Building drawings are essential tools for communication among architects, structural engineers, contractors, and site supervisors involved in building projects. Structural drawings, in particular, detail the framework that supports the building, ensuring safety, stability, and compliance with Kenya’s building codes. This chapter focuses on preparing structural drawings, emphasizing the key structural elements that form the backbone of any building project in the Kenyan context. Mastery of these drawings enhances accurate construction, reduces risks, and facilitates professional coordination on site.

2.1 Structural elements

Structural elements are the primary components that transfer loads safely to the ground, maintaining the building’s integrity and stability. In Kenya, understanding these elements is crucial for professionals working on commercial buildings, residential complexes, and public infrastructure projects. Structural drawings must clearly depict these elements with correct dimensions, reinforcement details, and specifications to guide construction teams effectively.

2.1.1 Beams

Beams are horizontal structural members designed to carry and transfer loads from slabs, walls, and roofs to columns or walls. They resist bending forces and shear stresses, playing a vital role in supporting floors and roofs in buildings across Kenya.

Types of beams commonly used in building construction

Technical drawings of three different steel beam cross-sections (I-bea (Source: Chudley)

• Simply supported beams: These rest on supports at both ends and carry loads between supports. They are widely used in small to medium-span floor systems in schools and retail outlets.

A simply supported beam (Source: STRUCTURES I-DBT-MOD I)

• Cantilever beams: Fixed at one end and free at the other, they are used for balconies and overhangs in hotels and residential buildings.
• Continuous beams: Extend over more than two supports, common in multi-storey buildings such as county government offices for enhanced load distribution.
• Fixed beams: Both ends are fixed to supports, providing higher resistance to bending moments, often used in structural frames of commercial buildings.
• T-beams: Formed by the slab and beam acting together, providing an efficient load path in reinforced concrete floor systems in hospitals.

Materials used for beams and their selection criteria

• Reinforced concrete: Preferred for its high compressive strength and durability in Kenya’s varied climate; suitable for most building types.

Construction of reinforced concrete roadway slab (Source: Wikimedia Commons)

• Steel beams: Used where high tensile strength and lighter weight are required, such as in industrial warehouses.

• Timber beams: Common in low-cost housing and rural constructions due to availability and ease of handling.

timber beams supporting building entrance roof (Source: Wikimedia Commons)

• Composite beams: Combine steel and concrete for enhanced performance in large-span structures like university halls.
• Pre-stressed concrete beams: Used in bridges and large commercial buildings for superior strength and reduced deflection.

Standard drawing conventions for beams

Cross-sectional diagrams of two types of steel beams (Source: Chudley)

• Line types: Continuous thick lines for beam outlines, dashed lines for hidden reinforcement or embedded parts.
• Dimensions: Length, width, and depth clearly indicated in millimeters, with reinforcement spacing and bar diameters specified.
• Symbols: Use of standard symbols for stirrups, main reinforcement bars, and concrete cover as per British Standard BS 8666.
• Scale: Typically drawn at 1:20 or 1:50 depending on complexity and sheet size.
• Title block: Includes beam designation, project details, drawing number, and revision history.

Procedure for drawing a reinforced concrete beam

1. Draw the beam outline to scale using continuous thick lines.
2. Indicate beam length, width, and depth with clear dimension lines.
3. Add the main reinforcement bars along the beam’s length, showing bar diameter and spacing.
4. Draw stirrups (shear reinforcement) with appropriate spacing and bar sizes.
5. Specify concrete cover distance from the outer surface to reinforcement.
6. Include notes on concrete grade and steel specifications.

2.1.2 Columns

Columns transfer compressive loads from slabs and beams down to foundations, ensuring vertical support and stability in buildings. Structural drawings for columns in Kenya must accurately show dimensions, reinforcement, and connection details to withstand axial loads and bending moments.

Types of columns used in building construction

• Rectangular columns: Most common type, easy to construct and suitable for most building layouts.

Reinforced concrete rectangular columns under construction (Source: Wikimedia Commons)

• Circular columns: Provide uniform strength in all directions and are used in aesthetic architectural designs.

• L-shaped columns: Used at building corners where two walls intersect, providing combined support.

• Composite columns: Made from steel encased in concrete, used in high-rise buildings for enhanced load capacity.
• Fluted columns: Mainly decorative but can also serve structural functions in public buildings.

Reinforcement detailing in columns

• Longitudinal bars: Vertical bars running the length of the column, typically 4 to 8 bars depending on load.

• Ties or hoops: Circular or rectangular stirrups that hold longitudinal bars in place and resist shear.

• Concrete cover: Minimum cover requirement of 40mm in Kenya to protect reinforcement from corrosion.
• Lap splices: Overlapping of bars to provide continuity where bars are cut.
• Column base: Details showing connection to foundation, including base plate or reinforcement anchorage.

Drawing standards for columns

• Line thickness: Thick continuous lines for column edges, thinner lines for reinforcement.
• Reinforcement symbols: Standardized symbols for bars and ties as per BS 8666.
• Dimensioning: Width and depth of columns shown on plan and elevation views.
• Section views: Cross-section drawings clearly indicating reinforcement layout.

A structural section of a building (Source: Design engineering structures)

• Notes: Concrete strength, bar grades, and construction sequence instructions.

Steps to prepare a column structural drawing

1. Draw the column outline in plan view at correct scale.
2. Add elevation view showing column height and section dimensions.
3. Insert longitudinal reinforcement bars with correct spacing and diameter.
4. Draw ties or hoops around longitudinal bars with spacing details.
5. Specify concrete cover and lap splice lengths.
6. Include annotations on concrete grade and steel specification.

2.1.3 Stairs

Stairs provide vertical circulation between floors and must be structurally sound and safe. Structural drawings for stairs in Kenya require precise depiction of treads, risers, stringers, and reinforcement to ensure compliance with building regulations.

Components of staircases

• Treads: Horizontal surfaces where users step, typically 250mm deep.

• Risers: Vertical faces between treads, usually 150mm high.

• Stringers: Structural members supporting the treads and risers on either side.

• Landing: Flat platforms between flights for rest and change of direction.

Rusty metal staircase landing platform (Source: Wikimedia Commons)

• Handrails and balustrades: Safety features often detailed separately but coordinated with structural elements.

Types of staircases commonly used

• Straight stairs: Simplest form, suitable for small buildings such as retail shops.
• L-shaped stairs: Change direction at a landing, common in offices and schools.
• Spiral stairs: Compact design used in limited spaces, such as in cooperatives’ offices.
• Dog-legged stairs: Two flights parallel but opposite directions with a landing in between.
• Curved stairs: Architecturally appealing, used in hotels and upscale residential buildings.

Structural drawing conventions for stairs

• Plan view: Shows tread and riser arrangement, flight length, and landing dimensions.
• Section view: Indicates stair slope, reinforcement in treads and risers, and stringer details.
• Reinforcement: Main bars along stringers and distribution bars in treads.
• Symbols: Use standard notation for bars, concrete cover, and dimensions.
• Scales: Typically 1:20 or 1:50 depending on detail level.

Procedure to draw a reinforced concrete stair

1. Draw stair outline in plan showing all flights and landings.
2. Indicate tread depth and riser height dimensions.
3. Add section view illustrating slope and reinforcement placement.
4. Detail main reinforcement bars along stringers with size and spacing.
5. Include distribution reinforcement in treads and risers.
6. Specify concrete cover, bar grades, and notes on construction sequence.

2.1.4 Foundation

Foundations transfer the building load safely to the ground, preventing settlement and structural failure. In Kenya, foundation design and drawings must consider soil conditions, load types, and building use.

Types of foundations used in Kenya

• Strip foundations: Continuous strips under load-bearing walls, common in residential buildings.

• Isolated pad foundations: Individual footings for columns, used in commercial and institutional buildings.

• Raft foundations: Large slabs supporting entire building footprint, applied in poor soil conditions such as in Nairobi’s soft soils.

• Pile foundations: Deep foundation using long columns driven into the ground, for heavy structures like hospitals.

A detailed cross-section of a typical reinforced concrete pile foundat (Source: substructure-1)

• Combined foundations: Footings shared by two or more columns, used where columns are closely spaced.

Factors influencing foundation selection

• Soil bearing capacity: Determines type and size of foundation; soil tests are mandatory.
• Load magnitude: Heavier buildings require deeper or more robust foundations.
• Water table level: High water tables may necessitate special foundation types.
• Building use: Public buildings require more stringent foundation design.
• Cost and availability: Budget and local materials impact foundation choice.

Drawing standards for foundation plans

• Plan view: Shows foundation layout, footing sizes, and column positions.
• Section view: Indicates footing depth, reinforcement, and ground level.
• Reinforcement: Detailed bar sizes and spacing for footings.
• Dimensions: Clear indication of footing length, width, and depth in millimeters.
• Notes: Include soil bearing capacity, concrete mix, and construction instructions.

Steps to prepare structural foundation drawings

1. Obtain soil test reports to determine bearing capacity.
2. Draw foundation plan showing all footings aligned with columns.
3. Add dimensions for footing length, width, and spacing.
4. Prepare section views showing footing depth and reinforcement layout.
5. Detail reinforcement bars with diameters and spacing.
6. Include notes on concrete grade, curing time, and excavation requirements.

2.1.5 Slabs

Slabs form the floors and roofs of buildings, distributing loads to beams and columns. Accurate structural drawings for slabs are critical in Kenya to ensure durability and load-bearing capacity compliant with building regulations.

Types of slabs used in building construction

• One-way slabs: Supported on two opposite sides, transferring loads in one direction; common in small residential buildings.

• Two-way slabs: Supported on all four sides, distributing loads in two directions; used in multi-storey buildings such as banks.

• Flat slabs: Supported directly on columns without beams, allowing flexible column placement in offices.
• Ribbed slabs: Consist of ribs running in one direction beneath a thin slab; used in large floor areas like university lecture halls.
• Hollow core slabs: Precast slabs with voids to reduce weight, increasingly used in commercial buildings.

Reinforcement detailing in slabs

• Main reinforcement: Bars placed in the direction of maximum bending moment.
• Distribution bars: Perpendicular to main bars to distribute loads evenly.
• Concrete cover: Minimum 25mm to protect reinforcement from corrosion.
• Shear reinforcement: Provided in areas of high shear stress, such as slab edges.
• Control joints: Incorporated to prevent cracking due to shrinkage and temperature changes.

Drawing conventions for slabs

• Plan view: Shows slab outline, thickness, and reinforcement layout.
• Section view: Indicates slab thickness, reinforcement positions, and support details.
• Symbols: Standard symbols for main and distribution reinforcement bars.
• Dimensions: Thickness, bar spacing, and slab span shown in millimeters.
• Notes: Concrete grade, curing instructions, and load specifications.

Procedure for preparing slab structural drawings

1. Draw the slab outline in plan view with dimensions.
2. Indicate slab thickness and support positions.
3. Layout main reinforcement bars with size and spacing.
4. Add distribution reinforcement bars perpendicular to main bars.
5. Show section views with slab thickness and reinforcement cover.
6. Include notes on concrete mix, curing, and load-bearing capacity.

Chapter Summary

This chapter covered the preparation of structural drawings by first examining key structural elements such as beams, columns, stairs, foundations, and slabs, highlighting their roles and representation in drawings. It emphasized adherence to relevant codes of practice to ensure safety, quality, and compliance with Kenyan and international standards. The importance of the bar bending schedule was detailed, showing how it guides the cutting and bending of reinforcement bars accurately. The chapter also explained how to develop detailed plans and sections, focusing on clear, precise, and scaled representations that communicate the design intent effectively. The design aspect was integrated to demonstrate how structural elements are dimensioned and arranged to meet functional and load-bearing requirements. Overall, the chapter provided a comprehensive approach to producing structural drawings that are accurate, standardized, and practical for construction use.