)||☐|
|other (explain below)||☐|
In each housing unit, there are no bathroom(s) without toilet(s), no toilet(s) only and no bathroom(s) including toilet(s).
According to a population census in 1998 (OPAS, 2001) around 80% of the minifalda houses in Nicaragua had a direct connection to the potable water supply system, 12% an indirect and 8% no connection. .
The type of ownership or occupancy is renting and outright ownership.
Most houses are owned by the residents; some are rented out.
|Type of ownership or|
|Most appropriate type|
|Ownership with debt (mortgage|
|Ownership by a group or pool of|
|other (explain below)||☐|
5.1 Structural and Architectural Features
|Statement||Most appropriate type|
|Lateral load path||The structure contains a complete load path for seismic|
force effects from any horizontal direction that serves
to transfer inertial forces from the building to the
|The building is regular with regards to both the plan|
and the elevation.
|Roof construction||The roof diaphragm is considered to be rigid and it is|
expected that the roof structure will maintain its
integrity, i.e. shape and form, during an earthquake of
intensity expected in this area.
|Floor construction||The floor diaphragm(s) are considered to be rigid and it|
is expected that the floor structure(s) will maintain its
integrity during an earthquake of intensity expected in
|There is no evidence of excessive foundation movement|
(e.g. settlement) that would affect the integrity or
performance of the structure in an earthquake.
|Wall and frame|
|The number of lines of walls or frames in each principal|
direction is greater than or equal to 2.
|Wall proportions||Height-to-thickness ratio of the shear walls at each floor level is:|
Less than 25 (concrete walls);
Less than 30 (reinforced masonry walls);
Less than 13 (unreinforced masonry walls);
|Vertical load-bearing elements (columns, walls)|
are attached to the foundations; concrete
columns and walls are doweled into the
|Exterior walls are anchored for out-of-plane seismic|
effects at each diaphragm level with metal anchors or
|Wall openings||The total width of door and window openings in a wall|
For brick masonry construction in cement mortar : less
than ½ of the distance between the adjacent cross
For adobe masonry, stone masonry and brick masonry
in mud mortar: less than 1/3 of the distance between
the adjacent cross
For precast concrete wall structures: less than 3/4 of
the length of a perimeter wall.
|Quality of building materials||Quality of building materials is considered to be|
adequate per the requirements of national codes and
standards (an estimate).
|Quality of workmanship||Quality of workmanship (based on visual inspection of|
few typical buildings) is considered to be good (per
local construction standards).
|Maintenance ||Buildings of this type are generally well maintained and there|
are no visible signs of deterioration of building
elements (concrete, steel, timber)
|Additional Comments|| |
5.2 Seismic Features
|Structural Element||Seismic Deficiency||Earthquake Resilient Features||Earthquake Damage Patterns|
|Walls (generally) ||- Use of different construction materials over wall height leads to stiffness and mass differences. || || |
|Wall bases (masonry)||- Brittle and heavy with possibly insufficient resistance to out-of-plan forces || || |
|Wooden wall frames||- Inadequate anchorage of wooden posts to the masonry base of the wall
- Lack of preservative treatment of timbers leading to deterioration due to vermin or insects ||- Flexibility, elasticity
- Light-weight construction ||- Anchorage / embedment failure of wooden posts |
|Roof||- No diaphragm action
- No strong connection to the walls
- Heavy dead loads in the case of heavy clay tiles
- Material deterioration of wooden trusses due to climate effects ||- Low dead loads in the case of corrugated iron or asbestos sheeting ||- Total and partial collapse of roof construction |
The minifalda construction type is not covered by the vulnerability table of the European Macroseismic Scale EMS-1998 (Grünthal (ed.), 1998). However, it is largely comparable with a timber wood frame construction. Timber structures are generally classified into vulnerability class D with a probable range between C and E, and in some exceptional cases B. However, since minifalda buildings basically consist of a mixed wall construction with different materials involved, their seismic behavior may not be as good as pure timber structures and may be classified as a higher vulnerability class. The different stiffness of the lower masonry and the upper wooden construction may lead to more damage.
This two-part construction technique does provide some advantages which mainly consist of protecting the wood from ascending earth-moisture and splash water.
5.3 Overall Seismic Vulnerability Rating
The overall rating of the seismic vulnerability of the housing type is C: MEDIUM VULNERABILITY (i.e., moderate seismic performance), the lower bound (i.e., the worst possible) is B: MEDIUM-HIGH VULNERABILITY (i.e., poor seismic performance), and the upper bound (i.e., the best possible) is D: MEDIUM-LOW VULNERABILITY (i.e., good seismic performance).
| ||very poor||poor||moderate||good||very good||excellent|
5.4 History of Past Earthquakes
|Date||Epicenter, region||Magnitude||Max. Intensity|
|1972 ||Managua ||6.2 ||VIII-IX |
|1985 ||Lago de Nicaragua, Rivas || || |
|1992 ||Pacific ocean || || |
|2000 ||Laguna de Apoyo ||5.4 ||V-VI (MMI) |
|2005 ||Isla de Ometepe ||5.6 || |
Compared to other dwelling types minifalda construction has behaved well during past earthquakes in Nicaragua, even though a considerable number of destructive earthquakes occurred (See table listing those events after 1972). After the 1972 Managua earthquake, minifalda houses became very popular. Some international aid organizations (e.g. German Red Cross, Guatemalan Red Cross, Asociación Christiana de Desarrollo) suggested the use of this construction technique for rebuilding residential and school buildings in Guatemala after the 1976 earthquake (Marroquin and Gándara, 1976).
6.1 Building Materials
|Structural element||Building material||Characteristic strength||Mix proportions/dimensions||Comments|
|Walls||For the wall base, masonry (adobe, clay bricks, or concrete blocks) is used. For the upper section of walls, wood is used.||No information is available on material strengths, mix of materials, etc. However, material properties of the base walls will not differ from those used for conventional adobe, clay brick or concrete block buildings in Nicaragua or entire Central America (See e.g., EERI-WHE contribution #144 by Lang et al. on adobe buildings in Guatemala).|| || |
|Foundation||For the foundations, mud, field stones, or concrete is used. ||No information is available on material strengths, or mix of materials. || || |
|Frames (beams & columns)|| || || || |
|Roof and floor(s)||For the roofs, wooden planks with clay tiles or corrugated sheets are used. Floors are made of earthen materials or cast plaster floor (screed). || || || |
The builder generally occupies the house and is the house owner.
6.3 Construction Process, Problems and Phasing
Structural engineers or architects are generally not involved in the design or erection process of this building type. As it is described earlier, the bases of these buildings do not differ from conventional adobe, clay brick or concrete block buildings (compare e.g., to EERI-WHE contribution #144 by Lang et al. on adobe buildings in Guatemala). Consequently the first steps of the construction process will be comparable with those for these building types. After the base walls are completed, i.e. the walls are brought up to approximately 1/3 to ½ of the story height, the vertical elements (wooden posts) of the wood frame are connected to or embedded into the wall bases (see Figure 6). As soon as the wood frames are completed with the horizontal elements (beams) and diagonal struts, the external wooden panels are connected to the frame. The wooden panels always are oriented in vertical direction (see Figures 1, 8, and 9). Later or in parallel to the mounting of wall panels, the timber beams and purlins of the roof construction are connected to the wall frame. Tiling is done afterwards with the roofing material as e.g., clay tiles, asbestos-cement or corrugated metal sheets. The construction process is finished by furnishing the wall bases with plaster and bringing a colorful paint the wooden walls. The construction of this type of housing takes place in a single phase. Typically, the building is originally designed for its final constructed size.
6.4 Design and Construction Expertise
No design or construction expertise can be found. Expertise may only be gained by word-of-mouth. Some international aid organizations suggested the use of this construction technique for rebuilding residential and school buildings in Guatemala after the 1976 earthquake (Marroquin and Gándara, 1976). However, guidelines for its design and construction have not yet been developed.
6.5 Building Codes and Standards
This construction type is not addressed by the codes/standards of the country.
6.6 Building Permits and Development Control Rules
This type of construction is a non-engineered, and not authorized as per development control rules. Building permits are not required to build this housing type.
6.7 Building Maintenance
Typically, the building of this housing type is maintained by Owner(s) and Tenant(s).
6.8 Construction Economics
A typical building of this type costs US $38/sqm. It typically takes 1-2 months to construct one housing unit.
Figure 8: Typical minifalda houses in Managua, Nicaragua. [Click to enlarge figures]
Figure 9: Typical minifalda houses in Masaya, Nicaragua. [Click to enlarge figures]
Earthquake insurance for this construction type is typically unavailable. For seismically strengthened existing buildings or new buildings incorporating seismically resilient features, an insurance premium discount or more complete coverage is unavailable. Earthquake insurance is only available for those buildings addressed in the code and which are constructed according to the code. For those buildings not meeting the requirements of the code, the insurance policies are higher. And since the owner or occupants of minifalda buildings are poor and unable to afford these higher rates, essentially none of these buildings are insured.
8.1 Description of Seismic Strengthening Provisions
There are no reports of minifalda houses in Central America having been damaged in past earthquakes. Consequently, strengthening or retrofitting measures are not known.
8.2 Seismic Strengthening Adopted
Has seismic strengthening described in the above table been performed in design and construction practice, and if so, to what extent?
Was the work done as a mitigation effort on an undamaged building, or as repair following an earthquake?
8.3 Construction and Performance of Seismic Strengthening
Was the construction inspected in the same manner as the new construction?
Who performed the construction seismic retrofit measures: a contractor, or owner/user? Was an architect or engineer involved?
What was the performance of retrofitted buildings of this type in subsequent earthquakes?
- HAZUS Earthquake Loss Estimation Methodology: User's Manual.
Federal Emergency Management Agency, Washington DC. 1999
- Desigualdades en el acceso, uso y gasto con el agua potable en Am
Washington, DC, United States 2001
- Estudio de la Vulnerabilidad S
Reinoso, E. (ed.)
SE-SINAPRED, INETER, Managua, Nicaragua August 200
- VIII Censo de Poblaci
Instituto Nacional de Estad
Departamentos/Regiones Aut November 2 Volumen I
- La vivienda popular en Guatemala - Antes y despues del terremoto de 1976
Marroquin, H., G
Universitaria de Guatemala 1976 Tomo I
- Manal de construcci
Forschungslabor f 2001
- European Macroseismic Scale 1998.
Cahiers du Centre Europ 1998
- Terremoto? Mi casa si resistente! Manual de construcci
- Dominik H. Lang
Senior Research Engineer, NORSAR
P.O. Box 53, Kjeller  2027, NORWAY
Email:email@example.com FAX: +47-63818719
- Alvaro Amador
M.Sc., Instituto Nicaraguense de Estudios Territoriales, Managua  , NICARAGUA
- Lisa Holliday
Engineer, Fears Laboratory, The University of Oklahoma, Norman, Oklahoma  73019, USA
- Claudio Romero L
M.Sc., Universidad National Aut, Managua  , NICARAGUA
- Armando Ugarte, Universidad Nacional de Ingenier, Managua  , NICARAGUA
- Andrew W. Charleson
School of Architecture, Victoria University of Wellington
Wellington 6001, NEW ZEALAND