Inadequate shear reinforcement could have contributed to the collapse of a 16-storey building in Taiwan following last week’s earthquake, according to a seismic and structures expert.
Arup UK, Middle East and Africa seismic skills leader Ziggy Lubkowski told New Civil Engineer that shear reinforcement links did not appear to bend as far as recommended for withstanding earthquakes.
Fifty nine people died in the city of Tainan following the magnitude 6.4 quake that struck early on Saturday morning.
Among the devastation the 16 year old, 16-storey Weiguan building collapsed. The steel reinforced concrete building has been a key focus of a ’reconnaisance report’ published on the US engineering and science research body Earthquake Engineering Research Institute’s (EERI) website.
Among the preliminary findings, the report highlights that there may have been no walls at the base floor, an apparent failure of steel coupler joints and potential lack of shear reinforcement for seismic design.
Disucussing the report and its findings on the structure, Lubkowski said: “In a column, you have the main steel and then the shear reinforcement links which confine that steel. There’s evidence of only 90º bend hooks being used in the shear, where in earthquake regions they should be 135º.
“There should be a square of steel, that when the concrete cover spalls [falls] off during the shaking, the shear links stay and hold the rest of the concrete together. But if you only have the 90º ones, it effectively unzips.”
Steel couplers, which join the reinforcement bars together, have been found in columns and are being investigated to see whether their failure contributed to the building’s collapse.
Earthquake 6 Feb 2016 Taiwan USGS map
Source: By Servicio Geológico de los Estados Unidos (USGS en inglés) [Public domain], via Wikimedia Commons
Lubkowski explained that the event would probably have been at the upper end of the design limits of the building, with the highest ground acceleration reported to have been 3.4m/s2.
He said that building codes for the area had been updated since the last major earthquake in 1999, but that many of the buildings which collapsed looked to have been built before this time and could therefore have potentially been designed to a lesser standard.
A large open plan market building also collapsed during the earthquake and Lubkowski speculated that this may have been built before the current codes and therefore have suffered from limited provision for earthquake design.
“There may have been poor conceptual design,” he said. “It may have been designed to [fulfil] an economic need to provide an open area, but that’s not very good from a seismic perspective.”
He also said that some buildings looked to have undergone “soft storey collapse” with either the basement or the floors of the building appearing not to have any walls to take the shear force exerted by the earthquake.
Hsinhua branch of King’s Town Bank
Source: By ScoutT7 (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia
The reconnaisance report also dismissed rumours that tin cans, allegedly found within the concrete, might have been the cause of one of the collapses, saying that these looked to be more like formwork used in a non-structural façade to reduce its weight.
The earthquake has damaged and caused the collapse of around 10 reinforced concrete buildings and several unreinforced masonry buildings. Other buildings have rotated due to liquifaction of the ground during the quake, these buildings are understood to be close to the Yansuei river.