Seismic-Proof Buildings in Developing Countries

Introduction

Article published August 2017, discusses seismic-proof buildings in developing countries.
Ductile seismic frames may not always be effective due to construction details and design rules.
Artwork "Straight" by Ai Weiwei denounces corruption in shoddy construction after the 2008 Sichuan earthquake.
Safe constructions are crucial in developing countries due to non-expert workers.
Paper provides a brief history of frame structures and wall-based structures within Earthquake Engineering.
Box-type wall structures offer superior structural properties compared to conventional frame structures.
Advocates for a shift from ductility-based Earthquake Engineering to strength-based design using box-type wall structures.
Keywords: seismic-proof buildings, developing countries, box-type structure, insulating concrete forms, numerical modeling.
Low- and middle-income countries experienced 53% of disasters globally but 93% of fatalities.
Earthquakes, tsunamis, cyclones, and flooding disproportionately affect poor populations in unsafe buildings.
Expected need for one billion new dwelling units by 2050, making safe building practices a main challenge.
High-income countries have improved building codes over the past century.
Low- and middle-income countries often adopt regulations from other countries without adaptation.
World Bank recognizes the need for specific adaptation to local factors affecting compliance.
Smart Shelter Research aims to establish reliable knowledge of semi-engineered construction techniques using local materials.
Between 2007 and 2012, Smart Shelter built schools and houses in India, Nepal, and Indonesia, instructing local communities.
Exterior walls of schools display explanations and illustrations of construction techniques.
Lessons learned reveal that modern reinforced concrete (RC) framed constructions are often not properly built in developing countries.
Collapse of RC school buildings in China during the 2008 Sichuan earthquakes caused numerous fatalities.
Ai Weiwei's artwork "Straight" denounces substandard building methods in China.

Frame Structures: The Need of "Ductility-Based" Seismic Design

Masonry-wall constructions were used for 10,000 years.
Steel and concrete introduced during the Industrial Revolution led to the development of frame structures.
First frame building structures were realized during the reconstruction of Chicago after the "Great Fire" of 1871.
Tall steel buildings made Chicago known as the "Skyscraper City."
Early steel frames were braced by diagonal elements for wind load resistance.
"Unity" building (steel frame, 1892) and "Monadnock" Building (tallest masonry building) in Chicago are examples.
First RC frame buildings were realized in France in the late nineteenth century.
Pioneers of RC frames were aware of limited lateral strength capabilities.
Frames were primarily conceived for gravity loads and internal space flexibility.
Robert Maillart conducted experimental tests on RC frames to assess flexural capabilities.
Experimental tests aimed at assessing lateral strength of RC frames predated recorded earthquake data.
1933 Long Beach Earthquake considered "the most important step toward the development of Earthquake Engineering."
Before modern Earthquake Engineering, RC frames were commonly adopted, leading to research on their seismic behavior.

"Ductility-Based" Seismic Engineering: Vision 2000 and Actual Seismic Codes

During the 1971 San Fernando Earthquake, RC frame structures experienced large inelastic responses and severe damages.
Highlighted the need for large ductility to ensure a good seismic behavior.
SEAOC Vision 2000 Committee (1995) proposed Performance-Based Seismic Design (PBSD).
Followed by research trends in Direct Displacement-Based Design procedures.
New design philosophies exploit non-linear deformation capacities, requiring knowledge of structural and non-structural components behaviors.
Framework addresses life-safety, damageability, and functionality issues in terms of Performance Objectives.
Performance Objectives define a building's performance level under a given hazard level.
For a building of "basic" importance (dwellings), seismic design must satisfy:
- Frequent earthquake: Fully Operational performance level.
- Occasional earthquake: Operational performance level.
- Rare earthquake: Life Safety performance level.
- Very rare earthquake: Near Collapse performance level.
Exploitation of complete inelastic capacity requires:
- Sophisticated design prescriptions in modern seismic codes.
- Application of precise and complex procedures during construction.
New trends introduce additional uncertainties in design and complex detailing in construction.
These issues are more relevant when dealing with buildings in developing countries.

Lateral Systems for Tall Buildings: The Premium for Height

In the late nineteenth century, most tall buildings employed steel rigid frames with wind bracings or rigid concrete frames.
Their significant height was accomplished through excessive use of material and overdesign.
Primary structural skeleton of a tall building is schematized as a cantilever beam.
Building must have adequate shear and flexural strength to resist lateral loads.
Fazlur Khan realized that as the building becomes taller, there is a high "premium for height" in terms of material required.
To reduce such premium, more efficient lateral resisting systems are to be used.
In 1969, he classified structural systems for tall buildings in the form of "Height for structural systems" diagrams.
Frames adequate for buildings up to around 10 stories.
For higher constructions, shear walls, core supported structures, tubular structures, modular or bundled tubes are much more efficient systems.
Examples of innovative structural solutions are in pioneering buildings, such as:
- John Hancock Building (completed in 1969) and Sears Towers (completed in 1974) designed by Khan.
- Onterie Center designed by Skidmore, Owings & Merrill (completed in 1986).
- 30 St. Mary’s Axe in London designed by Foster (completed in 2004).
- Agbar Tower in Barcelona designed by Jean Nouvel (completed in 2005).
- CCTV in Beijing designed by Office for Metropolitan Architecture (completed in 2008).
The "tube," identified by Khan as one of the most efficient earthquake-resistant systems, has the same working principle of low-rise masonry-wall constructions.
In Italy, more than 60% of the buildings are low-rise masonry buildings constructed before 1970.
These structures are characterized by regular plans with external perimeter walls and interior walls connected to each other to obtain a sort of "box" behavior.
Resistance against lateral loads was merely due to a box-type structural response: in-plane response of the walls parallel to the seismic input with negligible out-of-plane solicitations thanks to proper connections between orthogonal walls along with the presence of perimeter roof beams.
Box-type behavior is guaranteed only if proper connections between the adjacent vertical panels and between the panels and the floor slabs are present.
Coupling a "tubular" behavior with new high-performance construction materials would ensure optimal performances under earthquakes.
Since the use of shear wall buildings is not predominant in most earthquake-prone countries, researchers did not pay too much attention in the study of the seismic response of RC wall structures.
Buildings made by shear walls showed quite superior performances during strong earthquakes, such as the ones which stroke Chile on March 3rd, 1985 and February 27th, 2010.
Shear walls are typically designed to exhibit a ductile behavior by ensuring the ultimate shear strength being higher than the shear corresponding to develop flexural yielding in the vertical boundary reinforcement of the walls.
Detailing of RC elements to absorb and dissipate energy through the development of ductile flexural behavior is not the only way to achieve a satisfactory seismic response during severe earthquake events.
When the total cross section area of walls is large enough, the ductility demand is kept at a moderate level in tall buildings and practically does not develop in low-rise buildings, which thus practically remain within the elastic field.
Shear stresses are limited to values around 0.5 MPa.
Good seismic behavior is ensured by the very large lateral strength and stiffness of RC walls.
Simple rules ensuring the good seismic response of Chilean RC wall buildings have configured in what has been called the typical Chilean RC building.
Even though PBSD procedures were not included in Chilean building codes up to 2010, the experience showed that the typical Chilean building response was close to the Operational performance level under a rare earthquake and close to the Fully Operational performance level under an occasional earthquake.
A tube (or tubular structure) can be defined as a three-dimensional structural system that utilizes the entire perimeter to resist lateral loads.
Fazlur Khan was the first to realize that a structure could be treated in a holistic way.
He first introduced the concept of tubular structures in 1961.
Tubular structures involve a range of related structural forms: framed tube, tube-in-tube, bundled tube, braced tube, and composite tube.
The basic design philosophy is to centrifuge as much as possible the lateral resisting elements, by locating them around the perimeter so that the building flexural rigidity is close to the one of a tubular cantilever continuous beam.
The framed tube consists of four orthogonal rigidly jointed frame panels, having quite closely spaced columns connected by deep spandrel beams at each floor.
The exterior tube is designed to entirely carry the lateral loads.
The frames parallel to the lateral load direction act as the "webs" of the tube cantilever, while those perpendicular to the lateral load direction act as the "flanges."
The vertical gravity loads are resisted partially by the exterior frames and partially by some inner structures (columns or cores).
Given that the horizontal loads are resisted by the exterior tube, the tubular structure has noteworthy architectural advantages since more freedom in planning the interior spaces is permitted.
Even though the flexibility of the spandrel beams does not ensure the conservation of plane sections thus determining additional (or secondary) stresses concentrated in the outer columns (related to the shear lag phenomenon).
To reduce the shear lag effects, additional interior tubes may be added to create so-called bundled tube structures.
Another structural solution leading to a similar global behavior is based on the use of coupled shear walls to form the interior web of the framed tube.
An even more efficient structural system is the so-called braced tube.

From Nervi’s Ferrocement to Modern Insulating Concrete Forms (ICFs)

Ferrocement is a building material composed of cement, sand, water, aggregates and a metallic mesh.
The ACI Committee 549 (2010) has given the following definition for Ferrocement in their report in 1988: “Ferrocement is a form of reinforced concrete using closely spaced multiple layers of mesh and/or small diameter rods completely infiltrated with, or encapsulated in, mortar.”
It was invented by Joseph-Louis Lambot in the mid of nineteenth century for the construction of a boat.
Later in the twentieth century, the Italian engineer Pier Luigi Nervi patented the ferrocement (1943) and used the new material, together with innovative construction techniques (known under the name of “Nervi system”) for the realization of revolutionary constructions for the time, such as various soccer stadiums.
The main peculiarity of the ferrocement is the use of a fine and closely spaced mesh grid which allows to uniformly spread the steel mechanical properties within the entire structural element.
The presence of the metallic mesh grid leads to a structural material with good behavior both in compression and tension as well as superior deformation capacities with respect to the traditional RC.
Ferrocement, thanks to its enhanced mechanical properties, was also used to realize very thin shell structures such as the one covering the Nervi’s Palazzetto dello Sport in Rome (completed in 1960).
Insulating concrete forms are forms used to hold fresh concrete that remain in place permanently to provide insulation for the structure they enclose.
Their history dates back to after World War II, when blocks of treated wood fibers held together by cement were used in Switzerland.
In the 1940s and 1950s, chemical companies developed plastic foams, which by the 1960s allowed a Canadian inventor to develop a foam block that resembles today’s typical ICFs.
Around the same time, Europeans were developing similar products as well.
In the 1980s and 1990s, some American companies got involved in this technology, manufacturing blocks and panels or planks.
Even in Italy during the last decades several construction companies started to produce structural panels exploiting the properties of ICFs and ferrocement-like materials, obtaining wall systems capable of ensuring high structural, acoustic and thermal properties together with reduced costs, known under the name of SAAD (Italian acronym for structural systems based on EPS and smeared reinforcement).
The insulating layer of SAAD systems is typically realized in EPS (expanded polystyrene synthetized), a lightweight and resistant polymer, characterized by superior insulating properties and optimal benefit–cost ratios.
Two broad categories of panels based on the use of ICFs are produced by Italian companies:
- The structural wall is composed by a single inner insulating layer with two external structural RC layers appropriately connected by transversal connectors.
- The structural wall is composed by two external insulating layers with a central RC core.
Different insulating materials may be used, including EPS, NEO, XPS, LDR, and LDV.

RC Tubular Structures Made of Sandwich Walls: A Safe Solution for Developing Countries

During the last decades, the structural properties of a specific RC sandwich wall construction technology developed by a specific Italian company (Nidyon Costruzioni) have been extensively investigated at the University of Bologna.
The structural system is based on the production of prefabricated reinforced polystyrene panels (referred to as modular panels) with fixed length of 1,120 mm and height corresponding to the inter-storey building height.
The panels are made of a single expanded polystyrene (EPS) layer with thickness between 60 and 160 mm, reinforced by two nets of Ø 2.5 mm steel wire mesh (spaced at 5 cm) anchored to the external faces through Ø 19 mm steel ties, that connect also the opposite nets.
Once erected at the construction site, the modular panels act as support for the cast of the external 4 cm tick concrete layers (in most cases shotcrete).
Additional reinforcements (usually 1 + 1 Ø 12 bar and Ø 8/500 mm U-shaped bars, made up of B450C steel) are added around the openings (doors and windows) and close to the edges of the wall (to provide extra strength along the side).
The connections between the walls and the foundations are made through U-shaped Ø 8 mm anchor rods normally spaced at 500 mm.
The amount of reinforcement provided by the two steel meshes, together with the typical thickness of the two concrete layers, leads to a reinforcement ratio equal to 0.245%.
The solution allows to adequately couple thermal and structural performances as well as fast construction with reduced construction costs and appears particularly suitable for low-rise residential buildings, with squat structural walls characterized by an aspect ratio around 1.0.
For instance, after the 2009 l’Aquila Earthquake, a complex of seven 3-storey base-isolated residential buildings (700 m^2 surface) has been completed in 18 days using the Nidyon technology, with a construction cost in the order of 500 €/m^2 (reconstruction plan “Piano C.A.S.E.”).
The “Nidyon” structural solution is the result of a comprehensive research program that has been developed by the company in collaboration with the University of Bologna since the end of the 1990s.
The most significant experimental tests conducted during the years include:
- Tests at material level.
- Test at single panel level: uniaxial compression tests (with and without eccentricity), diagonal compression tests, slip tests, and out-of-plane tests.
- Connections tests (orthogonal walls and foundations).
- Seismic tests on single panels (in-plane reversed cyclic tests of specimens with and without opening).
- Seismic tests on a full-scale H-shaped structure (in-plane reversed cyclic tests).
- Seismic tests on a full-scale 3-storey building (shaking-table tests at Eucentre Lab, Pavia).
As far as the seismic behavior of a single panel is concerned, the results of the pseudo-static tests revealed that the in-plane seismic response of a single RC sandwich panel is comparable to that of conventional RC squat walls with similar dimensions and reinforcement ratios.
The main results of the experimental tests may be summarized as follows:
- The initial stiffness is approximately equal to 40 kN/mm.
- The maximum strength increases with the applied axial load and it is in the range of about 300–370 kN.
- The equivalent damping ratio is slightly larger than 0.05 for low values of the drift (0.1–0.2%.)
- The displacement ductility capacity is at least equal to 4.
- The drift at yielding is approximately equal to 0.1%.
- The panels are able to withstand horizontal load up to inter-storey drift equal to 1.3%.
- The tested panels are able to withstand large horizontal loads (approximately 100 kN/m, which roughly corresponds to the elastic seismic demand of a 5-storey residential building located in a high-risk seismic zone).
The shaking-table tests performed on the full-scale 3-storey building evidenced that the prototype building was able to sustain increasing levels of peak ground acceleration (PGA) up to 1.2 g without visible damages.
- The prototype building exhibited a dynamic response in terms of fundamental frequency that can be simulated using a linear elastic FE model assuming partially cracked concrete conditions.
- The prototype building exhibited “unexpected overstrengths” which were attributed to the pre-cracking concrete contribution in the shear strength (not manifested in the pseudo-static tests performed on the single panels).
All these results showed that the use of RC sandwich panels for low-rise buildings leads to superior seismic performances without particularly complex detailing and with quite competitive constructions costs.
In 2016, a residential complex made of 4-storey buildings to be realized in Mumbai (India) has been designed by a local company in partnership with Nidyon Costruzioni srl using the sandwich panel technology presented in the previous section.
The typical building is made of eight apartments per floor for a total number of 32 apartments per building.
Each apartment has a total surface of 47 m^2.
The construction cost of the building can be estimated around 65 €/m^2.

Case Study: Nepal’s School Buildings Made of RC Sandwich Walls

Between 2007 and 2012, Smart Shelter Foundation built 15 schools in Nepal, following general rules of thumb, which are commonly referred to as “best practice.”
- Hollow concrete blocks of 150 mm thickness.
- Heavy and traditional rubble stone walls of 40 cm thickness.
All buildings have withstood the 2015 Gorkha earthquakes (7.9 magnitude) without any significant damage.
The typical three-classroom school building is characterized by 1:3 width versus length ratio with almost square classrooms of 5 m × 5 m dimensions, resulting in a maximum length of the total building volume of 15 m.
In case, four or more classrooms are needed, they are divided in separate volumes of maximum three classrooms, with a gap in between the buildings of minimum 10 cm length.
Horizontal bands that tie up the walls are inserted at five different levels to provide additional lateral strength:
- Plinth beam foundation level.
- Sill beam under the window level.
- Stitches at the corners and t-sections.
- Plinth beam over all doors and windows.
- Top beam on top of the walls.
Both school designs follow these general rules of thumb:
- No more than 50% of wall openings are included, with sufficiently wide piers of at least 0.6 m, but preferably 0.9 m or more.
- For the heavy rubble stones, the preferred maximum wall height is 2.6 m, whereas the concrete block walls are maximum 3.0 m high.
One major difference between the two systems is the presence of vertical steel bars in the thinner walls with hollow cement blocks which are not inserted in the thick rubble stone walls.
The vertical steel bars introduced in the buildings made by hollow cement block walls are placed in the corners, t-sections, and next to the openings.
They are inserted to increase the wall flexural strength.
As part of the technology transfer in the villages in Nepal, all drawings and details are provided in Nepali language.
The seismic performances of the same school building designed by Smart Shelter Foundation but made of sandwich RC panels instead of rubber stones are investigated.
The use of ICFs would reach higher levels of seismic performance as well as thermal and acoustic insulation, still ensuring low construction cost and time, comparable to the traditional technique used to build the schools mentioned.
The single panels have a total resistant thickness of 100 mm.
The finite element model of a single block of the typical school layout by Smart Shelter Foundation is represented in Figure 14.
The building model is subjected to the 1940 El Centro earthquake base acceleration (Imperial Valley record, PGA approximately equal to 0.3 g).
From the envelope of the maximum shear stresses in the longitudinal wall (Figure 14) it can be noted that the peak shear stresses are around 0.1–0.2 N/mm^2, thus leading to first concrete cracking.
The reduced values of the shear stresses obtained from the numerical time-history analysis are a first indication that the building performs in a good way under seismic excitation.

Conclusion

Paper provides a historical overview from the first frame structures to modern RC tubular structures realized with sandwich walls within an Earthquake Engineering perspective.
The concept of ductility has been then introduced to overcome the intrinsic limited seismic capacity of framed structures.
Soon it was realized that frame structures are not an adequate lateral resisting system for tall buildings, whereas wall structures with box-type behavior due to their geometrical configurations have potential superior structural performances when subjected to lateral loads.
Such superior properties of box-type wall structures envisage a change of paradigm from the actual “ductility-based” Earthquake Engineering toward a “strength-based” design, exploiting the use of box-type wall-based structures even for the case of low-rise buildings.
use of this solution can easily yield to almost 100% safe buildings against earthquake, e.g., earthquake proof buildings.
the use of modern construction techniques, such as ICFs can even allow to couple optimal structural performances with good insulating properties and reduced construction costs, thus resulting an appealing solution for constrictions to be realized in developing countries.