Soft-Story Failure During Strong Earthquake, Potential and Its Solution

Soft-story is a condition of multi-story building structures with one or more story stiffness much smaller than those of the upper floor. The condition occurs in buildings with an open ground floor while the floors above contain rigid walls as room divider. This kind of soft-story is triggered by the interaction between the wall and the frame forming a composite structure called infilled frame (IF) which is far more rigid compared to the open frame structure underneath. In the event of a strong earthquake, this soft-story will fail first so that it will no longer capable of supporting the load from the floor above it. Eventually, it cause total failure of the building. Accordingly, soft-story failure is very dangerous and should be avoided.

               Criteria for soft-story have been set in the building codes of many countries including the SNI for earthquakes in Indonesia. However, soft-story failures always occur whenever strong earthquake hit. One of the causes of this soft-story mechanism is the "unexpected" interaction between the frame and the infill wall. For buildings with continuous wall position, from the bottom to the top level, there will be no soft-story issue. However, when the wall position is not continuous, then stiffness imbalance will cause soft-story mechanism. As an example, a hotel building with ground floor is open for parking while the upper floors are bedroom full of masonry walls, when analyzed as IF, it is found that the ground story stiffness is less than 70% of that above it. This means that this structure has the potential to experience soft-story failure. An example of such analysis can be found in the following link: http://ced.petra.ac.id/index.php/civ/article/view/18933.

               Researchers have known the interaction of the frames and the infill wall for a long time. In 1991 Smith and Coul published a book with a special chapter on Infilled Frame, which referred to related researches. However, until now many building engineers do not understand the true behavior of IF structure because, among other things, the current building codes in many countries (including Indonesia) have not explicitly regulated the IF structure. In addition, the existence of walls in multi-story buildings can easily be changed, and most of the times, the walls contain opening for windows and doors. The last reason is more than enough for the building engineers to “think” that the effect of wall is not significant and therefore, can be excluded in the design. Unfortunately, this thought is not true at all. A study on the effects of opening in the infill wall found that IF with opening ratio of up to 60% is still much stiffer than an open frame. Therefore, despite the large opening, the wall still needs to be considered in the analysis. See the following link: https://www.sciencedirect.com/science/article/pii/S1877705815034359.

               The method widely accepted for analyzing IF is the diagonal strut method. FEMA suggest strut width equations for solid walls. For walls with opening, there are researchers who suggest a reduction in the strut width from FEMA due to opening. In the author's observation, opening on the walls of high-rise buildings are always strengthened using practical columns and beams around the opening. It provides confinement effect so that the walls become stronger along the edges of the opening. For this reason, a simplified model was proposed to calculate the strut width of IF with confined opening, which can be easily used to design IF structure with wall opening. See this link: https://www.researchgate.net/publication/332654541_Analysis_and_Design_Methods_for_Infilled_Frames_with_Confined_Openings

               Cases of soft-story failure in the Kobe earthquake (1995), Padang (2009), and Palu (2018) and the analysis results of many researchers proved that structural analysis must be carried out carefully by modeling all structural components that may involve in resisting earthquake loads. Ignoring the walls in the analysis of frame structures, for example, makes the soft-story potential is undetectable. So, in the event of strong earthquake, we must be prepared with the risk of total failure. How is the potential for strong earthquake to occur and how is the readiness of building in Indonesia are yet to answer.

               Indonesia is in the ring of fire. Geologically, from both the tectonic plate and the fault line, Indonesia is at the confluence of three major world plates, namely Eurasia, Indo-Australia and the Pacific. Collision of these active plates has the potential to trigger a strong earthquake. Therefore, earthquakes can occur at any time with various magnitudes.

               One of the objectives of earthquake resistant design is to promote the design and construction of buildings that consider earthquake hazards and minimize the risk of damage and loss of life. In reality, most buildings built in Indonesia, were non-engineered buildings, not designed according to the earthquake resistant concept. Several engineered buildings also do not meet the requirements for earthquake resistant structures such as the presence of soft-story potential. The cases occur in many public buildings such as hotels, dormitories, offices and other buildings that utilized the ground floor as an open area while the floors above contain walls that were not considered in the design. How to mitigate the soft-story issue?

               The government, through the National Standard Agency (BSN), needs to immediately revise the building codes to include IF structure, referring to the latest research results and codes from other countries such as India, Mexico, Turkey, Japan and so on. Institutions that have experts on this field, such as universities, should involve their experts during design and construction process of building in their home campus. The reason is that, often time, the consultant with professional certification are not as skilled as expected. Authorities should not be confined to the existing laws and regulations. Creative efforts are necessary to anticipate the weaknesses of the prevailing system. Go the extra miles, it is worth the effort.