Building Construction Related to the Fire Service Chapter 3 — The Way Buildings are Built: Structural Design Features

Building Construction Learning Objective 1 Explain the various loads exerted on a building resulting from environmental sources. Building Construction

Building Construction Load and Gravity Load – Any effect or force that a structure must resist Gravity Force on building through weight of building components and contents Snow, ice, or water on building adds weight Total weight of building exerts force on soil beneath it Building Construction

Building Construction Wind Basic effects Direct pressure Aerodynamic drag Negative pressure Secondary effects Rocking effect Vibration Clean-off effect Building Construction

Building Construction Earthquakes Slippage between tectonic plates of earth’s surface Subjects buildings to seismic forces Occur most frequently in fault zones Seismic loads far more complex than those for wind Vibrational motion can be 3-dimensional (Continued) Building Construction

Building Construction Earthquakes Torsional, resonant forces Force magnitude depends on factors Most significant force is horizontal motion Buildings with geometric irregularities more susceptible to earthquake damage Building codes require stronger seismic bracing for buildings with larger occupancies (Continued) Building Construction

Building Construction Earthquakes (Continued) Building Construction

Building Construction Earthquakes Design methods to protect against forces Increase stiffness using symmetrical shear walls and cross bracing Continuous structures with high degree of redundancy in structural frames Damping mechanisms Base isolation Building Construction

Building Construction Soil Pressure Exerts horizontal pressure against foundation Can only be estimated Magnitude depends on several factors Active or passive Building Construction

Building Construction Other Forces Temperature Vibration Shrinkage Building Construction

Building Construction Learning Objective 2 Distinguish between the classifications of loads based on origin and movement. Building Construction

Live Loads and Dead Loads Classification of types of forces resulting from gravity Dead – Weight of any permanent part of a building Live – Any load that is not fixed or permanent (Continued) Building Construction

Live Loads and Dead Loads Distribution of loads Uniformly distributed loads Concentrated load Snow loads Water loads Building Construction

Static and Dynamic Loads Static – Steady or applied gradually Dead loads Many live loads Dynamic – Involves motion Wind, earthquakes, vibration, falling objects Capable of delivering energy in addition to weight Building Construction

Structural Equilibrium and Reactions Equilibrium – When support provided by structural system is equal to the applied loads Reactions – Forces that resist the applied loads Building Construction

Building Construction Learning Objective 3 Recognize and discuss the internal forces resulting from the loads and forces applied to a structural member. Building Construction

Interior Forces Created by Exterior Loads Tension – Pull material apart Compression – Squeeze material Shear – Slide one plane past an adjacent plane Strength of materials varies with direction of interior forces (Continued) Building Construction

Interior Forces Created by Exterior Loads (Continued) Building Construction

Interior Forces Created by Exterior Loads Stress – Quantity used to evaluate magnitude of interior forces Factors of safety Stresses typically occur in combination within a member Keeping stresses within allowable values determines shape and size of structural members Building Construction

Exterior Loads Classified by Manner Applied Axial – Applied to center of cross section of structural member and perpendicular to that cross section Eccentric – Perpendicular to the cross section of the structural member but does not pass through the center of the cross section (Continued) Building Construction

Exterior Loads Classified by Manner Applied Torsional – Offset from center of cross section of the structural member and at an angle to or in the same plane as the cross section Loads change under fire conditions due to thermal energy released (Continued) Building Construction

Exterior Loads Classified by Manner Applied Building Construction

Building Construction Learning Objective 4 Describe the basic structural components. Building Construction

Building Construction Beams Simply supported Cantilever Continuous Restrained Primary design consideration – Ability to resist bending from applied loads Courtesy of Ed Prendergast (Continued) Building Construction

Building Construction Beams Building Construction

Building Construction Columns Designed to support an axial compressive load Thin columns fail by buckling Short, squatty columns fail by crushing Building Construction

Building Construction Arches Interior stresses primarily compressive Produce inclined forces at end supports Carry loads across a distance Courtesy of Donny Howard Building Construction

Building Construction Cables Flexible members used to support roofs and brace tents and restrain pneumatic structures When used to support loads over distance, will assume shape of parabola Stresses are tension Building Construction

Building Construction Trusses Framed units made up of group of triangles in one plane If loads applied at only point of intersection of truss members, only compressive or tensile stresses If has curved top chord, subjected to bending forces (Continued) Building Construction

Building Construction Trusses Provide inherently rigid frame Potential for early failure under adverse conditions; failure of any portion of top or bottom chords results in failure of truss (Continued) Building Construction

Building Construction Trusses Building Construction

Building Construction Truss Connectors Wood truss – Pins, bolts, gusset plates, adhesives, brackets, metal straps Steel trusses – Steel gusset plates, rivets, welds If connector fails, truss will fail Building Construction

Building Construction Truss Connectors Building Construction

Building Construction Learning Objective 5 Describe the basic structural systems. Building Construction

Structural Bearing Walls Span elements such as beams, trusses, and precast concrete slabs Usually exterior walls with interior support system of columns and beams Provide lateral support along direction of the wall Subjected to compressive loads Building Construction

Frame Structural Systems Walls enclose frame but provide no structural support Steel stud wall framing Post and beam construction Rigid frames Truss frames Slab and column frames Courtesy of Ed Prendergast Building Construction

Shell and Membrane Systems Consist primarily of enclosing surface with stresses resulting from applied loads occur within the surface Courtesy of Ed Prendergast Building Construction

Building Construction Membrane Structure Thin stretched flexible material Addressed in building codes if life of 180 days or more Addressed in fire codes if life of 180 days or less Cannot resist compressive forces Advantages Building Construction

Building Construction Shell Structure Rigid three-dimensional structures having a thickness that is small compared to other structural materials Regular geometric shapes Most commonly constructed of concrete Building Construction