5.6 Timber and Masonry Structures

5.6 Timber and Masonry Structures

Introduction to Traditional and Composite Systems

Timber and masonry represent two of the oldest and most enduring construction materials. While modern engineering often focuses on steel and concrete, a significant portion of the global building stock, especially in residential and heritage contexts, utilizes these materials. Timber is a renewable, lightweight structural material with excellent strength-to-weight ratio. Masonry, the assemblage of units (brick, stone, block) with mortar, provides robustness, fire resistance, and thermal mass. This unit covers the design principles for timber elements, the codal guidelines for masonry (focusing on Nepal's context), and the characteristics of different mortars.

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1. Design Principles of Timber Beams and Columns

Timber design involves understanding the anisotropic nature of wood (properties differ along and across the grain) and accounting for natural defects and long-term effects.

1.1 Characteristics and Grading of Timber

1. Structural Properties:

   • Strength is Directional: Strong in tension and compression parallel to the grain, weak in shear and tension perpendicular to grain.

   • Orthotropic Material: Different elastic properties in three principal directions: longitudinal (parallel to grain), radial, and tangential.

   • Effect of Moisture Content (MC): Strength and stiffness increase as MC decreases below the fiber saturation point (~30%). Design uses properties at 12% MC.

   • Time-Dependent Effects:

     • Creep: Deformation under sustained load. Must be considered in deflection calculations.

     • Duration of Load (DOL) Effect: Timber can sustain higher short-term loads than long-term loads.

2. Grading:

   • Visual Grading: Based on observable characteristics like knots, slope of grain, checks, and wane. Assigned grades (e.g., Select Structural, No. 1, No. 2).

   • Machine Stress Rating (MSR): Non-destructive evaluation of stiffness to predict strength grades.

1.2 Design Philosophy (Limit State as per IS 883 & NBC)

Similar to steel/concrete, modern codes use Limit State Design with factored loads and reduced material strengths.

• Permissible Stress Design (PSD): Older method, still referenced. Uses working loads and permissible stresses (basic stresses modified for various factors).

• Key Adjustment Factors (in PSD, analogous to partial safety factors in LSM):

  • Load Duration Factor: Increases permissible stress for short-term loads (e.g., wind, earthquake).

  • Moisture Content Factor: Reduces stress for timber with MC > 19%.

  • Size/Depth Factor: For deep beams in bending.

  • Slenderness Factor: For columns.

1.3 Design of Timber Beams

1. Primary Checks:

   • Bending Stress (Flexure): $\sigma_b = \frac{M}{Z} \leq f_b'$ where $M$ = applied bending moment, $Z$ = section modulus, $f_b'$ = permissible bending stress (adjusted for grade, load duration, size, etc.).

   • Shear Stress: $\tau = \frac{1.5 V}{A} \leq f_v'$ where $V$ = shear force, $A$ = cross-sectional area, $f_v'$ = permissible shear stress (parallel to grain).

   • Deflection: Critical for serviceability. Must be within limits (e.g., span/360 for floors). Use effective modulus of elasticity (E') to account for creep.

2. Lateral Stability: Deep, narrow beams can buckle laterally (sideways). Requires lateral support at intervals or a check for lateral torsional buckling.

1.4 Design of Timber Columns

1. Classification:

   • Short Column: Fails by crushing. $\frac{L_{eff}}{d_{min}} \leq 11$ (approx).

   • Long Column: Fails by elastic buckling (Euler buckling).

   • Intermediate Column: Fails by inelastic buckling.

2. Design Stress:

   • The permissible compressive stress ($f_c'$) is reduced by a column stability factor that accounts for slenderness.

   • Slenderness Ratio: $\lambda = \frac{L_{eff}}{r_{min}}$ or $\frac{L_{eff}}{d_{min}}$, where $r_{min}$ = least radius of gyration.

   • For a rectangular section, $r = d/\sqrt{12}$.

   • The design follows an interaction formula for combined axial load and moment if eccentricity exists.

1.5 Connections in Timber

• Critical part of design. Types: Nailed, bolted, screwed, toothed-plate, or specialized metal connectors.

• Design involves checking for bearing in wood, yielding of fastener, and group action of multiple fasteners.

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2. Design of Masonry Structures

Masonry design, especially for low-rise buildings, often follows simplified empirical rules supplemented by engineered principles for seismic resistance.

2.1 Mandatory Rules of Thumb (MRT)

1. Concept: Prescriptive, experience-based guidelines that ensure basic stability without complex calculations. Mandated in many building codes for simple, small buildings.

2. Common MRT Provisions (as in NBC and other codes):

   • Maximum Wall Thickness: Minimum 230mm for load-bearing walls.

   • Maximum Story Height: Typically 3.5m.

   • Maximum Unsupported Wall Length (between cross-walls/buttresses): e.g., 7m for 230mm wall.

   • Maximum Building Height: e.g., 2 stories for unreinforced masonry in seismic zones.

   • Opening Sizes and Positions: Limits on size of door/window openings and their distance from corners (e.g., > 600mm from corner).

   • Lintel/Band Provision: Reinforced concrete bands (lintel, plinth, roof level) are mandatory to tie walls together and distribute loads.

2.2 Nepal Building Code (NBC) for Masonry

NBC 202:1994 (Mandatory Rules of Thumb) and NBC 203:2015 (Guidelines for Earthquake Resistant Construction) are key documents.

1. NBC 202:1994 (MRT):

   • Applicable to: Load-bearing masonry buildings up to 3 stories in non-engineered or partially engineered construction.

   • Provides simple dimensional rules for wall thickness, story height, opening sizes, and mandatory provision of Seismic Bands.

   • Seismic Bands: Horizontal reinforced concrete bands at various levels:

     • Plinth Band: At top of foundation.

     • Lintel Band: Over all openings (doors/windows).

     • Roof/Floor Band: At roof and floor levels.

     • Gable Band: For pitched roofs.

   • Purpose of Bands: To act as a "belt" that ties all walls together, ensuring integral box action and preventing separation during earthquakes.

2. NBC 203:2015:

   • Provides more detailed guidelines for engineered masonry.

   • Covers material specifications, design principles for vertical and lateral loads, and detailing for seismic resistance (including use of vertical reinforcement in pockets/pilasters).

2.3 Properties of Masonry

Masonry is a composite, non-homogeneous, and quasi-brittle material.

1. Compressive Strength ($f_m$):

   • The most important property. Depends on strength of unit (brick/block) and mortar.

   • $f_m = k \times (f_b)^a \times (f_j)^b$ where $f_b$ = unit strength, $f_j$ = mortar strength, $k, a, b$ = constants.

   • Typically determined by testing masonry prisms.

2. Tensile and Shear Strength:

   • Very low. Governs cracking and diagonal shear failure under lateral loads.

   • Shear strength depends on bond between mortar and unit, and axial pre-compression.

3. Modulus of Elasticity ($E_m$): Varies widely. Often estimated as $E_m = 550 f_m$ to $1000 f_m$.

2.4 Failure Modes of Masonry Structures

Understanding failure modes is key to designing for prevention, especially in seismic zones.

1. In-Plane Failure (Walls loaded parallel to their plane):

   • Flexural (Rocking) Failure: Wall rotates about its toe. Occurs in slender walls with low axial load. Characterized by horizontal cracks at bed joints at bottom/top.

   • Shear (Diagonal Tension) Failure: Diagonal stair-step cracks along head and bed joints. Occurs in squat walls or walls with high shear.

   • Sliding Shear Failure: Horizontal sliding along a bed joint. Occurs when shear force exceeds frictional resistance.

2. Out-of-Plane Failure (Walls loaded perpendicular to their plane):

   • Flexural Failure: Wall bends like a vertical beam. Most dangerous as it can lead to collapse. Prevented by limiting unsupported height/thickness ratio and providing lateral support (cross-walls, floors, roofs).

3. Connection Failures:

   • Wall-to-Wall Separation: Due to inadequate tying at corners (lack of proper bonding).

   • Wall-to-Roof/Floor Separation: Due to lack of proper anchorage, causing roofs to slide off.

4. Failure Due to Irregularities:

   • Soft-Story Collapse: Weak/open ground story fails.

   • Pounding: Adjacent buildings with insufficient separation hit each other.

   • Torsional Failure: Asymmetric building plans cause twisting.

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3. Mortars: Types and Properties

Mortar is the "glue" that binds masonry units, cushions them, and seals joints.

3.1 Functions of Mortar

• Binds units into a composite element.

• Cushions units, distributing loads uniformly.

• Seals joints against air/water penetration.

• Accommodates small dimensional variations.

3.2 Mud Mortar

1. Composition: Clayey soil, sometimes with additives like straw or lime for reduced shrinkage.

2. Properties:

   • Very Low Strength (negligible).

   • Poor Durability: Washes away easily with water, susceptible to erosion.

   • No Bond: Acts more as a filler than a binder.

   • High Shrinkage: Cracks upon drying.

3. Use: Only for non-structural, temporary, or very low-cost shelters in dry climates. Not permitted for any permanent or load-bearing construction in modern codes, especially in seismic areas.

3.3 Lime Mortar

1. Composition: Lime (fat lime or hydraulic lime) and sand.

2. Types:

   • Fat Lime Mortar: Made from high-calcium lime. Sets very slowly by carbonation (reaction with atmospheric CO₂).

   • Hydraulic Lime Mortar: Contains clay impurities. Sets underwater and gains strength faster.

3. Properties:

   • Moderate Strength: Lower than cement mortar but adequate for low-rise buildings.

   • High Workability and Water Retentivity.

   • Flexible: Can accommodate minor movements without cracking.

   • Self-Healing: Minor cracks can re-seal through carbonation.

   • Permeable: Allows moisture vapor transmission, preventing trapped moisture (good for historic buildings).

4. Use: Traditional construction, restoration of historic masonry, and in modern sustainable construction where flexibility and breathability are desired.

3.4 Cement Mortar

1. Composition: Cement (OPC/PPC) and sand. Proportions defined by volume (e.g., 1:4, 1:6).

2. Properties:

   • High Strength and Durability.

   • Rapid Hardening.

   • Low Permeability (good for waterproofing but can trap moisture).

   • Brittle: Prone to cracking if movements occur.

   • Poor Workability if not properly proportioned.

3. Grades: Designated by compressive strength at 28 days (e.g., M1, M2.5, M5, etc., where number indicates strength in MPa).

4. Selection of Proportion:

   • Rich Mix (1:3, 1:4): For high load-bearing walls, damp-proof courses.

   • Medium Mix (1:5, 1:6): For general above-ground masonry.

   • Lean Mix (1:8+): For non-load bearing or temporary work.

5. Use: Standard for modern load-bearing and reinforced masonry construction worldwide.

3.5 Composite Mortars

• Cement-Lime Mortar (Gauged Mortar): Combination of cement, lime, and sand.

• Advantages: Combines the high strength of cement with the workability, water retentivity, and flexibility of lime. Often considered the best general-purpose masonry mortar.

Conclusion: Timber design requires careful consideration of material anisotropy and time-dependent effects. Masonry design, particularly in seismic regions like Nepal, relies heavily on empirical rules (MRT) for simplicity and safety, mandating features like seismic bands. The transition to engineered masonry involves understanding composite material properties and failure modes. The choice of mortar is critical, moving from weak and permeable mud mortar to strong but brittle cement mortar, with lime offering a balanced, traditional alternative. Mastery of these principles is essential for the safe construction of a vast portion of the world's housing stock.