Earthquake Risks in Kenya and the Role of Eco-Concrete Beam and Block Slabs in Multistorey Buildings
In Kenya, most people do not consider the risk of an earthquake due to the low chance of its occurrence in the region. The earthquake events in Syria and Turkey that caused large-scale destruction have shown the risks of earthquakes on buildings and infrastructure. We recently experienced mild tremors in April 2023, even though Kenya is categorized as a low earthquake zone. This article focuses on the practicality of Eco-concrete beam and block slab use on multistorey buildings while addressing earthquake resistance concerns raised within the construction industry.
In Kenya, earthquake design is a requirement for buildings above 5 floors high. Beyond this limit design for wind and earthquake loading is a necessity. Earthquake design focuses on the ability of a structural system to resist damage under extreme loads, termed the robustness of a structure.
Additionally, when builders use precast elements in a structure, they must establish strong connections between members to ensure robustness and stability.
Earthquake load develops due to the inertia force generated within a building from seismic excitations, directly impacting structural stability.
Moreover, inertia force depends on mass, meaning a heavier structure experiences higher earthquake loading, increasing vulnerability to seismic forces.
Additionally, the magnitude of earthquake loading is influenced by a building’s weight, dynamic properties, floor stiffness variations, and earthquake intensity and duration.
Furthermore, when an earthquake load exceeds an element’s moment of resistance, the structure eventually sustains damage or even breaks.
Ground shaking induces inertia forces in a building where mass is present. The system transfers these inertia forces through the structure to maintain stability.
downward through horizontally and vertically aligned structural elements to foundations, which, in turn, transmit these forces to the soil underneath. The structure transfers these inertia forces along specific routes known as Load Paths.
Earthquake Resistant Building and Structure
Buildings may have multiple load paths running between locations of mass and foundations. Load paths are as much a concern for transmitting vertical loads (e.g., self-weight, live load) as for horizontal loads (e.g., earthquake and wind).
Structural elements in buildings that constitute load paths include:
(a) Horizontal diaphragm elements, such as roof slabs, floor slabs, trussed roofs, and bracing, are strategically laid in a horizontal plane to enhance stability.
(b) Additionally, vertical elements spanning the building’s height, including planar frames (interconnected beams and columns), reinforced concrete or masonry walls, and trusses, provide essential structural support.
(c) Foundations and Soils, Isolated and combined footings, mats, piles, wells, soil layers and rock.
(d) Connections between the above elements.
Engineers design earthquake loads on multistory structures to follow specific load paths through the building’s frame and shear walls. Additionally, horizontal floors efficiently resist and transfer earthquake forces along direct load paths, making robust connections between structural elements essential.
To enhance earthquake resistance, engineers incorporate moment-resisting frames with extra reinforcement to withstand horizontal loading. Moreover, every connection in an earthquake-resistant structure undergoes rigorous testing during strong shaking to ensure stability and durability. These connections must remain stiff and strong to ensure continuous load paths without sustaining damage.
The term robustness refers to the ability of a structural system to resist damage under extreme loads.
Monolithic Connection
Monolithic connections ensure continuous reinforcement across joints, anchored in adjacent elements, and cast homogeneously to enhance robustness in tall buildings.
This design improves resistance to horizontal loads from seismic activity and wind, ensuring structural stability in multistorey buildings.
Engineers must carefully detail reinforcement in supporting and supported members to achieve proper anchorage between precast and in-situ elements.
For a monolithic connection, anchor a U-shaped reinforced steel bar and cast it with in-situ beams, projecting reinforcement 600mm above.
Builders place the precast beam and block slab over the in-situ beam and bend the projecting reinforcement onto both slab sides.
Next, they lay top reinforcement across the in-situ beam and tie it securely to the anchored reinforcement, ensuring stability.
Cast 50mm thick screed homogenously over the connection to have continuity between the in-situ beam and the precast slab.
Conclusion
Use of EcoConcrete beam and block on multi-storey structures designed for earthquake-resistant structures is feasible considering all the principles of earthquake design. Their efficiency, speed, design flexibility, structural performance, and sustainability attributes make them highly desirable for construction projects.