Structural Steel Design Chapter 9
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Questions and Answers

What is the value of the bending coefficient Cb according to the text?

  • 2.5
  • 2.38 (correct)
  • 1.14
  • 1.5

    • What is the purpose of the equation for calculating nominal moment strengths?

  • To calculate the modulus of elasticity of the beam
  • To calculate the nominal moment strengths for unbraced lengths between Lp and Lr (correct)
  • To determine the cross-sectional area of the beam
  • To determine the warping properties of the beam

    • What is Lr a function of?

  • Several properties of the beam, including its cross-sectional area, modulus of elasticity, yield stress, and warping and torsional properties (correct)
  • Only the cross-sectional area of the beam
  • Only the yield stress of the beam
  • Only the warping and torsional properties of the beam

    • Where can the numerical values of Lr be found?

    <p>In the AISC Manual Table 3-2, entitled “W Shapes Selected by Zx”</p> Signup and view all the answers

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

    <p>To calculate Lr</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 name of the table in the AISC Manual that contains the numerical values of Lr?

    <p>W Shapes Selected by Zx</p> Signup and view all the answers

    Why are the formulas in the AISC Specification (F1) complex?

    <p>Because they involve many variables</p> Signup and view all the answers

    What can reduce friction in structures such as bridges and buildings?

    <p>Appreciable vibration</p> Signup and view all the answers

    What type of support can be provided by connecting beams or special members inserted for that purpose?

    <p>Lateral support</p> Signup and view all the answers

    What type of support is provided to the compression flange by beams that frame into the sides of the beam?

    <p>Full lateral support</p> Signup and view all the answers

    What should the designer check before assuming lateral support from connecting beams?

    <p>If the beams themselves are prevented from moving</p> Signup and view all the answers

    What is the condition for the buckling stress of doubly symmetric I-shaped members in AISC Specification?

    <p>Mn = FcrSx ≤ Mp</p> Signup and view all the answers

    What type of system may be desirable in one of the bays to provide lateral support?

    <p>X-bracing system</p> Signup and view all the answers

    What is the purpose of the beams represented with horizontal dashed lines in Fig. 9.7?

    <p>To provide questionable lateral support to the main beams</p> Signup and view all the answers

    Why is the limit state of lateral-torsional buckling not applicable for shapes bent about their minor axes?

    <p>Because the moment of inertia of the section about the bending axis is equal to the moment of inertia out of plane</p> Signup and view all the answers

    What is the condition for yielding to control the section?

    <p>The section is noncompact</p> Signup and view all the answers

    What is the benefit of providing lateral support to the compression flange of a beam?

    <p>It helps to prevent the compression flange from moving</p> Signup and view all the answers

    What is a consequence of not providing lateral support to the compression flange of a beam?

    <p>It may cause the compression flange to move</p> Signup and view all the answers

    What is the shape of the section for which the limit state of lateral-torsional buckling is not applicable?

    <p>Circular or square</p> Signup and view all the answers

    What is the equation for the buckling stress of doubly symmetric I-shaped members in AISC Specification?

    <p>Mn = FcrSx ≤ Mp</p> Signup and view all the answers

    What is the relation between the moments of inertia for which the limit state of lateral-torsional buckling is not applicable?

    <p>Ix ≤ Iy</p> Signup and view all the answers

    What controls the section if it is noncompact?

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

    What is the purpose of the AISC Specification equation for the buckling stress of doubly symmetric I-shaped members?

    <p>To calculate the buckling stress</p> Signup and view all the answers

    What is the steel type used in Example 9-9?

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

    What is the value of Lb in Example 9-9?

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

    What is the assumed value of Cb if the self-weight is neglected?

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

    What is the purpose of Example 9-9?

    <p>To select the lightest available section for a given situation</p> Signup and view all the answers

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

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

    What is the range of values that Cb can take if the self-weight is considered?

    <p>Between 1.30 and 1.67</p> Signup and view all the answers

    What is the purpose of Example 9-10?

    <p>To select the lightest available section for a given situation</p> Signup and view all the answers

    What is the figure number that shows the situation in Example 9-10?

    <p>Fig. 9.16</p> Signup and view all the answers

    What is the main purpose of design charts of beams?

    <p>To solve problems of beam design in a few seconds</p> Signup and view all the answers

    What is the range of unbraced lengths covered by the design charts?

    <p>In the plastic, inelastic, and elastic buckling ranges</p> Signup and view all the answers

    What is the value of Fy used in plotting the design charts?

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

    What is the value of Cb used in plotting the design charts?

    <p>1.0</p> Signup and view all the answers

    What is indicated by a solid circle ( ) in the LRFD curve?

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

    What is shown by a hollow circle (ο) in the LRFD curve?

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

    What happens to the values given in the design charts if Cb is greater than 1.0?

    <p>They will be magnified</p> Signup and view all the answers

    What is not considered in the development of the design charts?

    <p>All of the above</p> Signup and view all the answers

    What does the letter E represent in the classification of filler metal electrodes for shielded arc welding?

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

    What is the minimum length of a fillet weld in relation to the nominal leg size of the weld?

    <p>Four times the nominal leg size</p> Signup and view all the answers

    What is the maximum size of a fillet weld along edges of material less than 1/4 in thick?

    <p>The material thickness</p> Signup and view all the answers

    What is the recommended distance to keep the weld back from the edge of a plate with a thickness of 1/4 in or more?

    <p>1/16 in</p> Signup and view all the answers

    What is the minimum tensile strength of a weld specified by the electrode classification E70XX?

    <p>70 ksi</p> Signup and view all the answers

    What type of steel is typically used with E70 electrodes?

    <p>Fy = 36 to 60 ksi</p> Signup and view all the answers

    What determines the effective weld size if the length of a fillet weld is less than four times the nominal leg size?

    <p>One-quarter of the weld length</p> Signup and view all the answers

    What should be checked before assuming lateral support from connecting beams?

    <p>The type of connection</p> Signup and view all the answers

    What is the main characteristic of a compact section?

    <p>It is capable of developing a fully plastic stress distribution before buckling locally.</p> Signup and view all the answers

    What is the maximum value of b/t for a compact section?

    <p>0.38 E/Fy</p> Signup and view all the answers

    What is the maximum value of h/tw for a compact section?

    <p>3.76 E/Fy</p> Signup and view all the answers

    What happens if the width thickness ratio of the flanges exceeds λp = 0.38 E/Fy?

    <p>The section becomes noncompact.</p> Signup and view all the answers

    What is the main purpose of design charts of beams?

    <p>To provide a quick and efficient method for designing beams.</p> Signup and view all the answers

    What is the value of Fy used in plotting the design charts?

    <p>The value of Fy is not specified.</p> Signup and view all the answers

    What is indicated by a solid circle () in the LRFD curve?

    <p>The nominal moment strength.</p> Signup and view all the answers

    What happens to the values given in the design charts if Cb is greater than 1.0?

    <p>The values are increased.</p> Signup and view all the answers

    What is the definition of a noncompact section?

    <p>A section for which the yield stress can be reached in some, but not all, of its compression elements before buckling occurs</p> Signup and view all the answers

    What is the maximum value of the width-thickness ratio of the flanges of W- or other I-shaped rolled sections for noncompact sections?

    <p>1.0 E/Fy</p> Signup and view all the answers

    What is the nominal flexural strength Mn of noncompact beams?

    <p>The lowest of the lateral-torsional buckling strength and the compression flange local buckling strength</p> Signup and view all the answers

    What is the value of λr for the webs in flexural compression for noncompact sections?

    <p>5.70 E/Fy</p> Signup and view all the answers

    What is the purpose of the values of λp and λr provided in AISC Table B4.1b?

    <p>To determine the nominal flexural strength</p> Signup and view all the answers

    What is the significance of Fig.C-F1.1?

    <p>It shows the nominal flexural strength vs. the flange width to thickness ratio of rolled I-shapes</p> Signup and view all the answers

    What is the characteristic of a noncompact section that prevents it from reaching a fully plastic stress distribution?

    <p>It buckles before reaching the yield stress in all its compression elements</p> Signup and view all the answers

    What is the consequence of a noncompact section not reaching a fully plastic stress distribution?

    <p>It has a lower nominal flexural strength</p> Signup and view all the answers

    What is the primary purpose of weld symbols in fabrication and erection drawings?

    <p>To communicate the specific details and requirements of each weld to the welder</p> Signup and view all the answers

    Why are transversely loaded fillet welds stronger than ones loaded parallel to the weld's axis?

    <p>Both b and a</p> Signup and view all the answers

    What is the common assumption made by designers when determining the strength of fillet welds?

    <p>The loads are applied in the longitudinal direction</p> Signup and view all the answers

    What is the effect of deformation on the stress distribution of longitudinal fillet welds?

    <p>It leads to uneven stress distribution</p> Signup and view all the answers

    What is the reason for the difference in strength between transversely and longitudinally loaded fillet welds?

    <p>Both b and a</p> Signup and view all the answers

    What is the purpose of the Table presented in the content?

    <p>To show the various welding symbols developed by the American Welding Society</p> Signup and view all the answers

    What is the significance of the 45˚ angle in fillet welds?

    <p>It is the angle at which failure occurs</p> Signup and view all the answers

    Why do designers often use the method of determining the strength of fillet welds along their longitudinal axes?

    <p>To simplify computations</p> Signup and view all the answers

    What is the design strength of the connection shown in Figure, if the plates consist of A572 Grade 50 steel and E70 electrodes were used, and the 7/16-in fillet welds were made by the SMAW process?

    <p>150 kips</p> Signup and view all the answers

    Using 50 ksi steel and E70 electrodes, what is the minimum size of the SMAW fillet welds required to resist a full-capacity load on the 3/8 X 6-in member shown in Figure?

    <p>5/16 inch</p> Signup and view all the answers

    What is the total LRFD design strength of the 5/16-in E70 fillet welds shown in Figure?

    <p>220 kips</p> Signup and view all the answers

    What is the purpose of designing connections for members with both longitudinal and transverse fillet welds?

    <p>To increase the strength of the member</p> Signup and view all the answers

    What is the type of steel used in Example 5-3?

    <p>A572 Grade 50</p> Signup and view all the answers

    What is the size of the member used in Example 5-3?

    <p>3/8 X 6-in</p> Signup and view all the answers

    What is the type of welds used in Example 5-3?

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

    What is the purpose of designing connections for members with both longitudinal and transverse fillet welds?

    <p>To increase the strength of the member</p> Signup and view all the answers

    What is the design strength of the connection shown in Figure, if the plates consist of A572 Grade 50 steel and E70 electrodes were used, and the 5/16-in fillet welds were made by the SMAW process?

    <p>140 kips</p> Signup and view all the answers

    What is the minimum size of the SMAW fillet welds required to resist a full-capacity load on the 1/2 X 8-in member shown in Figure?

    <p>5/16 inch</p> Signup and view all the answers

    Study Notes

    Lateral Support of Beams

    • Lateral support of the compression flange is crucial in beams, especially when there is movement or vibration.
    • Lateral support can be provided by connecting beams or special members inserted for that purpose.
    • Beams that frame into the sides of the beam or girder can provide full lateral support at the connection.
    • However, if the connection is made primarily to the tensile flange, little lateral support is provided to the compression flange.
    • The designer should ensure that the beams themselves are prevented from moving before assuming lateral support.

    Bending Coefficients

    • Bending coefficients (Cb) are used to calculate the nominal moment strengths of beams.
    • Cb values are dependent on the loading conditions and can vary (e.g., Cb = 1.14, Cb = 2.38).

    Moment Capacities (Zone 2)

    • The nominal moment strengths for unbraced lengths between Lp and Lr are calculated using a specific equation.
    • Lr is a function of several section properties, such as its cross-sectional area, modulus of elasticity, yield stress, and warping and torsional properties.

    Elastic Buckling (Zone 3) and Moment Capacities

    • In Section F2.2 of the AISC Specification, the buckling stress for doubly symmetric I-shaped members is calculated using the equation: Mn = FcrSx ≤ Mp.
    • Lateral-torsional buckling is not applicable for shapes bent about their minor axes, for shapes with Ix ≤ Iy, or for circular or square shapes.
    • Yielding controls if the section is noncompact.

    Design Charts of Beams

    • The AISC provides design charts for beams, which enable quick solution of problems.
    • The charts cover unbraced lengths in the plastic range, inelastic range, and elastic buckling range (Zones 1-3).
    • The charts are plotted for Fy = 50 ksi and Cb = 1.0.
    • The charts cover almost all of the unbraced lengths encountered in practice.
    • The LRFD curve for a typical W section is shown in Fig. 9.12.

    Design of Beams for Moments

    • Bracing is provided only at the ends and at midspan.
    • A compact section is a section that has a sufficiently stocky profile so that it is capable of developing a fully plastic stress distribution before buckling locally (web or flange).
    • For a section to be compact, the width thickness ratio of the flanges of W- or other I-shaped rolled sections must not exceed a b/t value λp = 0.38 E/Fy, and the webs in flexural compression must not exceed an h/tw value λp = 3.76 E/Fy.
    • A noncompact section is one for which the yield stress can be reached in some, but not all, of its compression elements before buckling occurs.
    • For noncompact beams, the nominal flexural strength Mn is the lowest of the lateral-torsional buckling strength, the compression flange local buckling strength, and the web local buckling strength.

    Welded Connections

    • Welding symbols are used to communicate the specific details and requirements of each weld to the welder.
    • Weld symbols are included on fabrication and erection drawings.
    • The stress in a fillet weld is usually said to equal the load divided by the effective throat area of the weld, with no consideration given to the direction of the load.
    • Transverse fillet welds are stronger than longitudinal fillet welds due to more uniform stress distribution and larger effective throat areas.
    • The method of determining the strength of fillet welds along their longitudinal axes, regardless of the load directions, is usually used to simplify computations.
    • Filler metal electrodes for shielded arc welding are classified as E60XX, E70XX, etc., with the first set of digits indicating the minimum tensile strength of the weld in ksi.
    • For the usual situation, E70 electrodes are used for steels with Fy values from 36 to 60 ksi, while E80 is used when Fy is 65 ksi.
    • There are provisions applying to welding given in Section J2.2b of the LRFD Specification, including the minimum length of a fillet weld and the maximum size of a fillet weld along edges of material.

    Design of Connections for Members with Fillet Welds

    • Example 5-1: Design of a simple fillet weld connection.
    • Example 5-2: Design of a fillet weld connection with a plate consisting of A572 Grade 50 steel.
    • Example 5-3: Design of SMAW fillet welds to resist a full-capacity load on a 3/8 x 6-in member.
    • Example 5-4: Determination of the total LRFD design strength of the 5/16-in E70 fillet welds.

    Design of Connections for Members with Both Longitudinal and Transverse Fillet Welds

    • The design of connections for members with both longitudinal and transverse fillet welds involves calculating the total LRFD design strength of the welds.

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