A 7.8 magnitude earthquake hit off the coast of Sarangani, Mindanao on 8 June 2026. The recent quake further exposed the Philippines to the fury of earthquakes. Being in the Pacific Ring of Fire with many faults and subduction zones active, Philippines is highly seismic and this risk must be considered during planning, design, and construction of buildings and other structures.

While earthquakes cannot be prevented, their effect on buildings can be minimized by following proper engineering techniques, strict adherence to building codes, and good quality of construction. The effect of an earthquake on any structure is determined prior to the earthquake itself. Important decisions regarding design, detailing, and construction of building affect its seismic performance.

UNDERSTANDING SEISMIC DESIGN REQUIREMENTS

For building design in the Philippines, the minimum requirements for earthquake resistance are defined in the National Structural Code of the Philippines (NSCP).

Earthquake forces are dynamic in nature and in seismic design; the seismic forces induce movement in a building due to ground acceleration. This movement in turn causes the building to resist the movement due to its own mass, and this resistance is called an inertial force. This force acts laterally to the structure inducing the movement, and its magnitude is directly proportional to the mass of the building. The stress, deformation, and torsion induced in a building due to these forces can be very large compared to those induced by gravity loads.

Design for earthquake resistance today does not aim for 100% resistance to any amount of earthquake motion and consequent building damage. It aims for a performance from buildings in earthquakes which is acceptable. Acceptable performance for a building is for it to remain stable in strong earthquakes so that it does not collapse, and for its occupants to be able to evacuate the building in time to be safe.

Performance-based design principles generally focus on:

• Immediate Occupancy for minor earthquakes
• Life Safety during design-level earthquakes
• Collapse Prevention under maximum considered earthquakes

The Performance Objective(s) for a project will generally influence the choice of structural systems and detail design of the structural elements during the remainder of the design process.

STRUCTURAL SYSTEMS FOR SEISMIC RESISTANCE

A building’s performance during an earthquake largely depends on its lateral force-resisting systems.

Reinforced concrete shear walls are perhaps the most effective seismic system for high-rise residential, commercial, and institutional buildings in the Philippines. A very effective seismic system reinforced concrete shear walls can develop a large amount of lateral stiffness that will in turn limit inter-story drifts during an earthquake. Seismic forces are efficiently transferred to the foundations.

Moment-resisting frames also transfer forces through the beam-column connection. Architectural design of building exterior can be more easily handled with these systems. However, their resistance to large lateral loads is primarily based on flexure. Hence, these structures will experience more inter-story displacement during a strong earthquake than a building with shear walls laterally supported. Their behavior and safe design are heavily dependent on proper connection detailing to allow ductile failure as specified by earthquake resistant design.

Present structures are generally designed with a dual system comprising a shear wall system and a moment frame system to achieve an ideal combination of stiffness, strength, and seismic energy dissipation. Continuity of the structural system’s load path is important. Seismic forces developed by a diaphragm are transferred to the various structural elements that form the diaphragm. These forces are then transferred vertically through the various elements and down to the structural elements of the foundation system. To augment the performance objectives of high-rise buildings, Supplemental Energy Dissipation (SEDs) devices can be added to reduce its seismic response, thus enhancing the structural system’s resilience.

In addition to the fundamental lateral force-resisting system of a building, there are several types of Supplemental Energy Dissipation (SED) devices that are incorporated into modern high-rise buildings to further enhance their seismic performance. These systems allow for the structural members to remain largely in the elastic phase during strong earthquakes, with some part of the earthquake input energy being dissipated by the SEDs. Thus, the structural response, including the inter-story drift, can be significantly reduced while the potential damage to the structure can be minimized.

Buckling Restrained Braces, or BRBs, are being utilized in several of D.M. Consunji, Inc. (DMCI)’s high-rise residential projects. These energy dissipating devices are being utilized in The Viridian in Greenhills and in The Imperium at Capitol Commons. Viscoelastic Coupling Dampers, or VCDs, have been designed and applied in The Connor at Greenhills. On the other hand, the Outrigger Structural System has been incorporated into the design of The Empress at Capitol Commons. The primary objective of these advanced seismic technologies is to improve the wind and earthquake resistance of high-rise buildings. They increase structural stiffness, control inter-story displacement during strong wind and seismic forces and facilitate energy dissipation to prevent structural damage while allowing the main structural elements to remain in the elastic range.

The Imperium at Capitol Commons, a 62-story luxury residential tower has a lateral load resisting system consists of ductile reinforced concrete shear wall coupled with outrigger columns, connected by the Buckling Restrained Braces (BRBs). The BRBs of The Imperium are connected between five floors. Lower BRBs are between 27th and 31st floors and the remaining BRBs are between 51st and 55th floors.

The Outrigger Structural System was introduced to The Empress at Capitol Commons, a 56-story luxury residential tower with ductile reinforced concrete shear wall and Outrigger Structural System connected to the perimeter columns. The Outrigger Structural Systems are installed between the 36th and 40th floors.

The Viscoelastic Coupling Dampers (VCDs) is integrated in The Connor at Greenhills. The Connor is a 58-story residential tower consists of ductile reinforced concrete shear wall with VCDs between 32nd and 35th floors.

Although these advanced seismic technologies are generally used for taller buildings, DMCI had the privilege of working with a developer that implemented these advanced technologies in selected high-rise residential projects which showcases the latest advances in structural engineering that exceed minimum code requirements to provide better building performance and increased structural resilience.

DUCTILITY: THE FOUNDATION OF MODERN SEISMIC DESIGN

Historically, structures were designed based on strength, but earthquake resistant construction today is based on ductility or inelastic behavior with stability of the structure being the major objective. Designing structures for the full elastic earthquake force would be too expensive for most projects. Therefore, structures are designed to undergo some degree of inelastic behavior while remaining stable. To achieve this, the reinforcement must be properly detailed to allow for the large deformations that occur during an earthquake.

To form a strong structure in reinforced concrete that can undergo sufficient plastic deformations without collapse and at the same time maintain the stability of the structure, detailed reinforcement is required.

In reinforced concrete structures, ductility is achieved through:

• Proper reinforcement of columns and boundary components to confer confinement
• Adequate development and lap splice lengths
• Strong-column weak-beam design philosophy
• Enhanced detailing at beam-column joints
• Capacity-based design principles

Allowing a structure to dissipate most of the seismic energy by way of controlled yielding while remaining stable and not collapsing are the objectives of these measures.

THE ROLE OF GEOTECHNICAL ENGINEERING

The earthquake resistance of structures is significantly influenced by subsurface conditions.

During an earthquake, the structural performance of a building can be affected by the different seismic ground motions in various soils by means of soil-structure interaction and soil amplification. To determine the effects of seismic actions on a structure, a geotechnical investigation must be carried out in the initial design phase of a structure.

Site evaluations typically assess:

• Soil classification and bearing capacity
• Groundwater conditions
• Liquefaction susceptibility
• Settlement potential
• Slope stability concerns

Liquefaction of temporary nature is expected to occur in saturated granular soils during intense earthquakes and is of particular concern for coastal structures as well as structures built on reclaimed land. Various methods for ground improvement, deep foundations, as well as alternative specialized foundation systems are available to resist such loss of strength in the soil.

CONSTRUCTABILITY AND SEISMIC PERFORMANCE

Even the best design for earthquake resistance can be sabotaged by poor construction.

Structural design is implemented during the construction phase, and it is within this phase that the designed structures can achieve their performance goals. A variety of factors can affect this goal, however. These factors include structural elements being constructed outside of the intent of the approved design plans, reinforcement not being placed accurately, concrete not being properly compacted, and a general lack of proper quality control measures throughout the construction process.

A Constructability Review, to be conducted during the design phase of a project, covers several issues which affect the constructability of a building. These can include the method of construction, the detail of the structural reinforcement, dealing with heavily reinforced areas, accessibility to work areas and the necessary coordination between the structural, architectural, and MEP (mechanical, electrical, and plumbing) aspects of the building.

Building Information Modeling (BIM) can be very beneficial for the constructability review as it allows for a clash detection as well as for multidisciplinary coordination prior to the start of the construction. This way any field modifications that could possibly affect the structural performance of the building are avoided.

LESSONS FROM THE 2026 MINDANAO EARTHQUAKE

A strong earthquake hit Mindanao recently again to point out a combination of engineering, planning, and good implementation is necessary to achieve seismic resistance of structures. While earthquakes are unpredictable, their impact can be drastically reduced if buildings are designed and constructed in compliance with the latest provisions for seismic design and are analyzed and detailed correctly as well.

The recent earthquake in Mindanao also emphasized the importance of structural analysis and good detailing, as well as quality construction practice, aside from strict compliance with the requirements of the National Structural Code of the Philippines (NSCP). It emphasizes the need for sustained investment in earthquake resistant structures given the rapid urbanization happening in the country.

For the developer, the contractor, and building owner; earthquake resistance and earthquake resilience of buildings is not merely a matter of adhering to the minimum construction standards to protect investment against losses occasioned by a disaster. Their interest in earthquake resistance and resilience of buildings is more inclined towards a long-term view: to continue to operate the building and to protect the communities and environment in which the building is situated.

CONCLUSION

Earthquake-resistant design is increasingly a component of responsible building construction in earthquake affected countries as recent earthquakes are reshaping the Philippine built environment. With the right earthquake-resistant design comprising strong structural systems, ductile detailing, complete geotechnical analysis, and good construction practice, buildings can be earthquake-resistant.

What lessons can we learn from the June 2026 Mindanao earthquake to improve the earthquake resistance of the Philippine built environment? Earthquake resistant design of buildings to resist earthquakes requires more than one feature of a building to be designed for earthquake resistance. This is to include the earthquake-resistant design of the structure and ductile detailing of its components. This also includes a comprehensive geotechnical evaluation of the foundation of the building and good construction practice to implement the earthquake resistant design of the building. It also requires the coordination and teamwork of all engineers involved in the design of a building and good quality of their work.

D.M. Consunji, Inc. (DMCI) is grateful for developers who go beyond the minimum code requirements for structural design and performance. In DMCI, they work with clients in selecting the best seismic technology, in developing the best structural design, and in selecting the best project delivery method that results to a collaborative working relationship that results to a more resilient structure and in building for the future of Philippines’ construction industry.

ABOUT THE AUTHOR

Christine Ave V. Tragura, MBA, CLSSYB

Ave is a licensed Civil Engineer with over 16 years of international engineering and infrastructure experience across the construction, oil and gas, mining, manufacturing, and energy sectors in the Philippines and Singapore. She is currently serving as Digital Communications Senior Supervisor at D.M. Consunji, Inc. (DMCI).