Figure 1: A five-storey residential building collapsed in Ruiru last year on 11th November 2022
Source: Africanews.com
These are the reasons why most apartment buildings collapse in Kenya
1. Building small columns that cannot bear the weight of other structural elements of the building e. g. the beams and slabs
2. Muddy, loose soil, or black cotton soil with a low load bearing capacity. Those who dug the pits to build the foundation did not reach the hard rock or they did not remove the top soil.
Figure 1: A skeletal structure of a building with pad foundation. The columns, beams, and slabs are also shown.
3. Poor mixing of concrete used to make pad footings, columns, beams, and slabs. Failing to put enough water, ballast, sand, cement. The ratio of w:c:s is incorrect
4. Shear in the column footings such that the effective base area is small and hence not able to transfer the loads of the building to the ground safely
5. Adding more floors to an existing building that was not originally designed to bear the weight of the added floors. The foundation of the existing building was not designed to withstand the weight of columns, beams, slabs, wall added above
6. Failure to use the spirit level or plumb bob on columns and walls such that they do not stand upright at an angle of 90 degrees respective to the ground level.
Figure 2: The column is buckling because it does not stand at an angle of 90 degrees which is relative to the flat ground level
7. Poor workmanship-The people digging the foundation, laying it, constructing the beams, columns, slabs, walls are not skilled in the craft
8. Concrete is not allowed to cure by podding with water for at least 21 days. Another reason is poor compaction of concrete in slabs, beams, and columns such that during shrinkage, there are cracks in these structural elements. This lowers the strength of concrete to bear the lateral and vertical loads. The cracks allow entry of water and air-that causes the reinforcement steel to rust and hence reducing concrete's tensile strength
Figure 3: Cracks on the slab may cause it to swing downwards such that it deviates from the horizontal axis
9. Construction of floor slabs, columns, beams, and walls at different times and then adjoining them later-There may be inconsistences in terms of concrete mix ratio, type of reinforcement steel, stones used.
10. Most of these buildings collapse when they are under construction and that means the dead loads exceeds the safe bearing capacity of the soil. No significant live loads are present during construction
11. Another error is when masons build the walls first and columns later. They are supposed to build the skeletal structure first and then the walls
12. The landlords or clients build the apartment buildings in different times of the year because of the fluctuations in flow of rent income. You will find in some cases, a landlord can construct the ground floor fully and then rent it out. Then, he collects rent for year (s) so he can build the upper floors
13. Most fundis believe that each floor is separate from the other one but they are supposed to be uniform and understood to be one skeletal structure with walls
14. All the errors in construction happen because in these projects, the clients do involve construction professionals including the architects, structural engineer, construction manager, quantity surveyor, etc.
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