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CE 405: Design of Steel Structures – Prof. Dr. A. Varma1.0 INTRODUCTION TO STRUCTURAL ENGINEERING1.1 GENERAL INTRODUCTIONStructural design is a systematic and iterative process that involves:1) Identification of intended use and occupancy of a structure – by owner2) Development of architectural plans and layout – by architect3) Identification of structural framework – by engineer4) Estimation of structural loads depending on use and occupancy5) Analysis of the structure to determine member and connection design forces6) Design of structural members and connections7) Verification of design8) Fabrication & Erection – by steel fabricator and contractor9) Inspection and Approval – by state building officialIdeally, the owner and the architect, the architect and the engineer, and the engineer and thefabricator/contractor will collaborate and interact on a regular basis to conceive, develop, design,and build the structure in an efficient manner. The primary responsibilities of all these playersare as follows: Owner - primary responsibility is deciding the use and occupancy, and approving thearchitectural plans of the building. Architect - primary responsibility is ensuring that the architectural plan of the buildinginterior is appropriate for the intended use and the overall building is aesthetically pleasing. Engineer – primary responsibility is ensuring the safety and serviceability of the structure,i.e., designing the building to carry the loads safely and . Fabricator – primary responsibility is ensuring that the designed members and connectionsare fabricated economically in the shop or field as required.1

CE 405: Design of Steel Structures – Prof. Dr. A. Varma Contractor/Erector - primary responsibility is ensuring that the members and connections areeconomically assembled in the field to build the structure. State Building Official – primary responsibility is ensuring that the built structure satisfiesthe appropriate building codes accepted by the Govt.1.2 STRUCTURAL DESIGN Conceptually, from an engineering standpoint, the parameters that can be varied (somewhat)are: (1) the material of construction, and (2) the structural framing plan. The choices for material include: (a) steel, (b) reinforced concrete, and (c) steel-concretecomposite construction. The choices for structural framing plan include moment resisting frames, braced frames, dualframes, shear wall frames, and so on. The engineer can also innovate a new structuralframing plan for a particular structure if required. All viable material framing plan alternatives must be considered and designed to comparethe individual material fabrication / erection costs to identify the most efficient andeconomical design for the structure. For each material framing plan alternative considered, designing the structure consists ofdesigning the individual structural components, i.e., the members and the connections, of theframing plan. This course CE405 focuses on the design of individual structural components. The materialof construction will limited be steel, and the structural framing plans will be limited to bracedframes and moment resisting frames.1.3 STRUCTURAL FRAMEWORK Figure 1 shows the structural plan and layout of a four-story office building to be located inLansing. Figure 2 and 3 show the structural elevations of frames A-A and B-B, respectively,which are identified in Figure 1.2

CE 405: Design of Steel Structures – Prof. Dr. A. VarmaNA25 ft.25 ft.35 ft.Pin/hinge connectionFix/moment connection35 ft.35 ft.Frame B-BFrame A -AFigure 1. Structural floor plan and layoutRS10 ft.ETPJQO12 ft.DINHMGLFK12 ft.C12 ft.B15 ft.A25 ft.25 ft.Figure 2. Structural elevation of frame A-A3

CE 405: Design of Steel Structures – Prof. Dr. A. Varmag10 ft. fenub1mta1lszkryjqxipwhov12 ft.d12 ft.c12 ft.b15 ft.a35 ft.35 ft.35 ft.Figure 3. Structural elevation of frame B-B As shown in Figure 1, the building has two 25-ft. bays in the north-south direction and three35 ft. bays in the east-west direction. There are four structural frames in the north-south direction. These frames have structuralelevations similar to frame A-A shown in Figure 2. There are three structural frames in the east-west directions. These frames have structuralelevations similar to frame B-B shown in Figure 3. The building has a roof truss, which is shown in Figures 2 and 3. Frame A-A is a braced frame, where all members are connected using pin/hinge connections.Diagonal bracing members are needed for stability. Frame B-B is a moment frame, where all members are connected using fix/momentconnections. There is no need for diagonal bracing members. The north-south and east-west frames resist the vertical gravity loads together. The three moment frames in the east-west direction resist the horizontal lateral loads in theeast-west direction.4

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CE 405: Design of Steel Structures – Prof. Dr. A. Varma The four braced frames in the north-south direction resist the horizontal lateral loads in thenorth-south direction.1.4 STRUCTURAL MEMBERSStructural 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 and bending moment only. Theaxial force in a beam member is negligible. Beam-column member – member subjected to combined axial force and flexural loads (shearforce, and bending moments)In basic structural analysis (CE305) students have come across two types of structures,namely, trusses and frames. For example, Figure 2 shows a roof truss supported by a bracedframe. All the members of a truss are connected using pin/hinge connections. All external forces areapplied at the pins/hinges. As a result, all truss members are subjected to axial forces (tensionor compression) only. In braced and moment frames, the horizontal members (beams) are subjected to flexuralloads only. In braced frames, the vertical members (columns) are subjected to compressive axial forcesonly. In braced frames, the diagonal members (braces) are subjected to tension/compression axialforces only. In moment frames, the vertical members (beam-columns) are subjected to combined axialand flexural loads.5

CE 405: Design of Steel Structures – Prof. Dr. A. VarmaFor practice, let us categorize the member shown in Figures 2 and 3.RS10 ft.ETPOQJ12 ft.DINHMGLFK12 ft.C12 ft.B15 ft.A25 ft.25 ft.Figure 2. Structural elevation of frame A-Ag10 ft. fenub1mta1lszkryjqxipwhov12 ft.d12 ft.c12 ft.b15 ft.a35 ft.35 ft.Figure 3. Structural elevation of frame B-B635 ft.

CE 405: Design of Steel Structures – Prof. Dr. A. Varma1.5 STRUCTURAL CONNECTIONSMembers of a structural frame are connected together using connections. Prominentconnection types include: (1) truss / bracing member connections; (2) simple shear connections;(3) fully-restrained moment connections; and (4) partially-restrained flexible momentconnections. 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 forcontinuity. 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 bendingmoments are not transferred through the connection. Moment connections are fix connections used to connect beam to column members. Both theshear forces and bending moments are transferred through the connections with very smalldeformations (full restraint). Partially restrained connections are flexible connections used to connect beam to columnmembers. The shear forces are transferred fully through the connection. However, thebending moment is only transferred partially.SGussetFigure 4. Truss connection at S in Frame A-A.7

CE 405: Design of Steel Structures – Prof. Dr. A. VarmaBracing memberBeamBeamBracing memberFigure 5. Bracing connection and Simple Shear Connection at G in Frame A-A.8

CE 405: Design of Steel Structures – Prof. Dr. A. VarmaBeamColumnFigure 6. All-bolted double angle shear connection. Bevel Full penetration groove weld Field welding Weld access hole back-up barBeam fillet welds shear tabsColumnFigure 7. Directly welded flange fully restrained moment connection.9

CE 405: Design of Steel Structures – Prof. Dr. A. Varma Figure 4 shows an example truss connection. Figure 5 shows an example bracingconnection. Figure 6 shows an example shear connection. Figure 7 shows an examplemoment connection. Connections are developed using bolts or welds. Bolts are used to connect two or more plate elements that are in the same plane. Boltholes are drilled in the plate elements. The threaded bolt shank passes through the holes,and the connection is secured using nuts. Bolts are usually made of higher strength steel. Welds can be used to connect plate elements that are in the same or different planes. Ahigh voltage electric arc is developed between the two plate elements. The electric arccauses localized melting of the base metal (plate element) and the weld electrode. Aftercooling, all the molten metal (base and weld) solidifies into one continuum. Thus,developing a welded connection. In Figure 4, all the truss members are connected together by welding to a common gussetplate. The axial forces in the members are transferred through the gusset plates. Thissame connection can also be developed using bolts. How? In Figure 5, the bracing members are connected to gusset plates, which are alsoconnected to the beam and column. The bracing member can be connected to the gussetplate using bolts or welds. However, the gusset plate has to be welded to the beam /column. In Figure 6, two angles are bolted to the web of the beam. The perpendicular legs of theangles are bolted to the flange of the column. Thus, an all-bolted double-angle shearconnection is achieved. This all-bolted connection will be easier to assemble in the fieldas compared to welding. How is this a shear connection? In Figure 7, the beam flanges are beveled and welded directly to the flange of columnusing full penetration groove welds. This welding will have to be done in the field duringerection and it will require the use of back-up bars. Weld-access holes and skilledwelders are required to achieve a weld of acceptable quality. In Figure 7, the beam web is bolted to a shear tab (plate), which is fillet welded to thecolumn in the shop. This shear tab connection transfers the shear from the beam to thecolumn. How is Figure 7 a moment connection?10

CE 405: Design of Steel Structures – Prof. Dr. A. Varma1.6 Structural LoadsThe building structure must be designed to carry or resist the loads that are applied to it overits design-life. The building structure will be subjected to loads that have been categorized asfollows: Dead Loads (D): are permanent loads acting on the structure. These include the self-weightof structural and non-structural components. They are usually gravity loads. Live Loads (L): are non-permanent loads acting on the structure due to its use andoccupancy. The magnitude and 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 thebuilding. They cause horizontal lateral loads (forces) on the structure, which can be criticalfor tall buildings. Wind loads also cause uplift of light roof systems. Snow Loads (S): are vertical gravity loads due to snow, which are subjected to variability dueto seasons and drift. Roof Live Load (Lr): are live loads on the roof caused during the design life by planters,people, or by workers, equipment, and materials during maintenance. Values of structural loads are given in the publication ASCE 7-98: Minimum Design Loadsfor Buildings and Other Structures. The first phase of structural design consists of estimatingthe loads acting on the structure. This is done using the load values and combinationspresented in ASCE 7-98 as explained in the following sub-sections.11

CE 405: Design of Steel Structures – Prof. Dr. A. Varma1.6.1 Step I. Categorization of Buildings Categories I, II, III, and IV. See Table 1.1 below and in ASCE 7-98.12

CE 405: Design of Steel Structures – Prof. Dr. A. Varma1.6.2 Dead Loads (D)Dead loads consist of the weight of all materials of construction incorporated into thebuilding including but not limited to walls, floors, roofs, ceilings, stairways, built-in partitions,finishes, cladding and other similarly incorporated architectural and structural items, and fixedservice equipment such as plumbing stacks and risers, electrical feeders, and heating, ventilating,and air conditioning systems.In some cases, the structural dead load can be estimated satisfactorily from simple formulasbased in the weights and sizes of similar structures. For example, the average weight of steelframed buildings is 60-75 lb/ft2, and the average weight for reinforced concrete buildings is 110 130 lb/ft2.From an engineering standpoint, once the materials and sizes of the various components ofthe structure are determined, their weights can be found from tables that list their densities. SeeTables 1.2 and 1.3, which are taken from Hibbeler, R.C. (1999), Structural Analysis, 4th Edition.13

CE 405: Design of Steel Structures – Prof. Dr. A. Varma1.6.3 Live Loads Building floors are usually subjected to uniform live loads or concentrated live loads. Theyhave to be designe