Civil Engineering: Design of Beams for Moments
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Questions and Answers

What happens to the buckling moment as the unbraced length is further increased?

  • It becomes smaller and smaller (correct)
  • It remains unchanged
  • It becomes constant
  • It increases

    • What is the primary reason for the beam cross section to twist in elastic buckling?

  • The yield stress is exceeded
  • The moment capacity is reached
  • The warping resistance is exceeded
  • The compression flange moves laterally (correct)

    • What occurs when the compression flange of a beam begins to buckle?

  • The beam experiences longitudinal buckling
  • The beam undergoes lateral compression
  • The beam becomes stronger due to increased torsional resistance
  • The beam undergoes torsion or twisting (correct)

    • What is the primary purpose of checking other beam design criteria?

    <p>To ensure the beam's moment capacity is sufficient</p> Signup and view all the answers

    Which type of beam sections have a low resistance to lateral buckling and torsion?

    <p>W, S, and channel shapes</p> Signup and view all the answers

    What is the primary factor that determines the critical moment in elastic buckling?

    <p>The combination of torsional and warping resistances</p> Signup and view all the answers

    What type of beam sections have a high torsional resistance?

    <p>Built-up box shapes</p> Signup and view all the answers

    What is the primary reason for considering lateral bracing in beam design?

    <p>To prevent buckling of the compression flange</p> Signup and view all the answers

    When is a beam considered to have full lateral support?

    <p>When it is wholly encased in concrete or has its compression flange incorporated in a concrete slab</p> Signup and view all the answers

    What is the primary purpose of designing beams in Zone 1?

    <p>To ensure the beam's moment capacity is sufficient</p> Signup and view all the answers

    When can friction be assumed to provide full lateral support?

    <p>When the loads on the slab are fairly well fixed in position</p> Signup and view all the answers

    What happens to the beam as the moment is increased in elastic buckling?

    <p>The beam's deflection increases</p> Signup and view all the answers

    What is required when deciding what constitutes satisfactory lateral support for a steel beam?

    <p>Some judgment</p> Signup and view all the answers

    What is the primary design consideration in Zone 3 of beam design?

    <p>Preventing elastic buckling</p> Signup and view all the answers

    What happens to the beam when it undergoes torsion?

    <p>It undergoes rapid failure</p> Signup and view all the answers

    What determines the rate of failure of a beam undergoing torsion?

    <p>The torsional strength of the beam</p> Signup and view all the answers

    What is the purpose of using design charts of beams?

    <p>To select the lightest available section</p> Signup and view all the answers

    What is the value of Cb when the self-weight of the member is neglected?

    <p>1.67</p> Signup and view all the answers

    What is the length of the beam in Example 9-9?

    <p>17 ft</p> Signup and view all the answers

    What is the purpose of bracing in the design of beams?

    <p>To provide lateral support at specific points</p> Signup and view all the answers

    What is the value of the steel used in Examples 9-9 and 9-10?

    <p>50 ksi</p> Signup and view all the answers

    What is the effect of considering the self-weight of the member on the value of Cb?

    <p>It decreases the value of Cb</p> Signup and view all the answers

    What is the purpose of using the LRFD and ASD methods?

    <p>To select the lightest available section</p> Signup and view all the answers

    What is the figure number that represents the situation in Example 9-9?

    <p>Fig. 9.15</p> Signup and view all the answers

    What is a benefit of using fewer pieces in structural steel design?

    <p>Reduced fabrication time</p> Signup and view all the answers

    How many main types of welding procedures are used in structural applications?

    <p>Four</p> Signup and view all the answers

    What is Shielded Metal Arc Welding (SMAW) also known as?

    <p>Manual, stick, or hand welding</p> Signup and view all the answers

    What type of welding is also known as MIG welding?

    <p>Gas Metal Arc Welding (GMAW)</p> Signup and view all the answers

    What is a characteristic of Gas Metal Arc Welding (GMAW)?

    <p>Uses a continuous wire fed into the welding gun</p> Signup and view all the answers

    What is an advantage of Gas Metal Arc Welding (GMAW)?

    <p>It is fast and economical</p> Signup and view all the answers

    What type of welding uses an electric arc produced between the end of a coated metal electrode and the steel components to be welded?

    <p>Shielded Metal Arc Welding (SMAW)</p> Signup and view all the answers

    What is a characteristic of Submerged Arc Welding (SAW)?

    <p>Information not available in the content</p> Signup and view all the answers

    What is the desirable shape of a fillet weld?

    <p>Flat or slightly convex</p> Signup and view all the answers

    What is the ideal angle of a fillet weld with respect to the pieces being welded?

    <p>45˚</p> Signup and view all the answers

    What is the theoretical throat of the weld?

    <p>The shortest distance from the root of the weld to its diagrammatic face</p> Signup and view all the answers

    Why are concave surface fillet welds not desirable?

    <p>They have a tendency to crack upon cooling</p> Signup and view all the answers

    What is the characteristic of a 45˚ fillet weld?

    <p>Leg sizes are equal</p> Signup and view all the answers

    What is the effect of a concave surface on the stress distribution in a fillet weld?

    <p>It reduces stress concentration</p> Signup and view all the answers

    What is the desirable leg size of a fillet weld?

    <p>Equal</p> Signup and view all the answers

    What is the importance of the shape of a fillet weld?

    <p>It affects the strength of the weld</p> Signup and view all the answers

    What is the primary advantage of using welded connections over bolted connections in structural steel design?

    <p>They provide a more rigid structure</p> Signup and view all the answers

    What is a benefit of using welded structures over bolted structures in terms of design flexibility?

    <p>It is easier to make changes to the design</p> Signup and view all the answers

    What is a characteristic of the joints in welded structures?

    <p>They are as strong as or stronger than the base metal</p> Signup and view all the answers

    What is an advantage of welded connections in terms of noise level?

    <p>They are relatively quiet</p> Signup and view all the answers

    What is the primary reason for using welded connections in structural steel design?

    <p>To produce a one-piece construction</p> Signup and view all the answers

    What is the primary advantage of using welded connections in terms of structure flexibility?

    <p>They reduce the flexibility of the structure</p> Signup and view all the answers

    What is the primary characteristic of welded structures?

    <p>They are more rigid than bolted structures</p> Signup and view all the answers

    What is the primary difference between welded and bolted connections in terms of load transfer?

    <p>Welded connections transfer load directly between members</p> Signup and view all the answers

    What is the value of Cb when the self-weight of the member is neglected?

    <p>1.14</p> Signup and view all the answers

    What is Lr a function of in the calculation of moment capacities?

    <p>Cross-sectional area, modulus of elasticity, yield stress, and warping and torsional properties</p> Signup and view all the answers

    Where can numerical values for commonly used sections be found?

    <p>AISC Manual Table 3-2</p> Signup and view all the answers

    What is being referred to when calculating moment capacities in Zone 2?

    <p>Nominal moment strengths for unbraced lengths between Lp and Lr</p> Signup and view all the answers

    What is the purpose of the complex formulas in the AISC Specification (F1)?

    <p>To calculate the value of Lr</p> Signup and view all the answers

    What is the purpose of Table 3-2 in the AISC Manual?

    <p>To list the values of Lr for commonly used sections</p> Signup and view all the answers

    What is the equation used for in the calculation of moment capacities?

    <p>To calculate the nominal moment strengths for unbraced lengths</p> Signup and view all the answers

    What is the range of unbraced lengths for which the nominal moment strengths are calculated?

    <p>Between Lp and Lr</p> Signup and view all the answers

    What is the primary effect on the strength of a W shape beam when it is turned on its side?

    <p>It becomes 10 to 30 percent weaker</p> Signup and view all the answers

    Where can the proper plastic modulus about the y axis for a beam be found when it is turned on its side?

    <p>Table 3-4 of the Manual</p> Signup and view all the answers

    What is the primary reason for the reduction in strength of a W shape beam when it is turned on its side?

    <p>The change in the gravity loads</p> Signup and view all the answers

    What is the primary purpose of designing beams in Zone 1 of the AISC beam design?

    <p>To ensure that the beam is designed to resist both lateral and torsional buckling</p> Signup and view all the answers

    What is the range of percentage reduction in strength of a W shape beam when it is turned on its side?

    <p>10 to 30 percent</p> Signup and view all the answers

    What is the effect of turning a W shape beam on its side on its resistance to lateral buckling and torsion?

    <p>It decreases its resistance to lateral buckling and torsion</p> Signup and view all the answers

    What is the primary reason for considering lateral bracing in beam design?

    <p>To prevent lateral buckling and torsion</p> Signup and view all the answers

    What is the primary purpose of considering the orientation of a W shape beam in design?

    <p>To determine the beam's resistance to lateral buckling and torsion</p> Signup and view all the answers

    What is the primary factor that determines the critical moment in elastic buckling?

    <p>The modulus of elasticity of the material</p> Signup and view all the answers

    What is the primary factor that affects the strength of a W shape beam when it is subjected to gravity loads?

    <p>The beam's orientation</p> Signup and view all the answers

    What is the primary purpose of using design charts of beams?

    <p>To determine the design criteria for the beam</p> Signup and view all the answers

    What is the primary benefit of using Gas Metal Arc Welding (GMAW) in structural applications?

    <p>It produces high-quality welds</p> Signup and view all the answers

    What is the significance of the plastic modulus values in the table with respect to the orientation of a beam?

    <p>They are only applicable when the beam is in its upright position</p> Signup and view all the answers

    What is the characteristic of a 45˚ fillet weld?

    <p>It has a high strength-to-weight ratio</p> Signup and view all the answers

    What is the effect of a concave surface on the stress distribution in a fillet weld?

    <p>It increases the stress concentration</p> Signup and view all the answers

    What is the primary importance of the shape of a fillet weld?

    <p>It affects the strength of the weld</p> Signup and view all the answers

    What is the nominal strength of the base metal determined by?

    <p>The area of the base metal and nominal stress of the base metal</p> Signup and view all the answers

    What is the purpose of Table J2.5 in the AISC Specification?

    <p>To provide the weld values needed to use the equations for nominal strength</p> Signup and view all the answers

    What is the unit of FnBM and Fnw?

    <p>ksi</p> Signup and view all the answers

    What is the equation for the nominal strength of the weld metal?

    <p>Rn = Fnw Awe</p> Signup and view all the answers

    What is the term for the area of the weld?

    <p>Awe</p> Signup and view all the answers

    What is the purpose of determining the nominal strength of a particular weld?

    <p>To determine the design strength of the weld</p> Signup and view all the answers

    What is the term for the nominal stress of the weld metal?

    <p>Fnw</p> Signup and view all the answers

    What is the term for the area of the base metal?

    <p>ABM</p> Signup and view all the answers

    Study Notes

    Design of Beams for Moments

    • Elastic buckling occurs when the unbraced length (Lr) is greater than the section will buckle elastically before the yield stress is reached anywhere
    • As the unbraced length increases, the buckling moment becomes smaller
    • The beam will deflect transversely until a critical moment value (Mcr) is reached, causing the beam cross-section to twist and the compression flange to move laterally
    • Mcr is provided by the torsional resistance and warping resistance of the beam

    Full Plastic Moment (Zone 1)

    • In beam design, moments, shears, deflections, crippling, lateral bracing, fatigue, and others need to be considered
    • Beams are selected based on sufficient design moment capacities (Φb Mn) and then checked for other critical items
    • When the compression flange buckles, twisting or torsion occurs, and the smaller the torsional strength of the beam, the more rapid the failure
    • W, S, and channel shapes have limited torsional resistance, while built-up box shapes have more torsional resistance

    Lateral Support of Beams

    • Judgment is needed to decide what constitutes satisfactory lateral support for a steel beam
    • A beam wholly encased in concrete or with a compression flange in a concrete slab is well-supported laterally
    • When a concrete slab rests on the top flange of a beam, friction may provide full lateral support if the loads on the slab are fixed in position

    Design Charts of Beams

    • Example 9-9: Select the lightest available section for a beam with bracing at the ends and center line, using 50 ksi steel and both LRFD and ASD methods
    • Cb is 1.67 if the only uniform load is the member self-weight and it is neglected
    • Example 9-10: Using 50 ksi steel and both LRFD and ASD methods, select the lightest available section for a beam with a specific situation

    Welding Methods

    • There are four main types of welding procedures used in structural applications: Shielded Metal Arc Welding (SMAW), Gas Metal Arc Welding (GMAW), Flux Cored Arc Welding (FCAW), and Submerged Arc Welding (SAW)

    Shielded Metal Arc Welding (SMAW)

    • Also known as manual, stick, or hand welding
    • An electric arc is produced between the end of a coated metal electrode and the steel components to be welded

    Gas Metal Arc Welding (GMAW)

    • Also known as MIG welding
    • Fast and economical
    • A continuous wire is fed into the welding gun, and the process may be automated

    Fillet Welds

    • The root of the weld is the point at which the faces of the original metal pieces intersect
    • The theoretical throat of the weld is the shortest distance from the root of the weld to its diagrammatic face
    • For a 45˚ or equal leg fillet, the throat dimension is 0.707 times the leg of the weld
    • A concave surface is not desirable due to the tendency to crack upon cooling
    • A desirable fillet weld has a flat or slightly convex surface and an angle of around 45˚ with respect to the pieces being welded

    Design of Beams for Moments

    • The plastic moduli values in the table are given about the horizontal axes for beams in their upright positions.
    • If a beam is turned on its side, the proper plastic modulus about the y-axis can be found in Table 3-4 of the Manual or in the tables giving dimensions and properties of shapes in Part 1 of the AISC Manual.
    • A W shape turned on its side may be only 10 to 30 percent as strong as one in the upright position when subjected to gravity loads.

    Bending Coefficients

    • Bending coefficients are used to calculate the nominal moment strengths for unbraced lengths between Lp and Lr.
    • Lr is a function of several of the section's properties, such as its cross-sectional area, modulus of elasticity, yield stress, and warping and torsional properties.

    Moment Capacities (Zone 2)

    • The nominal moment strengths for unbraced lengths between Lp and Lr are calculated with a specific equation.
    • Numerical values for Lr have been determined for sections normally used as beams and are given in AISC Manual Table 3-2, entitled "W Shapes Selected by Zx".

    Noncompact Sections

    • Noncompact sections have specific design requirements.

    Welded Connections

    • Welding is a process by which metallic parts are connected by heating their surfaces to a plastic or fluid state and allowing the parts to flow together and join.
    • Welded structures are more rigid because the members are often welded directly to each other.
    • Advantages of welding include:
      • The process of fusing pieces together creates the most truly continuous structures.
      • It is easier to make changes in design and to correct errors during erection.
      • Welding is relatively silent.

    Advantages of Welding

    • Table 14.1 (AISC Specification Table J2.5) provides nominal strengths for various types of welds.
    • The design strength of a particular weld (ФRn) and the allowable strength (Rn /Ω) of welded joints shall be the lower value of the base material strength and the weld metal strength.

    Design Strength of Welded Joints

    • The nominal strength of the base metal is: Rn = FnBM ABM (AISC Equation J2-2)
    • The nominal strength of the weld metal is: Rn = Fnw Awe (AISC Equation J2-3)
    • In the equations: FnBM = nominal stress of the base metal, ksi; Fnw = nominal stress of the weld metal, ksi; ABM = area of the base metal, in2; Awe = area of the weld, in2.

    AISC Specification (Table J2.5)

    • Table J2.5 in AISC Specification provides the weld values needed to use these equations: Ф, Ω, FBM, and Fw.

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    Description

    This quiz covers the design of beams for moments in civil engineering, specifically elastic buckling and its relation to unbraced length and yield stress.

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