4.4 Design of steel and concrete structures

Steel and concrete structures form the backbone of civil engineering. This section dives into their design principles, focusing on limit state design and Load and Resistance Factor Design (LRFD) methodologies. These approaches ensure structures can withstand various loads while maintaining safety and functionality.

The design process for steel and concrete members is explored, covering tension and compression members, beams, columns, and connections. Reinforced concrete design is also examined, including beams, columns, and slabs. The section concludes with composite and prestressed concrete structures, highlighting their unique properties and applications.

Limit State Design vs LRFD

Limit State Design Philosophy

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• Considers various states at which a structure or its components cease to fulfill their intended function
• Ultimate limit state (ULS) refers to the state associated with collapse or failure of the structure
• Serviceability limit state (SLS) refers to the state associated with the functionality and appearance of the structure under normal use
• Other limit states may include fatigue, durability, and accidental situations

Load and Resistance Factor Design (LRFD) Methodology

• Applies separate factors to loads and resistances to account for uncertainties and variability in the design process
• Load factors are applied to nominal loads to determine factored loads (dead load, live load, wind load)
• Resistance factors are applied to nominal resistances to determine design resistances (material strength, member capacity)
• Design is considered satisfactory when the factored resistance is greater than or equal to the factored load combination for each applicable limit state
• Allows for a consistent level of reliability across different materials and structural systems

Steel Member Design

Tension Members

• Designed to resist axial tensile forces
• Considers gross cross-sectional area, effective net area, and the yielding and rupture limit states
• Yielding limit state ensures that the member does not yield under the applied tensile force
• Rupture limit state ensures that the member does not fracture at the net section (considering bolt holes or other discontinuities)
• Examples: truss members, bracing members, hanger rods

Compression Members

• Designed to resist axial compressive forces
• Takes into account the member's slenderness ratio, effective length, and buckling limit states
• Slenderness ratio affects the member's susceptibility to buckling (short vs. slender columns)
• Effective length depends on the member's end restraints (pinned, fixed, or partially restrained)
• Local buckling of individual elements (flanges, webs) must also be considered
• Examples: columns, struts, truss members

Beams and Flexural Members

• Designed to resist bending moments and shear forces
• Considers section modulus, moment of inertia, and the yielding, lateral-torsional buckling, and web local buckling limit states
• Yielding limit state ensures that the member does not yield under the applied bending moment
• Lateral-torsional buckling limit state prevents instability of the compression flange in slender beams
• Web local buckling limit state ensures the stability of the web under high shear stresses
• Examples: floor beams, girders, cantilevers

Connections

• Designed to transfer forces between members
• Considers strength and ductility of the connection elements and the connected members
• Bolted connections rely on the shear and bearing strength of the bolts and the connected plates
• Welded connections depend on the strength and quality of the weld material and the connected elements
• Connection design must ensure adequate stiffness, ductility, and capacity to transfer the required forces
• Examples: shear tabs, moment connections, base plates

Reinforced Concrete Design

Beams

• Designed to resist bending moments and shear forces
• Considers concrete compressive strength, reinforcement yield strength, and the distribution of stresses and strains across the section
• Determines the required amount and layout of longitudinal reinforcement for flexure (tension reinforcement, compression reinforcement)
• Determines the required amount and spacing of transverse reinforcement (stirrups) for shear
• Anchorage and development length of reinforcement must be provided to ensure proper force transfer
• Examples: simply supported beams, continuous beams, deep beams

Columns

• Designed to resist axial loads and bending moments
• Considers concrete compressive strength, reinforcement yield strength, and the interaction between axial loads and bending moments
• Determines the required amount and arrangement of longitudinal reinforcement (bars or tied reinforcement)
• Determines the required amount and spacing of transverse reinforcement (ties) for confinement and shear resistance
• Slenderness effects and second-order moments must be considered for slender columns
• Examples: tied columns, spiral columns, bridge piers

Slabs

• Designed to resist bending moments and shear forces
• Considers concrete compressive strength, reinforcement yield strength, and the distribution of stresses and strains across the section
• Determines the required amount and layout of reinforcement in both the primary and secondary directions (one-way slabs, two-way slabs)
• Punching shear around columns must be checked and reinforced if necessary
• Deflection control and crack width limitations must be satisfied for serviceability
• Examples: flat slabs, waffle slabs, ribbed slabs

Composite & Prestressed Concrete

Composite Structures

• Utilizes the advantages of both steel and concrete to achieve improved structural performance and efficiency
• Composite beams consist of a steel beam connected to a concrete slab through shear connectors
• Shear connectors (studs, channels) ensure the composite action between the steel beam and the concrete slab
• Effective width of the concrete slab contributes to the composite section properties
• Composite columns consist of a steel section encased in or filled with concrete
• Confinement effect provided by the steel section enhances the strength and ductility of the concrete
• Examples: composite floor systems, composite bridges, composite high-rise buildings

Prestressed Concrete

• Introduces compressive stresses into the concrete member before the application of external loads
• Counteracts the tensile stresses caused by the applied loads, improving the member's resistance to cracking and deflection
• Pretensioning involves stressing the tendons before casting the concrete and releasing them after the concrete hardens
• Post-tensioning involves stressing the tendons after the concrete has hardened and anchoring them at the ends of the member
• Determines the required amount and layout of prestressing tendons (strands, bars) and the magnitude of the prestressing force
• Considers the stress limits in both the prestressing tendons and the concrete to prevent excessive stresses and cracking
• Prestress losses due to elastic shortening, creep, shrinkage, and relaxation must be accounted for in the design
• Examples: prestressed beams, prestressed slabs, prestressed tanks, prestressed pavements