Building collapses

STABILITY OF BUILDINGS DURING CONSTRUCTION

PURPOSE

The purpose of this article is to emphasize to both commercial and housing builders of the need to adequately brace buildings and structures, during all stages of construction, to ensure they are stable

Kenyan’s have witnessed numerous incidents where buildings (includes structures) have partially or totally collapsed during the construction process or newly occupied buildings. These collapses have often resulted in death or injury to workers, occupants and other people in the vicinity; in addition to the damage to property and the significant financial cost to the builder from the clean-up and rectification works. The most recent was a six storey building that collapsed in Mathare, Nairobi Friday night, burying scores of occupants underneath its rubble. This happened just hours after a perimeter wall collapsed killing two people along Nairobi’s Lenana road after a down pour. The six storey building is just one of the several buildings, both commercial and residential which have collapsed over recent years.

There are several reasons which have been advanced to explain why buildings collapse.

STRUCTURAL FAILURE

Most buildings collapse mainly because of structural failure. This is when a building is designed to carry a specified load. However with time, the building’s owner decides to increase the load by say, increasing the number of occupants from those earlier planned for in the structural or architectural designs. Also, when structural designs are done poorly, then the building is bound to collapse.

“When considering over loading, it is important for people to understand that it is in three different types; dead load, live loads and wind loads which should be looked out for during construction lest the building collapses.”

The dead or permanent load is load of the building elements themselves like the weight of the walls, roofs and permanent partition floors.

The live or imposed load is the people and furniture that can be moved from one area to another. Once these loads are in excess, then a building will surely collapse due to the pressure exerted on the slabs and cannot hold the weight anymore.

The wind loads, though uncommon for buildings in Kenya to collapse because of this load, should also be considered. This particular load considers the direction in which the wind blows and here sufficient specifications are always catered for so that the building can withstand the wind pressure.

DESIGN INADEQUACIES

This requires for the builder to follow the structural designs to the neck. Once any of the instructions are not followed, for example loading more floors than the ones designed for, then a building will be weakened. Change of earlier planned for building structural design to a more complex design that was not accounted for, will increase on the weight hence putting tension on the columns leading to collapsing.

INCOMPETENCE IN ENGINEERING PRACTICES

Improper concrete mixing is the main incompetence done by engineers,  yet concrete determines how long the building will last for. The mixes give the building its durability.

Concrete before being approved is supposed to have passed through three different stages which are batching, testing and curing. Concrete has up to four other sub materials which are sand, cement, aggregate or stones and water and if not mixed appropriately then the strength will not be achieved.

“Different batching ratios lead to different concrete grades, for example, grade 20, 25, 30 and all these are applied on different parts of the building to achieve total strength.”

The curing process is the next stage of proper concrete verification. It is the drying process of concrete and this enables concrete to reach its strength but it is advisable to carry out this process under moist conditions.

Thereafter, the concrete should be taken to the structure laboratories where it is tested with cube tests to confirm whether the structure has reached the designed strength. But most engineers are reluctant to take such measures as required.

COUNTERFEIT MATERIAL

There is ample supply of counterfeit construction materials in the market today, ranging from the cement to steel bars. If the engineer is not cautious then a building will end up collapsing because surely counterfeit cement can never guarantee a building’s strength. It is the engineer’s responsibility to control quality of construction materials as well as look out for all these counterfeit products and not just leave the work to the relevant authorities.

“It is not wise to just assume that the products are genuine. Instead they should be tested to find out if they are of the best quality.”

POOR WORKMANSHIP

This is usually about poor role allocation. In this case, if the building owner or engineer does not supervise the builders, they may end up mixing the construction materials poorly.

There are also incidences when instructions of how to go about work on site are influenced by the property owners who quite oftenly have little knowledge or lack understanding in regards to the required construction standards. Such inconveniences lead to inefficiency and quack work on site since all specified measures and materials are not followed.

“Authorities and regulatory bodies are partly to blame for laxity and for not conducting their duties as required. Bodies such as the National Planning and Building Authority and other concerned local bodies are obligated to play their roles.”

WRONG BUILDING USE

Once most building owners in Kenya, realize that the business or use of the building they had earlier planned for is not earning them much or fetching the anticipated revenue, they switch to another form of use. The change of use could pose a threat to the building. When a building is changed from its originally planned use and instead used for a heavier purpose (for example-warehouse being turned into a factory with heavy machinery) then there is an overload beyond the building’s designed capabilities to handle.

BUILDING SETTLEMENT

Another vital issue often neglected by engineers is the soil strength test which is done to ascertain how hard the ground is. Different soil types have different characteristics and in instances where the building will contain more than four floors, then a geometric report is required for submission after a geotopical investigation is done to determine how hard the ground is which will in turn decide the foundation strength. Buildings are limited to 25 millimeters from foundation level and if they sink in then the buildings are bound to collapse, this goes to show that the soil is weakening and the building strength is pushing it down. That is why when considering building a high raised building, bores are dug up to 20 meters to determine the soil quality and know what quantity and quality of materials that need to be used to prevent the building from collapsing.

NATURAL CALAMITIES

Natural calamities though has led to a number of building collapsing in most foreign countries, this is rarely the case in Kenya or even in Africa at that. The strong earthquakes, strong tsunamis and cyclones are some of the well-known calamities that destroy buildings such that Africa rarely/does not experience regularly.

CORRECTING MISTAKES

Modifications (in special cases) can be carried out to correct the excess weight faults. Constructing additional columns can help in reinforcing the structure so as to support the excess weight.

OUTLIVING IT’S AGE

Building materials have a time frame at which they can hold up after use. That is why buildings are given expiring time frames, say 100 years. Once these years elapse then this particular building should be brought down. This is not always the case in Kenya where bringing down a building is never a consideration until it collapses. That is when it is deemed to expire.

RESORTS AND CITIES’ EXPECTATIONS

To prevent these types of structural collapses Resorts and Cities advocates for more engineer responsibility of the construction works by having systems in place, systems that ensure the buildings or structures are well stabilized and adequately braced against live and dead loads, including lateral loads during all stages of construction.

Any system should include the following:

  1. A competent person needs to check the proposed construction sequence and design appropriate temporary bracings or supports; some construction projects may require input from a structural engineer.
  2. Stability and bracing requirements should be documented, either in the work method statement or on the building plans, for each stages of construction.
  3. Stability and bracing arrangements are reviewed when structural changes are made to the building.
  4. Existing bracing is regularly checked; to ensure the continued robustness of the whole building.
  5. Ensure that all temporary bracing is installed correctly and is maintained in a serviceable condition until their use is no longer required.
  6. Controlling lateral instability
  7. Buildings and structures are usually stabilized against lateral loads by using any or a combination of the three following structural systems:
  • Triangulation – additional interconnecting diagonal components (bracing) connecting the columns and beams, which transfer the loads to the ground.
  • Rigid Structure – if the joints in the structure are made rigid, the structure can resist lateral loads. This can be accomplished by using knee-braces, deep footings etc..
  • Shear – if the supporting structure has walls rather than beams and columns, these walls can be used to resist the lateral loads, i.e. a shear wall.

When planning the construction process the builder needs to ensure that the installation of required lateral supports follows the construction sequence, so that all parts of the building or structure are adequately braced at all times.

Temporary bracing should be provided where:

  • The installation of permanent braces cannot occur until after the rest of skeleton structure is completed, especially when a building relies on these braces for lateral support, or
  • Shear walls are not built or in place and the structure relies wholly or partly on these walls for their lateral support.

It should also be noted that it is necessary in multi-storied buildings to provide lateral support at every level unless a lift or stair well is designed to act as a shear wall and provides lateral stability.

In the case of long buildings with multiple bays bracing of just one bay might not be adequate to provide the required stability against lateral forces.

Safety is no accident. It’s a choice we need to make throughout our entire lives. Whether it’s in construction of buildings, driving without passengers as a newly licensed teen, finding alternatives to prescription painkillers in middle-age or even fall-proofing the bathroom as an older adult, we’re all empowered to make safe decisions for ourselves and our communities at large.

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