World Housing Encyclopedia
an Encyclopedia of Housing Construction in
Seismically Active Areas of the World




an initiative of
Earthquake Engineering Research Institute (EERI) and
International Association for Earthquake Engineering (IAEE)


HOUSING REPORT
Vivienda de Minifalda (Wooden houses with heavy bases)


Report # 148
Report Date 24-02-2008
Country NICARAGUA
Author(s) Dominik H. Lang, Alvaro Amador, Lisa Holliday, Claudio Romero L, Armando Ugarte
Reviewer(s) Andrew W. Charleson

Important
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.

Summary

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.
 

1. General Information

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]

2. Architectural Aspects

2.1 Siting 
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 (

3. Structural Details

3.1 Structural System 
 
MaterialType of Load-Bearing Structure#SubtypesMost appropriate type
MasonryStone Masonry
Walls
1Rubble stone (field stone) in mud/lime
mortar or without mortar (usually with
timber roof)
2Dressed stone masonry (in
lime/cement mortar)
Adobe/ Earthen Walls3Mud walls
4Mud walls with horizontal wood elements
5Adobe block walls
6Rammed earth/Pise construction
Unreinforced masonry
walls
7Brick masonry in mud/lime
mortar
8Brick masonry in mud/lime
mortar with vertical posts
9Brick masonry in lime/cement
mortar
10Concrete block masonry in
cement mortar
Confined masonry11Clay brick/tile masonry, with
wooden posts and beams
12Clay brick masonry, with
concrete posts/tie columns
and beams
13Concrete blocks, tie columns
and beams
Reinforced masonry14Stone masonry in cement
mortar
15Clay brick masonry in cement
mortar
16Concrete block masonry in
cement mortar
Structural concreteMoment resisting
frame
17Flat slab structure
18Designed for gravity loads
only, with URM infill walls
19 Designed for seismic effects,
with URM infill walls
20Designed for seismic effects,
with structural infill walls
21Dual system – Frame with
shear wall
Structural wall22Moment frame with in-situ
shear walls
23Moment frame with precast
shear walls
Precast concrete24Moment frame
25Prestressed moment frame
with shear walls
26Large panel precast walls
27Shear wall structure with
walls cast-in-situ
28Shear wall structure with
precast wall panel structure
SteelMoment-resisting
frame
29With brick masonry partitions
30With cast in-situ concrete
walls
31With lightweight partitions
Braced frame32Concentric connections in all
panels
33Eccentric connections in a
few panels
Structural wall34Bolted plate
35Welded plate
TimberLoad-bearing timber
frame
36Thatch
37Walls with bamboo/reed mesh
and post (Wattle and Daub)
38Masonry with horizontal
beams/planks at intermediate
levels
39Post and beam frame (no
special connections)
40Wood frame (with special
connections)
41Stud-wall frame with
plywood/gypsum board
sheathing
42Wooden panel walls
OtherSeismic protection systems43Building protected with base-isolation systems
44Building protected with
seismic dampers
Hybrid systems45other (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 


MaterialDescription of floor/roof systemMost appropriate floorMost appropriate roof
Masonry Vaulted
Composite system of concrete joists and
masonry panels
Structural concreteSolid 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
topping (cast-in-situ)
Slabs (post-tensioned)
SteelComposite steel deck with concrete slab
(cast-in-situ)
TimberRammed earth with ballast and concrete or
plaster finishing
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
stones slates
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
OtherDescribed below

The floor is directly built on the ground.  The roof is not considered to act as a rigid diaphragm.  

3.6 Foundation 


TypeDescriptionMost appropriate type
Shallow foundationWall or column embedded in
soil, without footing
Rubble stone, fieldstone
isolated footing
Rubble stone, fieldstone strip
footing
Reinforced-concrete isolated
footing
Reinforced-concrete strip
footing
Mat foundation
No foundation
Deep foundationReinforced-concrete bearing
piles
Reinforced-concrete skin
friction piles
Steel bearing piles
Steel skin friction piles
Wood piles
Cast-in-place concrete piers
Caissons
OtherDescribed below




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]

4. Socio-Economic Aspects

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 classMost 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 incomeMost appropriate type
5:1 or worse
4:1
3:1
1:1 or better


What is a typical source of
financing for buildings of this
type?
Most appropriate type
Owner financed
Personal savings
Informal network: friends and
relatives
Small lending institutions / micro-
finance institutions
Commercial banks/mortgages
Employers
Investment pools
Government-owned housing
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.
.  

4.4 Ownership 

The type of ownership or occupancy is renting and outright ownership.  

Type of ownership or
occupancy?
Most appropriate type
Renting
outright ownership
Ownership with debt (mortgage
or other)
Individual ownership
Ownership by a group or pool of
persons
Long-term lease
other (explain below)
Most houses are owned by the residents; some are rented out.  

5. Seismic Vulnerability

5.1 Structural and Architectural Features 
Structural/
Architectural
Feature
StatementMost appropriate type
YesNoN/A
Lateral load pathThe 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
foundation.
Building
Configuration
The building is regular with regards to both the plan
and the elevation.
Roof constructionThe 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 constructionThe 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
this area.
Foundation
performance
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
structures-
redundancy
The number of lines of walls or frames in each principal
direction is greater than or equal to 2.
Wall proportionsHeight-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);
Foundation-wall
connection
Vertical load-bearing elements (columns, walls)
are attached to the foundations; concrete
columns and walls are doweled into the
foundation.
Wall-roof
connections
Exterior walls are anchored for out-of-plane seismic
effects at each diaphragm level with metal anchors or
straps
Wall openingsThe 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.
Quality of building materialsQuality of building materials is considered to be
adequate per the requirements of national codes and
standards (an estimate).
Quality of workmanshipQuality 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 ElementSeismic DeficiencyEarthquake Resilient FeaturesEarthquake 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).  

Vulnerabilityhighmedium-highmediummedium-lowlowvery low
  very poorpoormoderategoodvery goodexcellent
Vulnerability
Class
ABCDEF


5.4 History of Past Earthquakes
 

DateEpicenter, regionMagnitudeMax. 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. Construction

6.1 Building Materials 

Structural elementBuilding materialCharacteristic strengthMix proportions/dimensionsComments
WallsFor 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).  
FoundationFor 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).    


6.2 Builder 

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]

7. Insurance

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. Strengthening


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? 
Not applicable
.  

Was the work done as a mitigation effort on an undamaged building, or as repair following an earthquake? 
Not applicable
.  

8.3 Construction and Performance of Seismic Strengthening 


Was the construction inspected in the same manner as the new construction? 
Not applicable
.  

Who performed the construction seismic retrofit measures: a contractor, or owner/user? Was an architect or engineer involved? 
Not applicable
.  

What was the performance of retrofitted buildings of this type in subsequent earthquakes? 
Not applicable
.  

Reference(s)

  1. HAZUS Earthquake Loss Estimation Methodology: User's Manual.
    FEMA
    Federal Emergency Management Agency, Washington DC. 1999 
     
  2. Desigualdades en el acceso, uso y gasto con el agua potable en Am
    Organizaci
    Washington, DC, United States 2001 
     
  3. Estudio de la Vulnerabilidad S
    Reinoso, E. (ed.)
    SE-SINAPRED, INETER, Managua, Nicaragua August 200 
     
  4. VIII Censo de Poblaci
    Instituto Nacional de Estad
    Departamentos/Regiones Aut November 2 Volumen I
     
  5. La vivienda popular en Guatemala - Antes y despues del terremoto de 1976
    Marroquin, H., G
    Universitaria de Guatemala 1976 Tomo I
     
  6. Manal de construcci
    Minke, G.
    Forschungslabor f 2001 
     
  7. European Macroseismic Scale 1998.
    Gr
    Cahiers du Centre Europ 1998 
     
  8. Terremoto? Mi casa si resistente! Manual de construcci
    GTZ COPASA
     

Author(s)

  1. Dominik H. Lang
    Senior Research Engineer, NORSAR
    P.O. Box 53, Kjeller  2027, NORWAY
    Email:dominik@norsar.no  FAX: +47-63818719 
     
  2. Alvaro Amador
    M.Sc., Instituto Nicaraguense de Estudios Territoriales, Managua  , NICARAGUA
    Email:alvaro.amador@gf.ineter.gob.ni 
     
  3. Lisa Holliday
    Engineer, Fears Laboratory, The University of Oklahoma, Norman, Oklahoma  73019, USA
    Email:lisaholliday@ou.edu 
     
  4. Claudio Romero L
    M.Sc., Universidad National Aut, Managua  , NICARAGUA
    Email:claro@cigeo.edu.ni 
     
  5. Armando Ugarte, Universidad Nacional de Ingenier, Managua  , NICARAGUA
    Email:augarte@ibw.com.ni 
     

Reviewer(s)

  1. Andrew W. Charleson
    Associate Professor
    School of Architecture,  Victoria University of Wellington
    Wellington 6001, NEW ZEALAND
    Email:andrew.charleson@vuw.ac.nz