Beam and Beam-Portion Design Brief

 

The Beam and Beam Portion design brief is aimed at the design of beams. This brief will ignore any axial load on the member, whether there is axial tension or compression. As such, the beam and beam-portion design brief is not suitable for columns, or for members where the axial force is not negligible.

 

The Beam and Beam-Portion design brief is applicable for members with single axis bending moment. The Beam and Beam-Portion brief will check for local moment capacity, lateral torsional stability, shear capacity, deflection criteria and slenderness ratio. The user controlled inputs include specifying the restraint condition and, where appropriate, restraint positions to the minor axis (for lateral torsional buckling). Additional user inputs include specifying user determined C1-C3 factors (in the Eurocode design) and the vertical distance from the shear centre of the beam to the point of application of the load, to account for destabilising loads. There is also the option to consider the design of the section in torsion.

 

The Beam and Beam-Portion design brief ignores any axial forces in the member. It also ignores any minor axis bending. Both of these items are noted in the the design brief title at the top of the design output screen. The Member Forces and Maximum Deflection summary table will note the member forces, including axial force and major and minor axes bending. It is the responsibility of the design engineer to ensure that the correct design brief is applied to any particular member.

 

Once a Beam and Beam-Portion design brief is applied to a member, the basic screen layout is as follows.

 

 

 

 

 

 

Section AutoDesign

 

The Section AutoDesign tab contains options to amend the steel member section size, to make the member a compound member (which also provides options to provide plates to the member) and to add web openings. Within the Section AutoDesign tab, there are also options to carry out an autodesign on the selected member or on all members visible on screen.

 

The Beam to C&M icon  allows the design brief to be quickly changed to an Axial with Moment design brief. That would be used in cases where, for example, the member has axial force (which is ignored in the Beam and Beam-Portion brief) and the brief needs to be changed, but saves the need to delete the Beam and Beam-Portion brief before changing.

 

For more detailed information on the AutoDesign area, see the Designing a Member section.

 

 

Beam and Beam-Portion

 

The layout of the Beam and Beam-Portion tab is shown below.

 

 

C1, C2, C3 factors - Eurocode factors for modifying the bending moment equations. The factors are dependant on the shape of the bending moment diagram of the beam or part of the beam under consideration. The software will automatically determine the C1, C2 and C3 factors, values only need to be input in the Beam and Beam-Portion if the user wishes to overwrite the values.

 

zg - the zg factor modifies the bending moment equations to account for the height of the point of load application above the shear centre of the beam. The zg value is input in millimetres (mm) and a positive value represents a load applied above the shear centre. The zg input only needs to be used for destabilising or stabilising loads. With a default value of 0 the software takes the load to be applied at the shear centre of the member

 

Theta Limit - this is the rotation limit for use in designs where torsion is present. IF the "Ignore Torsion" option is set, the Theta limit is not active and the value input is not relevant. When the design is set to include the torsion, the Theta Limit defaults to a value of 2 degrees. This is the SCI recommended limit.

 

Lambda limit - Slenderness limit. This was originally a BS 5950 check, but was dropped in later version of the code and does not appear in the Eurocodes. However, it was opted to maintain this input in the steel design.

 

Spacing - for use with double members. The spacing defines the gap between double members. The input value is in millimetres (mm). The default setting is zero.

 

Lv - the distance between connection locations in a double member. The input is specified in metres (m). If a value of zero is used, the software will default to using the actual member length.

 

FL/UL - the ratio between the factored loads (FL) and unfactored loads (UL). This is used in the British Standard design as part of the check to ensure that Mc = Py*S <= 1.2 py. This check is not used in the EC design so the FL/UL factor is not required.

 

Def Limit - The Span/Deflection limit used for the serviceability deflection check. The default value is 360. The deflection is checked for the worst case serviceability load case. Clicking on the input box will show a small icon at the right of the input. Using the mouse and clicking on this icon will bring up the Deflection Limits table. In this table, up to 30 user defined deflection limits can be specified. With a user specified set of limits added to the table, these new deflection limits can be used by inputting the line number of the table into the Def Limit input in place of a span/deflection limit.

 

 

For further information on the nodal and member and in-span deflections see this technical note.

 

Effective Length:

 

The effective lengths of the beam for use in the lateral torsional buckling check are controlled through the drop downs to the right of the bottom pane. The layout is shown below.

 

 

The top drop down controls the effective length setting for the start (lower node numbered end) of the beam. The middle drop down controls the effective length setting of the beam at the end of the beam (higher node numbered end). This allows different end conditions to be considered. Where different end conditions exist the software will use the average of the two settings. To set the end of the beam to be the same as the start, either select the same effective length factor, or set the middle drop down to "As End1". The bottom drop down allows the user to control the tables used to determine the LTB factors ( determined using the British Standard method). Setting this drop down to 'Autoselect' means the software will automatically pick the appropriate table from BS 5950-1.

 

Non doubly symmetric members:

 

For members which are not doubly symmetric, for example, angles or channel sections, setting whether the flange is in tension or compression, or whether the web is in tension or compression are set using the drop downs below the LTB settings. If the section selected is an angle, the drop down contents will modify to Long Leg in Compression or Tension and WEB in Compression or Tension.

 

Torsion Design:

 

For members with torsional loading, torsion can be considered in the design using the torsional inputs.  These are located at the right of the Beam and Beam-portion tab. The torsion inputs are shown below.

 

 

The default setting is that the torsion design checks are off. To consider Torsion, the top drop down needs to be changed from "Ignore Torsion". The drop down allows the setting of the degree of restraint to warping at the member ends. It should be noted that obtaining restraint to warping is often not practical or possible. For further details, reference should be made to a relevant design guide or SCI publication.

 

To display a diagram related to the torsion design, use the bottom drop down.

If you wish to turn off the torsion design for the whole model, click on the "check Torsion (Global Key)" icon.

 

Additional note:-

When an eccentric load is applied to a beam or beams, the frame analysis takes account of the torsional stiffness of the beams, but doesn’t take account of the warping stiffness, which is essentially a second order analysis. When you go into the steel design the warping stiffness is now taken account of – see SCI P385 for further details on the design of steel members with torsion.

Therefore, the values in the steel design are more accurate where torsional rotation is concerned than those derived directly from the initial frame analysis.

 

Lateral Restraints

 

The lateral restraints tab provides inputs which control the restraints to the member for use in the lateral torsional bending design checks. The lateral restraints only restrain the minor axis in bending.

The layout of the Lateral Restraint tab in the Beam and Beam Portion design brief is shown below.

 

 

 

The position of up to 9 lateral restraints can be defined on any member using the Portion inputs. Restraints can be grouped into sets of three restraints, each set controlled by the inputs in each of the three areas. The "Start at" input controls the distance to the start of each set of restraints, with the start distance being defined from the start of the member. The start of any member is defined as the end with the lowest numbered node. The position of the restraints are then defined by the length between restraints as defined by the Portion input.

 

The Start at inputs do not have to be set. This will then treat all restraints as acting as a single set of restraints. That is, the restraint 4 will then start the Portion 4 length from the end of Portion 3, and Portion 7 will continue from the end of Portion 6. However, for longer beams, it may be desirable to have restraints defined for the start, middle and end of a member, and in this case it will likely be necessary to input the start point of each set of restraints. In this case, there will not be a portion assigned from the end of Portion 3 and the start of Portion 4, each set will therefore act independently.

 

If the portions are all of equal length, this can be specified by inputting a single portion length in the Portion 1 input, with the start at distance set to zero. This will create a set of equally spaced portions. There will either be 9 equally spaced portions, or the portions will end at the last full portion before the end of the member, depending on the portion and member lengths.

 

Where a member has other members framing into it, for example, a primary beam supporting secondary beams, the portions can be created automatically at the position of the connecting members. This can be done by either clicking on the

 icon, or, alternatively, by checking the "Auto Restrain at Connected Members" checkbox. The Generate Restraint at Joining Members icon will automatically input the Portion lengths in the Portion 1-9 inputs. This means that the position of the restraints can be amended manually. When the "Auto Restrain at Connected Members" check box is selected, the Portion input and the Generate Restraint at Joining Members options are disabled. Any amendments to the position of the incoming members in the Masterframe model will then be automatically accounted for in the design brief on the amended members.

 

A typical layout of the Lateral Restraints on a primary beam is shown below.

 

 

 

 

The position of the portions are indicated in the results diagram at the top of the results area. To view the design results for any particular portion, the portion can be selected either graphically, by using the mouse to click on the portion in the results diagram at the top of the design output area. The active portion by highlighted in blue in the results diagram. The current selected portion is also noted in the Portion drop down. The current portion is displayed as . The Portion drop down also provides an alternative method for selecting the portion to check, by expanding the drop down. The drop down notes the Portion number along with the position of the portion with the distances noted from the start of the member. For the above member, the expanded drop down appears as shown below.

 

 

 

 

The selected portion is indicated with a blue highlight in the graphics. The relevant portion input also highlights.