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Questions and Answers
What happens to the buckling moment as the unbraced length is further increased?
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?
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?
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?
What is the primary purpose of checking other beam design criteria?
Which type of beam sections have a low resistance to lateral buckling and torsion?
Which type of beam sections have a low resistance to lateral buckling and torsion?
What is the primary factor that determines the critical moment in elastic buckling?
What is the primary factor that determines the critical moment in elastic buckling?
What type of beam sections have a high torsional resistance?
What type of beam sections have a high torsional resistance?
What is the primary reason for considering lateral bracing in beam design?
What is the primary reason for considering lateral bracing in beam design?
When is a beam considered to have full lateral support?
When is a beam considered to have full lateral support?
What is the primary purpose of designing beams in Zone 1?
What is the primary purpose of designing beams in Zone 1?
When can friction be assumed to provide full lateral support?
When can friction be assumed to provide full lateral support?
What happens to the beam as the moment is increased in elastic buckling?
What happens to the beam as the moment is increased in elastic buckling?
What is required when deciding what constitutes satisfactory lateral support for a steel beam?
What is required when deciding what constitutes satisfactory lateral support for a steel beam?
What is the primary design consideration in Zone 3 of beam design?
What is the primary design consideration in Zone 3 of beam design?
What happens to the beam when it undergoes torsion?
What happens to the beam when it undergoes torsion?
What determines the rate of failure of a beam undergoing torsion?
What determines the rate of failure of a beam undergoing torsion?
What is the purpose of using design charts of beams?
What is the purpose of using design charts of beams?
What is the value of Cb when the self-weight of the member is neglected?
What is the value of Cb when the self-weight of the member is neglected?
What is the length of the beam in Example 9-9?
What is the length of the beam in Example 9-9?
What is the purpose of bracing in the design of beams?
What is the purpose of bracing in the design of beams?
What is the value of the steel used in Examples 9-9 and 9-10?
What is the value of the steel used in Examples 9-9 and 9-10?
What is the effect of considering the self-weight of the member on the value of Cb?
What is the effect of considering the self-weight of the member on the value of Cb?
What is the purpose of using the LRFD and ASD methods?
What is the purpose of using the LRFD and ASD methods?
What is the figure number that represents the situation in Example 9-9?
What is the figure number that represents the situation in Example 9-9?
What is a benefit of using fewer pieces in structural steel design?
What is a benefit of using fewer pieces in structural steel design?
How many main types of welding procedures are used in structural applications?
How many main types of welding procedures are used in structural applications?
What is Shielded Metal Arc Welding (SMAW) also known as?
What is Shielded Metal Arc Welding (SMAW) also known as?
What type of welding is also known as MIG welding?
What type of welding is also known as MIG welding?
What is a characteristic of Gas Metal Arc Welding (GMAW)?
What is a characteristic of Gas Metal Arc Welding (GMAW)?
What is an advantage of Gas Metal Arc Welding (GMAW)?
What is an advantage of Gas Metal Arc Welding (GMAW)?
What type of welding uses an electric arc produced between the end of a coated metal electrode and the steel components to be welded?
What type of welding uses an electric arc produced between the end of a coated metal electrode and the steel components to be welded?
What is a characteristic of Submerged Arc Welding (SAW)?
What is a characteristic of Submerged Arc Welding (SAW)?
What is the desirable shape of a fillet weld?
What is the desirable shape of a fillet weld?
What is the ideal angle of a fillet weld with respect to the pieces being welded?
What is the ideal angle of a fillet weld with respect to the pieces being welded?
What is the theoretical throat of the weld?
What is the theoretical throat of the weld?
Why are concave surface fillet welds not desirable?
Why are concave surface fillet welds not desirable?
What is the characteristic of a 45Ëš fillet weld?
What is the characteristic of a 45Ëš fillet weld?
What is the effect of a concave surface on the stress distribution in a fillet weld?
What is the effect of a concave surface on the stress distribution in a fillet weld?
What is the desirable leg size of a fillet weld?
What is the desirable leg size of a fillet weld?
What is the importance of the shape of a fillet weld?
What is the importance of the shape of a fillet weld?
What is the primary advantage of using welded connections over bolted connections in structural steel design?
What is the primary advantage of using welded connections over bolted connections in structural steel design?
What is a benefit of using welded structures over bolted structures in terms of design flexibility?
What is a benefit of using welded structures over bolted structures in terms of design flexibility?
What is a characteristic of the joints in welded structures?
What is a characteristic of the joints in welded structures?
What is an advantage of welded connections in terms of noise level?
What is an advantage of welded connections in terms of noise level?
What is the primary reason for using welded connections in structural steel design?
What is the primary reason for using welded connections in structural steel design?
What is the primary advantage of using welded connections in terms of structure flexibility?
What is the primary advantage of using welded connections in terms of structure flexibility?
What is the primary characteristic of welded structures?
What is the primary characteristic of welded structures?
What is the primary difference between welded and bolted connections in terms of load transfer?
What is the primary difference between welded and bolted connections in terms of load transfer?
What is the value of Cb when the self-weight of the member is neglected?
What is the value of Cb when the self-weight of the member is neglected?
What is Lr a function of in the calculation of moment capacities?
What is Lr a function of in the calculation of moment capacities?
Where can numerical values for commonly used sections be found?
Where can numerical values for commonly used sections be found?
What is being referred to when calculating moment capacities in Zone 2?
What is being referred to when calculating moment capacities in Zone 2?
What is the purpose of the complex formulas in the AISC Specification (F1)?
What is the purpose of the complex formulas in the AISC Specification (F1)?
What is the purpose of Table 3-2 in the AISC Manual?
What is the purpose of Table 3-2 in the AISC Manual?
What is the equation used for in the calculation of moment capacities?
What is the equation used for in the calculation of moment capacities?
What is the range of unbraced lengths for which the nominal moment strengths are calculated?
What is the range of unbraced lengths for which the nominal moment strengths are calculated?
What is the primary effect on the strength of a W shape beam when it is turned on its side?
What is the primary effect on the strength of a W shape beam when it is turned on its side?
Where can the proper plastic modulus about the y axis for a beam be found when it is turned on its side?
Where can the proper plastic modulus about the y axis for a beam be found when it is turned on its side?
What is the primary reason for the reduction in strength of a W shape beam when it is turned on its side?
What is the primary reason for the reduction in strength of a W shape beam when it is turned on its side?
What is the primary purpose of designing beams in Zone 1 of the AISC beam design?
What is the primary purpose of designing beams in Zone 1 of the AISC beam design?
What is the range of percentage reduction in strength of a W shape beam when it is turned on its side?
What is the range of percentage reduction in strength of a W shape beam when it is turned on its side?
What is the effect of turning a W shape beam on its side on its resistance to lateral buckling and torsion?
What is the effect of turning a W shape beam on its side on its resistance to lateral buckling and torsion?
What is the primary reason for considering lateral bracing in beam design?
What is the primary reason for considering lateral bracing in beam design?
What is the primary purpose of considering the orientation of a W shape beam in design?
What is the primary purpose of considering the orientation of a W shape beam in design?
What is the primary factor that determines the critical moment in elastic buckling?
What is the primary factor that determines the critical moment in elastic buckling?
What is the primary factor that affects the strength of a W shape beam when it is subjected to gravity loads?
What is the primary factor that affects the strength of a W shape beam when it is subjected to gravity loads?
What is the primary purpose of using design charts of beams?
What is the primary purpose of using design charts of beams?
What is the primary benefit of using Gas Metal Arc Welding (GMAW) in structural applications?
What is the primary benefit of using Gas Metal Arc Welding (GMAW) in structural applications?
What is the significance of the plastic modulus values in the table with respect to the orientation of a beam?
What is the significance of the plastic modulus values in the table with respect to the orientation of a beam?
What is the characteristic of a 45Ëš fillet weld?
What is the characteristic of a 45Ëš fillet weld?
What is the effect of a concave surface on the stress distribution in a fillet weld?
What is the effect of a concave surface on the stress distribution in a fillet weld?
What is the primary importance of the shape of a fillet weld?
What is the primary importance of the shape of a fillet weld?
What is the nominal strength of the base metal determined by?
What is the nominal strength of the base metal determined by?
What is the purpose of Table J2.5 in the AISC Specification?
What is the purpose of Table J2.5 in the AISC Specification?
What is the unit of FnBM and Fnw?
What is the unit of FnBM and Fnw?
What is the equation for the nominal strength of the weld metal?
What is the equation for the nominal strength of the weld metal?
What is the term for the area of the weld?
What is the term for the area of the weld?
What is the purpose of determining the nominal strength of a particular weld?
What is the purpose of determining the nominal strength of a particular weld?
What is the term for the nominal stress of the weld metal?
What is the term for the nominal stress of the weld metal?
What is the term for the area of the base metal?
What is the term for the area of the base metal?
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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 45Ëš 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 45Ëš 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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