Report: 160

Building Type: Combined and Confined Masonry Construction

Country: Mexico

Author(s): Arturo Tena-Colunga, Artemio Juarez-Angeles, Victor Hugo Salinas-Vallejo

Last Updated:

Regions Where Found: Buildings of this construction type can be found in most parts of Central Mexico, in particular in the states of Puebla, Tlaxcala, Estado de Mexico, Hidalgo, Queretaro, Morelos, Oaxaca, Colima and is starting to be used in Mexico City as well (Figure 2). The earthquake hazard in this region of Mexico is high. This type of housing construction is commonly found in rural, sub-urban and urban areas.

Summary: It is defined as combined and confined masonry structures those where the bearing/seismic walls are made by alternating courses of lightweight concrete blocks (inexpensive in Mexico) with courses of fired clay bricks (more expensive) and they are confined with cast-in place reinforced-concrete tie-beams and tie-columns (Figure 1). The impact of confining elements in masonry walls includes: a) enhancing their stability and integrity for in-plane and out-of-plane earthquake loads, b) enhancing their strength (resistance) under lateral earthquake loads and, c) reducing their brittleness under earthquake loads and hence improving their earthquake performance. Although combined masonry construction has historical background in Mexico and worldwide (i.e., Tena-Colunga et al. 2009), combined and confined masonry became popular in recent times by the initiative of the inhabitants of the central Mexican states of Puebla, Tlaxcala and Oaxaca. This modern version of combined and confined masonry has been used since the early 1990s. Different arrangements to combine and alternate brick courses with block courses have been used (Juarez-Angeles 2009, Salinas-Vallejo 2009), but the one that it is most commonly used is the one depicted in Figure 1, where three courses of clay bricks alternate with a course of concrete blocks. Usually, this type of construction is being used for housing in rural and urban regions of Mexico, but it has also being used for warehouses and apartment buildings up to three stories high. The most common floor systems used with combined and confined masonry are: a) cast-in-place reinforced-concrete slabs 10 to 12 cm thick and, b) precast beams with concrete block infill and concrete topping (cast-inplace) and, c) cast-in-place waffle flat slab with polystyrene infill. Because of the poor quality of the concrete blocks produced in the central regions of Mexico, combined and confined masonry walls have similar behavior but lower shear strength and ductility compared to traditional confined masonry walls ma

Length of time practiced: Less than 25 years

Still Practiced: Yes

In practice as of:

Building Occupancy: Single dwellingMixed residential/commercial

Typical number of stories: 1-3

Terrain-Flat: Typically

Terrain-Sloped: Typically

Comments: The main function of this building typology is mixed use (both commercial and residential use). The main use of buildings of thi

Plan Shape: Rectangular, solid

Additional comments on plan shape: The typical plan layout is rectangular or nearly-rectangular (trapezoid, etc). It is common to have one-story and two story structures for housing.

Typical plan length (meters): 6-14

Typical plan width (meters): 8-20

Typical story height (meters): 2.2 - 2.4

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

Additional comments on structural system: Vertical loads on the building are resisted by the combined and confined masonry wall system. Lateral loads on the building are resisted by the combined and confined masonry wall system, mostly with rigid and semi-rigid diaphragms made with cast-in-place RC floor systems. Different arrangements to combine and alternate brick courses with block courses have been used (Figure 1 and Figures 3 to 9), as briefly illustrated here and explained in detail elsewhere (Ju#rez-Angeles 2009, Salinas- Vallejo 2009). The most commonly used combination pattern is the one depicted in Figures 1 and 9, where three courses of clay bricks alternate with a course of concrete blocks. The characteristics, dimensions, reinforcement and spacing of the RC confining elements (tie-beams and tie-columns) are identical of the Mexican practice for traditional confined masonry structures made with fired-clay bricks or concrete blocks (for example, NTCM-2004 2004).

Gravity load-bearing & lateral load-resisting systems: This construction type is defined as Combined and Confined Masonry, where the walls are made by alternating courses of concrete blocks with courses of fired clay bricks, confined by reinforced-concrete tie-beams and tiecolumns.

Typical wall densities in direction 1: 5-10%

Typical wall densities in direction 2: 5-10%

Additional comments on typical wall densities: The typical structural wall density is up to 10 %. Usually, higher structural wall density is in the direction perpendicular to the facade.

Wall Openings: The facade walls always have openings for doors and windows, ranging for 30% to 50% of the total area of the wall. Perimeter perpendicular walls to the facade sometimes have openings, but most commonly they do not have openings; therefore, the highest wall density of these houses is commonly in the direction perpendicular to the facade.

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

Modifications of buildings: Extensions to houses (additional rooms built after the initial construction) are common practice in rural and suburban areas of Mexico, particularly when the family grows, for example, extra rooms needed for a married son/daughter with children. Depending on the land availability, these additional rooms are built on the ground level (preferred, primarily in rural areas) or in upper stories (primarily in suburban areas).

Type of Foundation: Shallow Foundation: Rubble stone, fieldstone strip footingShallow Foundation: Reinforced concrete strip footing

Additional comments on foundation: Depending on the availability of materials and the soil conditions, rubble stone strip footing, reinforced-concrete strip footing or a reinforced-concrete slab are used.

Type of Floor System: Other floor system

Additional comments on floor system: The most common floor systems used with combined and confined masonry are: a) cast-in-place reinforced-concrete slabs 10 to 12 cm thick and, b) precast beams with concrete block infill and concrete topping (cast-in-situ) and, c) castin- place waffle flat slab with polystyrene infill. These floor systems can be classified as rigid or semi-rigid diaphragms for the typical spans of the floor systems used in this type of construction, according to recent analytical studies (Tena- Colunga and Cortes 2009, Cortes 2009).

Type of Roof System: Roof system, other

Additional comments on roof system: For single story construction, sometimes metal, asbestos or industrialized cardboard corrugated sheets are used as roof system (Figure 9), usually anchored directly in the walls using nails or screws. Metal corrugated sheets are also used as roof systems in warehouses (Figure 3). Then, for such conditions, these structures should be considered as having no diaphragm or a very flexible diaphragm.

Additional comments section 2: They do not share common walls with adjacent buildings. This is particularly common in rural areas. When separated from adjacent buildings, the typical distance from a neighboring building is several meters.

Who is involved with the design process? ArchitectOther

Roles of those involved in the design process: Combined and confined masonry houses are mostly designed empirically by extrapolating the Mexican construction practice for confined masonry houses made with fired-clay bricks or concrete blocks. Low-income people usually hire a 'maestro' (master bricklayer) to help them 'design' their home and to built it (if the master bricklayer is not a relative or a friend of the family). Middle-income people usually hire an architect or an engineer to design their home to suit their needs (basically an architectural design) and then to take charge of the construction process. If an architect is in charge, it is likely that no structural calculations would be done. If an engineer is in charge, most likely he would do a quick calculation of wall density using the simplified method for seismic analysis of Mexican building codes and would assume that the shear strength of combined and confined masonry is the smallest one of the two involved material, which in Mexico would be the concrete blocks.

Expertise of those involved in the design process:

Who typically builds this construction type? OwnerBuilderOther

Roles of those involved in the building process: Among low-income class the houses are built by the homeowner or a relative of the homeowner. For middle-income people, usually the builder is hired by the owner.

Expertise of those involved in building process: Depending on the budget, the building team can be composed of: a) a team of independent bricklayers or, b) an engineer/architect that directs a team of bricklayers.

Construction process and phasing: Combined and confined masonry is typically built by a team of bricklayers. Sometimes they are supervised the most experienced bricklayer called 'maestro' (master) who is the one responsible for taking charge of construction decisions. In some other cases, an architect/engineer are in charge for the design of the structure and being responsible for the construction process. Conventional manual tools are used by bricklayers to build these structures. The construction of this type of housing takes place in a single phase. Typically, the building is originally not designed for its final constructed size. Depending on the budget of the owner, the construction of this type of housing takes place in a single phase (good budget) or incrementally over time (limited budget). Typically, the building is originally not designed for its final constructed size.

Construction issues:

Is this construction type address by codes/standards? No

Applicable codes or standards: This construction type is not addressed by the codes/standards of the country yet. However, since combined and confined masonry structures are basically sister structures of traditional confined masonry structures made with fired-clay bricks only or with concrete blocks only, most of the procedures and recommendations available in NTCM-2004 (2004) can be used as reference for improving the design of combined and confined masonry. In absence of experimental data to assess design properties for combined and confined masonry (for example, fm*, vm*), one may conservatively assume that these properties would be the same of the weakest of the two materials (in Mexico, concrete blocks), granted that the masonry units (bricks and blocks) and the mortar used fulfill all the requirements set in NTCM-2004.

Process for building code enforcement:

Are building permits required? No

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: In rural and suburban zones, building permits are not required to build this housing type. However, in urban zones, particularly in important cities (for example, Mexico City, Queretaro or Puebla), building permits are required to build this housing type. Perhaps that is one of the reasons that combined and confined masonry has not yet extended in important cities such as Puebla or Queretaro, but in smaller cities, towns and villages of those states.

Typical problems associated with this type of construction:

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

Additional comments on maintenance and building condition: Usually, little or no maintenance is done on the facade (exterior walls).

Unit construction cost: Rural construction cost: 120-150 $US/m2, suburban construction cost: 180-250 $US/m2. This estimated cost includes all that is required in the construction process (plumbing, electricity, finishing, etc.). Labor cost is similar to traditional confined masonry construction made with only fired clay bricks.

Labor requirements:

Additional comments section 3:

Patterns of occupancy: Usually, a single family occupies a housing unit. Nevertheless, poor people may have large families (the original parents and the families of their married sons/daughters with children) sheltered in a housing unit. For residential use, each building has 1 housing unit in it. For apartment buildings, each structure consists of 5 to 10 housing units.

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

Number of inhabitants in a typical building of this construction type during the evening/night: 44474

Additional comments on number of inhabitants:

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

Additional comments on economic level of inhabitants: For poor people, the members of the family usually have either low-income formal job (less than $300.00 U.S. per month) and/or are in the informal economy (self-employed in low-income commercial or manufacturing activities). Therefore, the family needs many of them to work in order to afford living. In such families, it is common that at least one of their relatives has experience as a bricklayer.

Typical Source of Financing: Owner financedPersonal savings

Additional comments on financing: For poor people, the ratio of the Housing Unit Price to their Annual Income is between 3:1 to 4:1. Their typical source of financing for the purchase of a house is almost nonexistent. Poor people usually start buying building materials with their own very small personal savings and/or a loan from a relative, and then start building their home by their own means.

Type of Ownership: RentOwn outrightUnits owned individually (condominium)Owned by group or poolOther

Additional comments on ownership: Ownership with debt, usually to a relative.

Is earthquake insurance for this construction type typically available?: No

What does earthquake insurance typically cover/cost: Earthquake insurance is available as supplement of other insurance (fire, robbery), but people living in these houses usually do not have money to pay for it. As a matter of fact, Mexican household usually do not buy insurance for their home, even if they have the money for that. The reason is that insurance companies (national and international) do not have a good reputation in Mexico.

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: No damage was observed/reported for one and two stories combined and confined masonry houses at small towns in Puebla (Cholula) and Tlaxcala states during the moderate June 15, 1999 Tehuac#n earthquake (Tena-Colunga et al. 2009). This earthquake was particularly damaging for unreinforced masonry churches (known in Mexican Architecture as Colonial Churches ), primarily built from centuries XVII to XIX. Many of these churches experienced partial or total collapses. In Cholula, the main two churches experienced heavy partial collapses, whereas nearby combined and confined masonry houses did not crack. In small towns and villages in Tlaxcala, Colonial Churches experienced extensive shear cracking of the walls of front towers whereas nearby combined confined masonry houses remained undamaged.

Additional comments on earthquake damage patterns:

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: The building is often regular with regards to its plan and somewhat irregular in elevation because of the location of windows openings. At each storey level, the height-to-thickness ratio (h/t) of shear walls is adequate for confined masonry construction (h/t less than 20). Up to date, there is no reliable information about the quality of maintenance for buildings of this type; however, there are no visible signs of deterioration of building materials, particularly in the weaker material, the concrete blocks. This is an important aspect to highlight, as for this type of construction, house owners frequently do not use painted mortar/lime or stucco finishing for facades and therefore building materials (bricks and blocks) are directly exposed to weathering effects (rain, temperature, etc).

Vertical irregularities typically found in this construction type: Other

Horizontal irregularities typically found in this construction type: Other

Seismic deficiency in walls: These walls are made with non-industrial fired clay bricks and lightweight concrete blocks with no quality control and therefore, they do not fulfill the Mexican standards for structural use. The mortar mix used by the people has volumetric proportions:1:2:6 (cement:lime:sand), clearly a mix that is out of what it is recommended in the Mexican and other international masonry codes for seismic zones. Limited shear strength and deformation capacity (ductility). Cannot develop a ductile flexural failure mode. It is a common poor construction practice to build window openings without tie-column (unconfined), which results in lower shear strength, deformation capacity, and walls integrity. Another common deficiency is to leave slender ending walls (H/L>4) know n as mochetas in Mexico without tie-columns at the free end. Typically, tie-columns and tiebeams have stirrups spaced every 20 cm. Therefore, after the initial cracking of the walls, shear cracks propagate through the tie-columns reducing considerably the stability of the wall (besides its strength, stiffness and deformation capacity). To prevent these effects closer stirrups should be used at the ends of tie-columns and tie-beams.

Earthquake-resilient features in walls: High wall density and a reasonably good confinement practice according to Mexican standards (tie-columns separated up to 4 m and tie-beams placed at top and bottom of the walls or separated up to 3m). Good quality of workmanship. These structures have had good performances during moderate and strong earthquakes, such as the M=6.5 June 15, 1999 Tehuac#n earthquake and the M=7.6 January 21, 2003 Tecom#n earthquake.

Seismic deficiency in frames:

Earthquake-resilient features in frame:

Seismic deficiency in roof and floors: Some single story houses and warehouses have industrialized light corrugated sheets as roof system, then performing similar to structures with no diaphragm that are very vulnerable to out-of-plane failures of the walls, specially if orthogonal walls are not well tied together.

Earthquake resilient features in roof and floors: Most structures have rigid, cast-in- place reinforced concrete diaphragms, that are rigid and strong.

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: NTCM-2004 (2004) has general provisions for the rehabilitation of masonry structures and for the correct confinement of masonry structures, including the placement and detailing of tie-columns and tie-beams. Besides, it has recommendations for the seismic design of confined masonry walls strengthened with a welded wire mesh and mortar.

Has seismic strengthening described in the above table been performed? The addition of tie-columns to properly confine windows and door openings and 'mochetas' (very slender walls [h/L>4] with an unconfined or free end) has been successfully conducted in traditional confined masonry structures made with fired clay bricks only or concrete blocks only. In fact, the addition of tie-beams and tie-columns has also been done in very old, former unreinforced masonry structures in Mexico.

Was the work done as a mitigation effort on an undamaged building or as a repair following earthquake damages?: This practice has been done both as a mitigation effort on an undamaged building and as repair following an existing damage due to soil settlements or the action of an earthquake. The use of a wall jacketing by using a welded-wire mesh anchored to the walls through nails and covered with mortar has been used for decades in Mexico to repair cracked confined masonry walls due to soil settlements or the action of an earthquake. This technique has been proved to be very effective for confined masonry structures during experimental tests as it increases both their shear strength and their deformation or ductility capacity (Ruiz and Alcocer 1998). This is one of the reasons that wall jacketing is now included in NTCM-2004 (2004) as an option for original design as well.

Was the construction inspected in the same manner as new construction? The inspection level is similar to the one for a new construction.

Who performed the construction: a contractor or owner/user? Was an architect or engineer involved? Usually, for a retrofit or a strengthening, an engineer is involved. However, for low-income people, the master bricklayer ('maestro') and his team are the only ones involved.

What has been the performance of retrofitted buildings of this type in subsequent earthquakes?: Some confined masonry houses made with fired clay bricks were retrofitted using additional tie-columns or wall jacketing in Mexico City previous to the 1985 Michoacan earthquake. None of them were reported/observed to experience any damage as a consequence of the Ms 8.1, September 19, Michoac#n earthquake.

Additional comments section 6:

• Normas Tecnicas Complementarias para y construccion de Estructuras de Mamposteria NTCM-2004 Gaceta Oficial del Distrito Federal, Octubre 2004

• Mecanismos de resistencia y de deformacion de muros de mamposteria combinada y confinada Juarez-Angeles, A. Tesis de Maestria, Posgrado en Ingenieria Estructural, Division de Ciencias Basicas e Ingenieria, Universidad Autonoma Metropolitana Azcapotzalco 2009

• Evaluacion de la flexibilidad de diafragma para sistemas de piso utilizados en estructuras de mamposteria Cortes, J. A. Proyecto Terminal II, Departamento de Materiales, Universidad Aut#noma Metropolitana, agosto 2009

• Chapter 6: Housing, The Tecomun, Mexico Earthquake January 21, 2003. An EERI and SMIS Learning from Earthquakes Reconnaissance Report Reyes, C., L. Flores, S. M. Alcocer, A. Lang, R. Duran, O. A. Lopez, M. A. Pacheco, H. Juarez, R. Martin-del-Campo, J. Tejada, A. Echavarria and J. Cuenca Research Institute and Sociedad Mexicana de Ingenieria a 206 pp 109-190

• Desempeno experimental de estructuras de mamposteria confinada rehabilitadas mediante el uso de malla de alambre Ruiz, J and S M Alcocer Revista de Ingenieria a, SMIS 1998 No. 59, pp 59-79

• Comportamiento ante cargas laterales de muros de mamposteria combinada unidos con morteros de autoconstruccion Salinas-Vallejo, V. H. Tesis de Maestria, Posgrado en Ingenieria Estructural, Division de Ciencias Basicas e Ingenieria, Universidad Autonoma Metropolitana Azcapotzalco, mayo 2009

• Cyclic behavior of combined and confined masonry walls Tena-Colunga, A., A. Juarez-Angeles and V. M. Salinas-Vallejo Engineering Structures 2009 Vol. 31, No. 1, pp. 240-259

• Evaluacion de la condicion de diafragma rigido o flexible para el empleo del metodo simplificado en estructuras de mamposteria Tena-Colunga, A. and J. A. Cortes Proceedings, XVII Congreso Nacional de Ingenieria a, Puebla, Puebla, CDROM 2009 Paper No. V-23, pp. 1-13