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

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

The compression flange moves laterally

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

The beam undergoes torsion or twisting

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

To ensure the beam's moment capacity is sufficient Signup and view all the answers

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

W, S, and channel shapes Signup and view all the answers

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

The combination of torsional and warping resistances Signup and view all the answers

What type of beam sections have a high torsional resistance?

Built-up box shapes Signup and view all the answers

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

To prevent buckling of the compression flange Signup and view all the answers

When is a beam considered to have full lateral support?

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

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

To ensure the beam's moment capacity is sufficient Signup and view all the answers

When can friction be assumed to provide full lateral support?

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

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

The beam's deflection increases Signup and view all the answers

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

Some judgment Signup and view all the answers

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

Preventing elastic buckling Signup and view all the answers

What happens to the beam when it undergoes torsion?

It undergoes rapid failure Signup and view all the answers

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

The torsional strength of the beam Signup and view all the answers

What is the purpose of using design charts of beams?

To select the lightest available section Signup and view all the answers

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

1.67 Signup and view all the answers

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

17 ft Signup and view all the answers

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

To provide lateral support at specific points Signup and view all the answers

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

50 ksi Signup and view all the answers

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

It decreases the value of Cb Signup and view all the answers

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

To select the lightest available section Signup and view all the answers

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

Fig. 9.15 Signup and view all the answers

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

Reduced fabrication time Signup and view all the answers

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

Four Signup and view all the answers

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

Manual, stick, or hand welding Signup and view all the answers

What type of welding is also known as MIG welding?

Gas Metal Arc Welding (GMAW) Signup and view all the answers

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

Uses a continuous wire fed into the welding gun Signup and view all the answers

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

It is fast and economical 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?

Shielded Metal Arc Welding (SMAW) Signup and view all the answers

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

Information not available in the content Signup and view all the answers

What is the desirable shape of a fillet weld?

Flat or slightly convex Signup and view all the answers

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

45˚ Signup and view all the answers

What is the theoretical throat of the weld?

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

Why are concave surface fillet welds not desirable?

They have a tendency to crack upon cooling Signup and view all the answers

What is the characteristic of a 45˚ fillet weld?

Leg sizes are equal Signup and view all the answers

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

It reduces stress concentration Signup and view all the answers

What is the desirable leg size of a fillet weld?

Equal Signup and view all the answers

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

It affects the strength of the weld Signup and view all the answers

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

They provide a more rigid structure Signup and view all the answers

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

It is easier to make changes to the design Signup and view all the answers

What is a characteristic of the joints in welded structures?

They are as strong as or stronger than the base metal Signup and view all the answers

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

They are relatively quiet Signup and view all the answers

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

To produce a one-piece construction Signup and view all the answers

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

They reduce the flexibility of the structure Signup and view all the answers

What is the primary characteristic of welded structures?

They are more rigid than bolted structures Signup and view all the answers

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

Welded connections transfer load directly between members Signup and view all the answers

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

1.14 Signup and view all the answers

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

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

Where can numerical values for commonly used sections be found?

AISC Manual Table 3-2 Signup and view all the answers

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

Nominal moment strengths for unbraced lengths between Lp and Lr Signup and view all the answers

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

To calculate the value of Lr Signup and view all the answers

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

To list the values of Lr for commonly used sections Signup and view all the answers

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

To calculate the nominal moment strengths for unbraced lengths Signup and view all the answers

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

Between Lp and Lr 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?

It becomes 10 to 30 percent weaker 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?

Table 3-4 of the Manual 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?

The change in the gravity loads Signup and view all the answers

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

To ensure that the beam is designed to resist both lateral and torsional buckling 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?

10 to 30 percent 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?

It decreases its resistance to lateral buckling and torsion Signup and view all the answers

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

To prevent lateral buckling and torsion Signup and view all the answers

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

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

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

The modulus of elasticity of the material 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?

The beam's orientation Signup and view all the answers

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

To determine the design criteria for the beam Signup and view all the answers

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

It produces high-quality welds 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?

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

What is the characteristic of a 45˚ fillet weld?

It has a high strength-to-weight ratio Signup and view all the answers

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

It increases the stress concentration Signup and view all the answers

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

It affects the strength of the weld Signup and view all the answers

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

The area of the base metal and nominal stress of the base metal Signup and view all the answers

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

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

What is the unit of FnBM and Fnw?

ksi Signup and view all the answers

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

Rn = Fnw Awe Signup and view all the answers

What is the term for the area of the weld?

Awe Signup and view all the answers

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

To determine the design strength of the weld Signup and view all the answers

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

Fnw Signup and view all the answers

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

ABM 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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