Beam Design Defaults dialog

Dialog Controls

Design default criteria may be changed for the current beam data by typing the desired values in the appropriate edit boxes.

Setting	Description
Maximum Span/Depth Ratio	The minimum beam or joist depth can be controlled by specifying a value for Maximum Span/Depth Ratio. When selecting optimized sizes the program selects members that are at least deep enough to satisfy this criteria. This is a common rule-of-thumb, not a Code requirement. Common values are 20 to 24. Note that you must use consistent units - ft/ft or m/m. A value of 0.0 indicates no limit.
Check Unbraced Length	Select this option to calculate the bending capacity considering the bracing conditions of the flanges. Left unselected the top and bottom flange will be considered fully braced, irrespective of actual physical conditions, when determining the sections bending capacity.
Consider Point of Inflection	This option does not mean that the Point of Inflection will or will not be considered a brace point; rather, it affects the way the program looks at brace points of the flanges on either side of the point of inflection when determining the unbraced length. Refer to the manual (Help - Manual) for a complete explanation.
Noncomposite/Precomposite Beam Design	For composite beams in their pre-composite state, the program can consider the top flange as being braced by a supported deck, provided that the appropriate checkbox is selected. Selecting Deck perpendicular to Beam Braces flange results in the top flange of the beam being braced if the beam supports a deck, which is more than 10 degrees off the axis of the beam. Selecting Deck parallel to Beam Braces flange results in the top flange of the beam being braced if the beam supports a deck which is less than 10 degrees off the axis of the beam.

Additional Design Defaults for AISC 360

Setting	Description
Reduce Ieff per AISC 360 Commentary (0.75)	Historically, Eq. (C-I3-3) has been used to calculate Ieff. The errata to the Commentary to AISC 360-05 modifies that equation to be the calculation of Iequiv, and indicates that I_eff = 0.75×I_equiv. Thus the Commentary is indicating that the value of Ieff historically used be reduced by 0.75: Comparisons to short-term deflection tests indicate that the effective moment of inertia, Ieff, is 15 to 30 percent lower than that calculated based on linear elastic theory (Iequiv). Therefore, for realistic deflection calculations, Ieff should be taken as 0.75 Iequiv.

Setting

Description

Reduce Ieff per AISC 360 Commentary (0.75)

Historically, Eq. (C-I3-3) has been used to calculate Ieff. The errata to the Commentary to AISC 360-05 modifies that equation to be the calculation of Iequiv, and indicates that I_eff = 0.75×I_equiv. Thus the Commentary is indicating that the value of Ieff historically used be reduced by 0.75:

Comparisons to short-term deflection tests indicate that the effective moment of inertia, Ieff, is 15 to 30 percent lower than that calculated based on linear elastic theory (Iequiv). Therefore, for realistic deflection calculations, Ieff should be taken as 0.75 Iequiv.

Additional Design Defaults for AS 4100-1998

Setting	Description
Use Moment Modifi. Factor = 1 on Simple Beam Spans	Setting this option sets the equivalent uniform moment factor, α_m, to 1.0 for all simple beam (pinned supports with no cantilevers). The α_m is defined in the AS 4100-1998, Section 5.6.
Use Moment Modifi. Factor = 1 on Cantilevers	Setting this option sets the equivalent uniform moment factor, α_m, to 1.0 for the cantilever portion of a beam. If left unselected the α_m will be calculated according to Section 5.6. Note: RAM Steel Beam assumes the end of a cantilever is braced against translation for purposes of calculating capacity considering lateral torsional buckling.

Additional Design Defaults for BS 5950

Setting

Description

Use mLT=1.0 on Simple Span Beams

Select this option to set the equivalent uniform moment factor to 1.0 for all simple beam (pinned supports with no cantilevers). The mLT is defined in the BS 5950 Draft Amendment, Dated April 1998, Section 4.3.6.2. and BS 5950-1:2000, Section 4.3.6.6.

Use mLT=1.0 on all Cantilevers

Select this option to set the equivalent uniform moment factor to 1.0 for the cantilever portion of a beam. Note that if left unselected the mLT will still be set to 1.0 on cantilever when BS 5950:1990 is selected (as required by BS 5950 Draft Amendment, Dated April 20th, 1998, Table 4.4.). However, if BS5950-1:2000 is the design code, mLT will be calculated according to Table 18 footnotes unless the engineer explicitly sets mLT=1.0 for cantilevers.

According to 4.3.5.3c of BS5950-1:2000, the lateral buckling capacity of the unsupported flange of beams directly supporting a concrete or composite floor is calculated according to G.2. Where this check is required in a structure, RAM Steel Beam will assume that mLT is 1.0 and will calculate an nt value according to G.4.2.

Beam Effective Length

In calculating the effective length for lateral torsional buckling the end conditions of the unbraced segment must be considered. A beam fixed at a column (as in the case of a beam cantilevering through a column) is provided an effective length factor of 0.7 for that end. A beam continuous through a lateral support (due to a framing beam) is provided an effective length factor of 1.0 for that end. The effective length factor for the segment is taken as the average of the effective length factors from the two ends. As no connection details are available to RAM Structural System the engineer is responsible for providing the effective length factor to be used at the end of cantilevers and at pin supports.

Vibration of Composite Beams

Setting	Description
Minimum Frequency, Short /Long Span Beams	Different frequency limits can be set for both short span and long span beams.
Long spans are longer than	The transition span length between short span and long span.
Increase Ig 10% to account for increased dynamic stiffness	The deflection is based on the full composite section using the short term modular ratio under Dead Load and 10% of the Live Load. Ig may be increased 10% to account for increased stiffness of the beam under dynamic loading, such as for the effects of continuity. If the size fails the specified limit, a larger size will be used when optimizing beam sizes. A value of 0.0 specified for the minimum frequency means that there is no minimum limit on the frequency; specify 0.0 if the simplified vibration approach is not to be used.

Additional Design Defaults for CAN/CSA S16

Setting	Description
Use w2 = 1.0 on Simple Span Beams	Select this option to set the equivalent uniform moment factor to 1.0 for all simple beam (pinned supports with no cantilevers). The w2 factor is defined in CAN/CSA-S16-01, S16-09, and S16-14 Section 13.6.
Use w2 = 1.0 on all Cantilevers	Select this option to set the equivalent uniform moment factor to 1.0 for the cantilever portion of a beam. Note that even if left unselected the w2 will always be 1.0 on a cantilever as specified by the SSRC Stability Design Criteria for Metal Structures, Galambos, 1998. This reference specifies a w2 of 1.0 but increases the unbraced length by 1.5 for the cantilever beam segment.

Additional Design Defaults for Eurocode

Setting	Description
Use C = 1.0 on Simple Span Beams	Select this option to set the C1, C2, and C3 factors to 1.0 for calculating the Mcr value for all simple beams (pinned supports with no cantilevers).
Use C = 1.0 on all Cantilevers	Select this option to set the C1, C2, and C3 factors to 1.0 for the cantilever portion of a beam. These factors are used to calculate the Mcr value for all simple beams (pinned supports with no cantilevers).

See the RAM Steel Beam manual for additional information on the C factors.

Additional Design Defaults for IS 800-07

Setting	Description
Use LLT=1.0L on Simple Span Beams	Refer to Section 8.3 of the RAM Steel Beam manual for details on effective length for lateral torsional buckling.
Use LLT=0.8L on Simple Span Beams	Refer to Section 8.3 of the RAM Steel Beam manual for details on effective length for lateral torsional buckling.
Beam Effective Length	In calculating the effective length for lateral torsional buckling the end conditions of the unbraced segment must be considered. A beam fixed at a column (as in the case of a beam cantilevering through a column) is provided an effective length factor of 0.7 for that end. A beam continuous through a lateral support (due to a framing beam) is provided an effective length factor of 1.0 for that end. The effective length factor for the segment is taken as the average of the effective length factors from the two ends. As no connection details are available to RAM Structural System the engineer is responsible for providing the effective length factor to be used at the end of cantilevers and at pin supports.