Lecture 9

1. Lecture 9. Beams in Structural Engineering

Prepared by: S. Niyetbay

2. Timber Beams: Applications and Types

Timber beams are widely used in low-rise construction, including
residential, public, and agricultural buildings.
They are also applied in pitched roofs of multi-storey buildings, temporary
structures, and in aggressive industrial environments.
Timber beams can be:
Solid wood beams (rectangular or round sections)
Built-up beams (connected by nails, dowels, or splines)
Glued laminated beams (glulam) made of boards or plywood
Key limitations:
Maximum span for solid timber beams is typically up to 6 m
Load-bearing capacity is limited by cross-section size and timber quality
Advantages:
Lightweight
Easy to install
Sustainable material

3. Structural Behavior and Installation of Timber Beams

Solid timber beams used in floor systems are:
Placed on walls with spacing up to 1.5 m
Connected using steel anchors
Protected with insulation and mortar at supports
Structural elements:
Beams (main load-bearing elements)
Insulation layers
Anchorage systems
For roof systems:
Purlins support rafters
Typically designed as single-span beams
Span reduction achieved using struts or secondary beams
Glued laminated beams (glulam):
Manufactured using synthetic adhesives
Allow spans up to 15 m or more
Provide higher strength and optimized cross-sections
Timber beams are efficient for small and medium spans, while glued systems extend their
application to larger structures.

4. Structural Systems of Timber Purlins and Beams

• In roof structures, purlins act as horizontal beams supporting
rafters and transferring loads to vertical supports.
• Two main structural systems:
• Purlins with struts (braces)
• Reduce effective span
• Improve load distribution
• Increase structural stability
• Purlins with secondary beams (sub-beams)
• Provide additional support at intermediate points
• Connected using bolts and steel fasteners
• More rigid and suitable for higher loads
• Main elements:
• Purlin (main beam)
• Struts or posts
• Fastening systems (bolts, clamps)
• Conclusion:
Using auxiliary elements significantly reduces bending
moments and deflections, allowing more efficient structural
design.

5. Glued Timber Beams and Advanced Sections

• Glued laminated timber beams (glulam) allow flexible and optimized structural
forms:
• Types of beams:
• Parallel chord beams
• Pitched (cambered) beams
• Variable height beams
• Common cross-sections:
• Rectangular
• T-shaped (T-beams)
• I-shaped (double-T beams)
• Advanced systems:
• Beams with plywood webs
• Flat web beams
• Corrugated web beams
• Components:
• Flanges (top and bottom chords)
• Web (plywood panel)
• Stiffeners and braces
• Advantages:
• Efficient material distribution according to stress diagram
• Increased span capacity
• Reduced self-weight
• Industrial prefabrication
• Conclusion:
Modern timber beams combine high strength, efficiency, and adaptability,
making them competitive with steel and reinforced concrete in medium-span
structures.

6. Behavior of Timber Beams under Bending

• Failure of timber beams is mainly caused by bending,
which induces:
• Compression stresses in the upper fibers
• Tension stresses in the lower fibers
• The behavior of timber beams develops in three
stages:
• Elastic stage
• Linear stress distribution across the section
• Material works elastically
• Neutral axis remains at the centroid
• Elastic–plastic stage
• Plastic deformations appear in compression zone
• Crushing of compressed fibers
• Neutral axis shifts downward
• Failure stage
• Rupture of tensile fibers
• Sudden structural failure
• Design assumption:
Strength calculations are usually performed based on
the first (elastic) stage, assuming linear stress
distribution.

7. Failure Modes and Serviceability of Timber Beams

• Main failure modes:
• Bending failure (most common)
• Tensile rupture in extreme fibers
• Shear failure (less common)
• Occurs due to transverse force (Q)
• Typical for short beams (l/h ≤ 5)
• Often near supports under concentrated
loads
• Failure in glued I-beams
• Delamination or web failure
• Higher risk when web thickness is small
• Serviceability considerations:
• Timber beams may experience large deflections
• Excessive deflection leads to:
• Loss of functionality
• Structural discomfort
• Important design requirement:
• Check both strength and deflection limits
• Key feature of timber:
• Significant deflection before failure → provides
warning behavior

8. Strength Calculation of Timber Beams (Bending)

Design of timber beams is performed considering:
Strength
Stability
Stiffness
Key factors affecting strength:
Natural defects (knots, grain deviation)
Fiber discontinuity after cutting
Cross-section size
Bending strength condition: