Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Formwork and Centring
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Formwork and Centring
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Subject: Carpentary And Joinery
Class: Senior Secondary 2
Term: 3rd Term
Week: 2
Theme: Construction Principles And Techniques
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Construct various types of form work. Design and construct centres for arches. State purpose of timbering.
This section provides in-depth explanations of the core concepts, supported by practical considerations relevant to the Nigerian context. This section outlines practical activities for the teacher and students to ensure comprehensive understanding and engagement. This section provides scaffolded practice questions to check understanding, with detailed solutions.
Question 1: A contractor in Enugu is about to pour concrete for the ground floor slab, some supporting columns, and the foundation footings of a new residential building. Identify and describe the specific type of formwork required for each of these structural elements.
Solution 1: Foundation Footings: Foundation Formwork would be used. This typically involves simple timber planks or steel plates forming a rectangular or square box directly within the excavated trench. The planks are secured by driving pegs or stakes into the ground outside and reinforced with spreaders across the top to maintain the width during concrete pouring.
Supporting Columns: Column Formwork would be used. This usually consists of four panels (often plywood or timber planks) assembled to form a vertical box. These panels are held together and prevented from bulging by external clamps or yokes (timber battens or steel bands) placed at regular intervals. The formwork must be adequately plumbed and braced to ensure verticality and stability. * Ground Floor Slab: Slab Formwork would be used. This involves a horizontal deck (plywood or timber planks) supported by a grid of joists, which in turn rest on primary bearers. These bearers are then supported by an array of vertical props (steel or timber) extending from the ground or lower level. Edge formwork is also required around the perimeter of the slab.
Commentary: This question assesses the ability to link structural elements to their appropriate formwork types, a core objective of the lesson. The solution details the construction principles for each, reinforcing practical understanding. --- Question 2: A local government is constructing a small culvert for a stream crossing in a rural community in Osun State. The culvert will have a segmental arch opening. Describe the key components and sequential steps involved in designing and constructing the timber centre for this segmental arch.
Solution 2: Key Components of the Arch Centre:
1. Ribs: Curved timber members cut to the exact profile of the inner curve (intrados) of the segmental arch.
2. Lagging Strips: Narrow timber pieces laid longitudinally over the ribs to create a continuous, smooth curved surface.
3. Ties/Braces: Horizontal timber members connecting the ribs to maintain their spacing and rigidity.
4. Wedges: Pairs of opposing tapered timber blocks placed under the centre to facilitate gradual lowering and removal.
5. Sole Plate: A horizontal timber member beneath the ribs, distributing the load to the props.
6. Props/Standards: Vertical timber posts or steel adjustable props supporting the sole plate and the entire centre.
Sequential Steps for Construction:
1. Setting Out the Profile: Accurately draw the full-scale segmental arch profile (span, rise, radius) on a large, flat sheet of plywood or timber board. This serves as a template.
2. Cutting the Ribs: Cut several identical curved ribs from strong timber (e.g., 25-50mm thick planks) using the prepared template. The number of ribs depends on the arch span and load.
3. Assembling the Ribs: Space the cut ribs equally along the span of the arch and connect them securely with horizontal timber ties (battens) nailed or bolted across their bottom edges. Ensure the assembly is rigid.
4. Attaching Lagging: Nail or screw the lagging strips across the curved top edges of the assembled ribs. Ensure they are evenly spaced, forming a continuous, smooth curved surface. Leave small gaps between lagging for swelling and easy striking.
5. Incorporating Wedges and Sole Plate: Position the sole plate beneath the assembled ribs. Place pairs of timber wedges between the sole plate and the supporting props. These wedges are crucial for controlled striking.
6. Erecting and Bracing: Carefully raise the assembled centre (with sole plate and wedges) onto its vertical props. Ensure the centre is perfectly aligned, level, and at the correct height relative to the arch abutments. Brace it securely from all sides to prevent any movement during arch construction.
7. Preparation: Apply a formwork release agent (e.g., used engine oil, form oil) to the lagging strips to prevent the concrete/mortar from sticking and to ensure easy removal.
Commentary: This question tests the detailed understanding of arch centre components and the practical steps striking.
6. Erecting and Bracing: Carefully raise the assembled centre (with sole plate and wedges) onto its vertical props. Ensure the centre is perfectly aligned, level, and at the correct height relative to the arch abutments. Brace it securely from all sides to prevent any movement during arch construction.
7. Preparation: Apply a formwork release agent (e.g., used engine oil, form oil) to the lagging strips to prevent the concrete/mortar from sticking and to ensure easy removal.
Commentary: This question tests the detailed understanding of arch centre components and the practical steps for their construction, reinforcing the second performance objective. The Nigerian context (culvert in Osun State) makes it relatable. --- Question 3: A construction company is excavating a 3-meter deep trench in soft, sandy soil in the coastal region of Lagos for laying a new water pipeline. State four critical purposes of timbering this trench, and briefly describe how "Open Timbering" would be implemented in this scenario.
Solution 3: Four Critical Purposes of Timbering the Trench:
1. Preventing Soil Collapse: The primary purpose is to stop the soft, sandy soil from collapsing into the trench, which is a high risk in such soil conditions.
2. Ensuring Worker Safety: To protect workers inside the trench from being buried or injured by collapsing soil, thereby creating a safe working environment.
3. Maintaining Trench Dimensions: To keep the trench at its specified width and depth throughout the pipeline laying process, preventing erosion of the trench sides.
4. Protecting Adjacent Structures: If applicable, to prevent ground movement that could undermine the stability of nearby roads, fences, or buildings. Implementation of "Open Timbering":
1. Sheeting/Poling Boards: Vertical timber planks (e.g., 50mm thick, 200-300mm wide, and slightly longer than the trench depth) would be placed against the inner face of the excavated trench sides, running along its length. These can be driven into the soil as excavation proceeds or placed as sections of the trench are dug.
2. Walers (Waling Pieces): Horizontal timbers (e.g., 100x100mm or 150x50mm depending on pressure) would be placed against the sheeting boards, running parallel to the trench at specific vertical intervals (e.g., 1m to 1.5m, depending on soil stability).
3. Struts (Cross Braces): Horizontal timbers (e.g., 100x100mm) would be placed between the opposite walers, spanning the width of the trench. These struts are tightened using wedges or jacks to apply outward pressure on the walers and sheeting, pressing them firmly against the trench sides to prevent inward movement. Struts would be spaced horizontally (e.g., 1.5m to 2m apart) to provide continuous support.
4. Staged Installation: The timbering would be installed in stages as the excavation proceeds downwards, ensuring that an unsupported trench face is never left for too long, especially in soft soil.
Commentary: This question directly addresses the third performance objective and the evaluation guide. It requires stating the purpose of timbering and explaining a specific method of implementation in a relevant Nigerian context, highlighting practical application and safety.
This topic has extensive real-life applications in Nigeria, connecting directly to various aspects of community development, infrastructure, and career opportunities.
Housing and Building Construction: Every concrete building constructed in Nigeria, from small residential bungalows to multi-storey commercial complexes in cities like Lagos, Abuja, or Port Harcourt, relies heavily on formwork for its foundations, columns, beams, and slabs. Students can observe these ongoing projects in their communities and appreciate the carpenter's role in creating these moulds. This knowledge is directly applicable to vocational skills and self-employment in the building industry.
Infrastructure Development: Bridges and Culverts: The construction of bridges over rivers and streams, especially in rural areas, and culverts for drainage under roads (e.g., across the Niger Delta or major highways), directly involves both formwork (for abutments, piers, deck slabs) and centring (for arch bridges/culverts). Understanding these processes can inform students about the engineering marvels that facilitate transportation and reduce flooding.
Water and Sanitation Projects: Laying water pipelines, sewerage systems, or constructing deep foundations for water treatment plants often requires extensive trench excavation and critical use of timbering to ensure worker safety and trench stability, particularly in challenging soil conditions found across Nigeria (e.g., loose sand in coastal areas, unstable lateritic soils).
Environmental and Safety Awareness: The emphasis on timbering highlights the critical importance of safety protocols on construction sites. This knowledge instils in students an understanding of occupational hazards and the responsibility of builders to protect workers, a significant concern in the Nigerian construction sector. It can also integrate discussions on sustainable timber sourcing and alternative materials to reduce environmental impact.