68402: Structural Design of Buildings II61420: Design of Steel Structures62323: Architectural Structures II Monther Dwaikat Assistant Professor Department of Building Engineering An-Najah National University Introduction to Structural Design of Steel
Contents • Structural Design • Design Loads • Structural Steel - Properties • Design philosophies • Determining load and resistance factors • Load and resistance factors
Introduction to Design of Steel Structures • General Introduction • Structural design is a systematic & iterative process that involves: • Identification of intended use & occupancy of a structure – by owner • Development of architectural plans & layout – by architect • Identification of structural framework – by engineer • Estimation of structural loads depending on use & occupancy • Analysis of the structure to determine member & connection design forces • Design of structural members & connections • Verification of design • Fabrication & Erection – by steel fabricator & contractor • Inspection & Approval – by state building official
Primary Responsibilities • The primary responsibilities are: • Owner - primary responsibility is deciding the use & occupancy, & approving the arch. plans of the building. • Architect - primary responsibility is ensuring that the architectural plan of the building interior is appropriate for the intended use & the overall building is aesthetically pleasing. • Engineer – primary responsibility is ensuring the safety & serviceability of the structure, i.e., designing the building to carry the loads safely.
Primary Responsibilities • Fabricator – primary responsibility is ensuring that the designed members & connections are fabricated economically in the shop or field as required. • Contractor/Erector - primary responsibility is ensuring that the members & connections are economically assembled in the field to build the structure. • State Building Official – primary responsibility is ensuring that the built structure satisfies the appropriate building codes accepted by the Govt.
Structural Design • Conceptually, from an engineering standpoint, the parameters that can be varied (somewhat) are: • The material of construction • The structural framing plan. • The choices for material include: • Steel • Reinforced concrete • Steel-concrete composite construction. • The choices for structural framing plan include: • Moment resisting frames. • Braced frames. • Dual frames • Shear wall frames, and so on. • The engineer can also innovate a new structural framing plan for a particular structure if required.
Structural Design • All viable material + framing plan alternatives must be considered & designed to compare the individual material + fabrication / erection costs to identify the most efficient & economical design for the structure. • For each material + framing plan alternative considered, designing the structure consists of designing the individual structural components, i.e., the members & the connections, of the framing plan.
Assume dimensions, structural conditions and cross sections Structural Analysis Selection of cross sections to satisfy structural requirements Does the design violate the initial assumptions? YES NO Final Design Structural Design • Determination of dimensions and selection of cross sections. • The design process is a loop:
Strength Serviceability Optimal design Economy Structural Design • Optimal structural design shall achieve balance between the following requirements:
Roles and responsibilities of the structural steel designer • Arrange and proportion the members of the structures, using engineer’s intuition and sound engineering principles, so that they can be practically erected, have sufficient strength (safe), and are economical. • Practicality: Ensure structures can be fabricated and erected without problems • Safety: Ensure structures can safely support the loads. Ensure deflections and vibrations are controlled for occupants comfort. • Cost: Minimize costs without sacrifice of strength (consider labor costs in fabrication and erection, not just material costs)
Basic Structural Shapes • Trusses • Frames ( Beam-Column) • Beams • Girders • Columns • Space trusses/frames
Purlins Columns Beams-Frames Bracing Steel Structures
Steel Structures Industrial/Parking structures “Frames”
Steel Structures Joists/Trusses
Steel Structures High rise buildings
Steel Structures • Girder bridges
Steel Structures • Truss bridges
Steel Structures • Cable stayed & suspended bridges
Structural Members • Structural members are categorized based up on the internal forces in them. For example: • Tension member –subjected to tensile axial force only • Column or compression member –subjected to compressive axial force only • Tension/Compression member –subjected to tensile/compressive axial forces • Beam member –subjected to flexural loads, i.e., shear force & bending moment only. The • axial force in a beam member is negligible. • Beam-column member – member subjected to combined axial force & flexural loads (shear • force, & bending moments)
Structural Members • In trusses: • All the members are connected using pin/hinge connections. • All external forces are applied at the pins/hinges. • All truss members are subjected to axial forces (tension or compression) only. • In frames: • The horizontal members (beams) are subjected to flexural loads only. • In braced frames: • The vertical members (columns) are subjected to compressive axial forces only. • The diagonal members (braces) are subjected to tension/compression axial forces only. • In moment frames • The vertical members (beam-columns) are subjected to combined axial & flexural loads.
Structural Connections • Members of a structural frame are connected together using connections. Prominent connection types include: • Truss / bracing member connections are used to connect two or more truss members together. Only the axial forces in the members have to be transferred through the connection for continuity. • Simple shear connections are the pin connections used to connect beam to column members. Only the shear forces are transferred through the connection for continuity. The bending moments are not transferred through the connection. • Moment connections are fix connections used to connect beam to column members. Both the shear forces & bending moments are transferred through the connections with very small deformations (full restraint).
Structural Connections Truss connection Simple Shear connection Moment resisting connection
Structural Loads • The building structure must be designed to carry or resist the loads that are applied to it over its design-life. The building structure will be subjected to loads that have been categorized as follows: • Dead Loads (D): are permanent loads acting on the structure. These include the self-weight of structural & non-structural components. They are usually gravity loads. • Live Loads (L): are non-permanent loads acting on the structure due to its use & occupancy. The magnitude & location of live loads changes frequently over the design life. Hence, they cannot be estimated with the same accuracy as dead loads. • Wind Loads (W): are in the form of pressure or suction on the exterior surfaces of the building. They cause horizontal lateral loads (forces) on the structure, which can be critical for tall buildings. Wind loads also cause uplift of light roof systems.
Structural Loads • Snow Loads (S): are vertical gravity loads due to snow, which are subjected to variability due to seasons & drift. • Roof Live Load (Lr): are live loads on the roof caused during the design life by planters, people, or by workers, equipment, & materials during maintenance. • Values of structural loads can be computed based on the design code.
Dead Loads (D) • Dead loads consist of the weight of all materials of construction incorporated into the building including but not limited to walls, floors, roofs, ceilings, stairways, built-in partitions, finishes, cladding & other similarly incorporated architectural & structural items, & fixed service equipment such as plumbing stacks & risers, electrical feeders, & heating, ventilating, & air conditioning systems. • In some cases, the structural dead load can be estimated satisfactorily from simple formulas based in the weights & sizes of similar structures. For example, the average weight of steel framed buildings is 3 - 3.6 kPa, & the average weight for reinforced concrete buildings is 5 - 6 kPa.
Dead Loads (D) • From an engineering standpoint, once the materials and sizes of the various components of the structure are determined, their weights can be found from tables that list their densities. See Tables 1.2 & 1.3, which are taken from Hibbeler, R.C. (1999), Structural Analysis, 4th Edition.
Live Loads – Summary Table • Building floors are usually subjected to uniform live loads or concentrated live loads. They have to be designed to safely support these loads.
Wind Loads • Design wind loads for buildings can be based on: (a) simplified procedure; (b) analytical procedure; & (c) wind tunnel or small-scale procedure. • Refer to ASCE 7-05 for the simplified procedure. This simplified procedure is applicable only to buildings with mean roof height less than 18 m or the least dimension of the building. • The wind tunnel procedure consists of developing a small-scale model of the building & testing it in a wind tunnel to determine the expected wind pressures etc. It is expensive & may be utilized for difficult or special situations. • The analytical procedure is used in most design offices. It is fairly systematic but somewhat complicated to account for the various situations that can occur:
Wind Loads • Wind velocity will cause pressure on any surface in its path. The wind velocity & hence the velocity pressure depend on the height from the ground level. Equation 1.3 is recommended by ASCE 7-05 for calculating the velocity pressure (qz) in SI qz= 0.613 Kz KztKd V2 I (N/m2)
Wind Loads qz – Static wind pressure V - the wind velocity in m/s Kd- a directionality factor (= 0.85 see Table 6.4 page 80) Kzt - a topographic factor (= 1.0) I - the importance factor (=1.0) Kz - varies with height z above the ground level (see Table 6.3 page 79) exposure B structure surrounded by buildings/forests/… at least 6m height exposure C open terrain
Wind Loads • A significant portion of Palestine has V = 100 km/h. At these location qz= 402 Kz(N/m2) The velocity pressure qzis used to calculate the design wind pressure (p) for the building structure conservatively as follows: p = q GCp (N/m2)
ASCE 7-05 pg. 79 Kz - varies with height z above the ground level A – large city centers B – urban/ suburban area C – open terrain with scattered obstructions D – Flat unobstructed surface
Wind Loads G - gust effect factor (= 0.85) Cp - external pressure coefficient from Figure 6-6 page 48-49 in ASCE 7-05 or Cp = 0.8 windward Cp = -0.5 leeward Cp = -0.7 sidewalls Cp = -0.7 slope<0.75 (1.5) • Note that: • A positive sign indicates pressure acting towards a surface. • Negative sign indicates pressure away from the surface
15 m 15 m 15 m 15 m Plan Example 1.1 – Wind Load • Consider the building structure with the structural floor plan & elevation shown below. Estimate the wind loads acting on the structure when the wind blows in the east-west direction. The structure is located in Nablus.
6 @ 3m 6 @ 3m Example 1.1 – Wind Load
Example 1.1 – Wind Load • Velocity pressure (qz) • Kd- directionality factor = 0.85 • Kzt- topographic factor = 1.0 • I - importance factor = 1.0 • V = 100 kph in Nablus qz= 402 Kz(N/m2) • Kz - varies with height z above the ground level • Kz values for Exposure B, Case 2
Example 1.1 – Wind Load • Wind pressure (p) • Gust factor = G = 0.85 for rigid structures • External pressure coefficient = Cp= +0.8 for windward walls • Cp= -0.5 for leeward walls • Cp= -0.7 for side walls • External pressure = q G Cp • External pressure on windward wall = qz GCp= 402 Kzx 0.85 x 0.8 = 273.4 Kz Pa toward surface • External pressure on leeward wall = qh GCp= 402 K18x 0.85 x (-0.5) = 145.2 Pa away from surface • External pressure on side wall = qh GCp= 402 K18x 0.85 x (-0.7) = 203.3 Pa away from surface • The external pressures on the structure are shown in the following two figures.
203.3 273.4 Kz 145.2 203.3 Example 1.1 – Wind Load
3 m 232.4 221.5 3 m 3 m 207.8 145.2 191.4 3 m 180.4 169.5 3 m 155.8 3 m Example 1.1 – Wind Load
Background of Structural Steel • Economical production in large volume not available until mid 19th century and the introduction of the Bessemer process. Steel became the principal metallic structural material by 1890. • Steels consists almost entirely of iron (over 98%) and small quantities of carbon, silicon, manganese, sulfur, phosphorus, and other elements. • The quantities of carbon affect properties of steel the most. • Increase of carbon content increases hardness and strength • Alloy steel – has additional amounts of alloy elements such chronium, vanadium, nickel, manganese, copper, or zirconium. • The American Society for Testing of Materials (ASTM) specifies exact maximum percentages of carbon content and other additions for a number of structural steels. Consult Manual, Part 2, Table 2-1 to 2-3 for availability of steel in structural shapes, plate products, and structural fasteners.
ASTM classifications of structural steels • Carbon steels – A36, A53, A500, A501, A529, A570. Have well-defined yield point. Divided into four categories: • Low-carbon steel (< 0.15%) • Mild steel (0.15 to 0.29%, structural carbon steels) • Medium-carbon steel (0.3 to 0.59%) • High-carbon steel (0.6 to 1.7%) • High-Strength Low-Alloy steels – A242, A572, A588, A606, A607, A618, A709 • Well-defined yield point • Higher strengths and other properties • Alloy Steels – A514, A709, A852, A913. • Yield point defined as the stress at 0.2% offset strain • Low-alloy steels quenched and tempered → 550 to 760 MPa yield strengths
Advantages and disadvantages of steel as a structural material • Advantages • High strength per unit of weight → smaller weight of structures • Uniformity • Elasticity • Long lasting • Ductility • Toughness • Easy connection • Speed of erection • Ability to be rolled into various sizes and shapes • Possible reuse and recyclable
Advantages and disadvantages of steel as a structural material • Disadvantages • Maintenance costs • Fire protection/Fireproofing costs • Susceptibility to buckling failure • Fatigue • Brittle fracture
Three basic types of steel used for structural steel Plain Carbon Steel Low-alloy steel High-alloy “specialty steel” The most commonly used is mild steel - ASTM A36 Typical high strength steel: The higher the steel strength, the higher the carbon content and the less ductile it is. Types of Steel ASTM A242 ASTM A992
Standard Plain Carbon Steel Stress “f” Necking & Fracture Strain Hardening Fu Fy Yield plateau E Strain “e” Elastic Stress-strain curve
What is a Limit State • When a structure or structural element becomes unfit for its intended purpose it has reached or exceeded a limit state • Two categories of limit states: • Strength limit states • Serviceability limit states
Limit States Strength Limit States a) Loss of Equilibrium b) Loss of load bearing capacity c) Spread of local failure d) Very large deformations Serviceability Limit States a) Excessive deflection b) Excessive local damage c) Unwanted vibration
Design Philosophies • Allowable Stress Design (ASD) • Plastic Design (PD) • Load and Resistance Factor Design (LRFD)
Allowable Stress Design • Service loads are calculated as expected during service life. • Linear elastic analysis is performed. • A factor of safety (FOS) of the material strength is assumed (usually 3-4) • Design is satisfactory if (maximum stress < allowable stress) • Limitations • Case specific, no guarantee that our design covers all cases • Arbitrary choice of FOS?!