Nigerian engineers said the 21-storey building that collapsed in Lagos early this month was originally designed only for six floors before more were added to the structure.
The high-rise building was still under construction in the upscale Ikoyi district of the city when it crumbled on November 1, trapping dozens of site workers and others inside.
At least 45 people, including the owner of the building, were killed, while 15 others were pulled out alive from the debris, according to the latest tally by the state government.
Emergency agencies had initially put the death toll at 42, but state authorities said last week three more corpses had been recovered under the rubble.
Although the government has set up a panel to determine the cause of the collapse within one month, the Nigerian Institution of Structural Engineers (NIStructE) said variations on the original design were to blame.
“There are clear indications of several design brief changes on the project and the engineering and management of these changes appear to have been seriously inadequate,” said institute president Kehinde Osifala in a statement late Tuesday.
“The building that collapsed was initially designed for just six floors, and later to 12 floors, before this was further changed to 15 floors,” he said.
“It could not yet be established the adequacy of any properly designed and documented further revision to the eventual (and tragically, final) 21 floors that was being implemented and which collapsed.”
Osifala said there were indications more than two structural engineering design firms worked on the project at different times.
“The preliminary investigation also revealed some evidence of structural inadequacy in the construction and that signs of some structural distress had already started to show within certain elements of the building,” he said.
He said although some measures were already being taken to address the defects, “the method of implementation of this was not in accordance with sound structural engineering practices.”
Building collapses are common in Africa’s most populous country, where millions live in dilapidated properties and construction standards are routinely ignored.
Poor workmanship and materials and a lack of official oversight are often blamed.
Since 2005, at least 152 buildings have collapsed in Lagos, the city of some 20 million people, according to data collected by researcher Olasunkanmi Habeeb Okunola, from the university of Witwatersrand in South Africa.
One of those incidents that particularly angered Nigerians was in 2014 when dozens of people died in a church collapse in Lagos.