Mexico City’s tragic anniversary
On 19 September 2017, a powerful earthquake rocked Mexico exactly 32 years to the day that a similar quake struck the country. Building codes introduced following the 1985 quake proved invaluable, as 95% of the buildings that collapsed or were damaged in 2017 were built before 1985.
As a result of the lessons learned following the 1985 earthquake, a major drill is carried out in Mexico City every year on 19 September. During the drill, the sirens of the seismic alert system sound, and people evacuate buildings in an orderly fashion and proceed to places designated as free or safe zones. This year, the drill started at 11 a.m. Two hours later, the seismic alarms sounded again. But this time it was for real – a 7.1 magnitude earthquake with its epicentre close to Axochiapan on the border between the states of Puebla and Morelos.
Just a few seconds’ advance warning
In the country’s capital, the alarms went off only 15 seconds before the ground started to shake. The Mexican early warning system is able to give longer warnings of subduction earthquakes originating on the Pacific coast, but not of intraplate earthquakes like the one that occurred that afternoon. Nevertheless, this short notice was sufficient for many people to get outside or to take suitable protection measures inside buildings.
The earthquake caused damage in Mexico City, as well as in other Mexican states. 369 people died. Overall losses amounted to US$ 6bn, of which US$ 2bn were insured. Various sectors incurred significant damage, such as commerce, industry, schools, roads, and water utilities. In terms of economic losses, the worst-affected sector was housing. In the states of Morelos and Puebla, dwellings with vulnerable structures such as unreinforced masonry, buildings made of adobe and many old cultural buildings like churches or convents suffered severe structural damage.
Not since 1985 had Mexico City seen a similar total collapse of buildings and so many people trapped and killed. A large number of fatalities (two thirds of the total) occurred in the city. Most of the severe damage was concentrated in the so-called Transition and Lake zones, as defined in the seismic microzonation of the city (see map on page 41). In similar earthquakes that have hit Mexico City in the last century – i.e. 1957, 1979 and 1985 with magnitudes of 7.6, 7.6 and 8.0 respectively – these areas also suffered the worst damage. How the ground responded to the earthquake obviously played an important part in the damage to buildings.
Almost 95% of the buildings that collapsed or were severely damaged were constructed prior to 1985, which means that they had previously withstood the ground motion caused by the 1985 earthquake. We do not know for sure whether those structures were reinforced after the 1985 quake. Based on the number of collapsed or partially collapsed buildings, it can be said that new buildings (later than 1987) withstood the earthquake well.
The problem of soft storeys
About half of the totally or partially collapsed buildings displayed one of the following features: (1) they had a soft storey, e.g. ground floor columns with little deformation capacity; (2) they were located on corners and thus fundamentally affected by torsion, a rotational movement of the building which the structure has little capacity to withstand; (3) their structural system was based on reinforced concrete columns with flat slabs, without any beams between the columns.
The use of a flat-slab system is limited in earthquake zones in the building codes of some countries, for example in New Zealand. The American Concrete Institute code (ACI 318) places restrictions on the use of flat slabs in areas considered to be highly seismic, such as California. The Mexican code restricts the use of flat slabs through the limitation of the relative horizontal displacement between adjacent storeys. The tolerable displacement is dependent on the storey height. In the 1985 earthquake, many flat-slab buildings had collapsed.
Seismic reinforcement possible but difficult
The practice of using soft storeys in earthquake zones as a construction option to accommodate parking spaces or commercial stores has usually proved highly problematic. Furthermore, the lack of column ductility, i.e. insufficient ability to bend without breaking, has caused dramatic building collapses. Trying to resolve this problem through structural measures is not easy. Strengthening the storey with additional walls of reinforced concrete or with additional steel frames in order to increase rigidity, or through the steel-plate jacketing of columns in order to increase their ductility, are among the structural options used.
The corner buildings that totallycollapsed evidently lacked good torsional design. As was also observed in 1985, ignoring the necessary torsional and shear strength of column or wall elements is a serious error. However, collapses are not always due to insufficient structural design. In other cases of severe damage, the absence of supervision during construction, poor quality of materials, a change in the original use of the building, or alteration of the original structure (e.g. removing interior walls) without the additional requisite structural reinforcements could have had an influence. The municipal government of Mexico City was quick to react, launching an initiative in November 2017 to include seismic retrofitting in the building code.