Steel beam design

Tekla Structural Designer
2021
Tekla Structural Designer

Steel beam design

Overview

Tekla Structural Designer will design steel beams for an international range of doubly symmetric I-sections, C-sections, rectangular and square hollow sections, single angles, double angles and tees for many different countries and also for many specific manufacturers. Plated beam design is also available for some head codes.

Steel beams are designed using a set of design forces obtained from 3D Analysis. (Grillage Chasedown Analysis and FE Chasedown Analysis results are not required.)

In addition to major axis bending, minor axis bending and axial loads are also considered.

In its simplest form a steel non-composite beam can be a cantilever, or a single member between supports to which it is pinned.

It can also be a continuous beam consisting of multiple members that do not, with the exception of the remote ends, transfer moment to the rest of the structure. Each span of a continuous beam can be of different section size, type and grade.

At the remote ends of the beam there are a number of options for the end fixity depending upon to what the end of the beam is connected. These are:

  • Free end
  • Moment connection
  • Pin connection
  • Fully fixed
  • Partially fixed (% of 4EI/L)
  • Partially fixed (linear spring)

The beam may have incoming beams providing restraint and may or may not be continuously restrained over any length between restraints.

Fabrication

The Fabrication type is specified in the beam properties.

Fabrication types that can be designed as non-composite steel beams in Tekla Structural Designer are dependent on the design code.

Refer to the following table for details.

Head code Rolled Plated Westok cellular Westok plated FAB SEC DELTA BEAM
AISC Yes No No No No No
EC Yes Yes Yes Yes No No
BS Yes Yes Yes Yes No No
IS Yes Yes No No No No
AS Yes Yes No No No No

For guidance in relation to design of plated steel beams to a specific head code, see: Plated steel beams

Restraints

Each beam may have incoming beams providing restraint and may or may not be continuously restrained over any length between restraints.

Conditions of restraint can be defined in- and out-of-plane for compression (strut) buckling and top and bottom flange for lateral torsional buckling (LTB). The effective lengths used in the buckling checks depend on the type of restraint, particularly at supports.

In all cases, the program sets the default effective length to 1.0L, it does not attempt to adjust the effective length (between supports for example) in any way. You are expected to adjust the effective length factor (up or down) as necessary. Any compression or LTB effective length can take the type “Continuous” to indicate that it is continuously restrained over that length.

LTB and Compression restraints

LTB and Compression restraints are determined from the incoming members described within the Tekla Structural Designer model.

The default assumption is that an incoming member connecting to either side of the beam provides lateral restraint to the top flange only, and compression restraint about the minor axis. An incoming member connecting from above or below doesn't provide lateral restraint but provides compression restraint about the major axis.

By right-clicking a member to edit its properties in the Properties dialog box, you are then able to edit the restraints. You can indicate also continuously restrained sub-beams and edit length factors.

Note that the same level of restraint editing is not provided in the Properties window (although it does allow you to independently set both the top and bottom flanges as continuously restrained for the entire member length via the Top/Bottom flange cont. rest. properties).

Tip: As an alternative to using the Steel Beam Properties dialog box, restraint settings for steel beams can be easily reviewed and adjusted graphically via Review View > Show/ Alter State. For more details see: Review and modify restraints

Torsion restraints

Torsion restraints are currently displayed in the Properties dialog box for information only. In design the beam is treated as unrestrained against warping and restrained against torsion.

Continuously Restrained Flanges

In the Properties window you can independently set both the top and bottom flanges as continuously restrained. By setting Top flange cont. rest. and/or Bottom flange cont. rest. to "Yes" the relevant buckling checks are not performed during the design process.

Deflection limits

Serviceability criteria often control the design of normal composite beams. This is because they are usually designed to be as shallow as possible for a given span.

Deflection Limits can be specified in the beam properties. These allow you to control the amount of deflection in both composite beams and steel beams by applying either a relative or absolute limit to the deflection under different loading conditions.

A typical application of these settings might be:

  • not to apply any deflection limit to the slab loads, as this deflection can be handled through camber,

  • to apply the relative span/over limit for live load deflection, to meet code requirements,

  • possibly, to apply an absolute limit to the post composite deflection to ensure the overall deflection is not too large.

Torsion

A Check for torsion can be requested in the beam properties.

Torsion can only be checked to the AISC or Eurocode head code and only under the following conditions:

  • Section geometry
    • doubly symmetric rolled I-sections
    • closed RHS/SHS/CHS sections (steel and cold formed)
    • closed HSS sections
  • pinned
    • single span beams
    • applied torsion moment
    • no web openings

If any of the above conditions are contravened the check is beyond scope.

Only a check design is performed, (no auto-design for torsion).

The procedure and scope are different for Open sections (I’s) vs closed sections (HSS’s)
  • Open Sections:
    • Torsion design and angle rotation check will be carried out for applied torsion forces only. “Inherent” torsion is not checked
  • Closed Sections
    • “Inherent” torsion and/or physically applied torsion loads checked
    • Angle of rotation check carried out for applied forces only

Camber

You can introduce camber by selecting Apply camber in the beam properties.

Camber is primarily used to counteract the effects of dead load on the deflection of a beam. This is particularly useful in long span composite construction where the self-weight of the concrete is cambered out. It also ensures little, if any, concrete over pour occurs when placing the concrete.

The amount of camber can be specified either:

  • As a value

  • As a proportion of span

  • As a proportion of deflection

    • If this option is selected, the engineer should identify the combination to be used for the calculations by selecting Camber adjacent to the appropriate gravity combination on the Loading combination dialog page.

    • If no combination is selected then the first gravity combination in the combination list is used.

In the latter case, if 100% of the dead load deflection is cambered out, it is also possible to include a proportion of the live load deflection if required.

Lower limits can be set for span length and calculated camber below which camber is not applied. This ensures that unwanted or impractical levels of camber are not specified.

Limits to the maximum allowable camber as well as to the minimum section web thickness to avoid web crippling can also be defined. These will be accounted for in the automatic design process affecting the section size selection.

See also: Support article: Composite Beam deflections including camber

Natural frequency

For steel beams the natural frequency calculation can optionally be requested by selecting Calculate natural frequency in the beam properties. When requested it can optionally be checked against a minimum frequency if required.

When the calculation is activated, a simple (design model) approach is taken based on uniform loading and pin supports. This fairly simple calculation is provided to the designer for information only. The calculation can be too coarse particularly for long span beams and does not consider the response side of the behavior i.e. the reaction of the building occupants to any particular limiting value for the floor system under consideration. In such cases the designer has the option to perform a 1st Order Modal Analysis.

See also:

Instability factor

Long members in a model that have axial force in them can be unstable during second-order analysis because their individual elastic critical buckling load factor is lower than the elastic critical buckling load factor of the building as a whole and is less than 1.0.

However, often such members, for example the rafters in a portal frame, are stable in design because, for example, there are many smaller members or sheeting, that restrain the member in reality. They fail in the analysis because it is too resource intensive to model all the individual restraining members in the model which would also add unwanted clutter.

To prevent or to reduce the incidence of such failures during the analysis a multiplier can be applied to the minor axis inertia of these members which caters for the effect of the restraining members.

This multiplier is defined by selecting Prevent out of plane instability in the member properties and then entering a suitable value in the Instability factor field.

When activated, the minor axis inertia of the member is multiplied by the factor, thus increasing its minor axis (theoretical elastic) buckling capacity. This only occurs in the 2nd order analysis solver model. The default value for the factor is 20. This is just a sensible initial value, not a 'magic' number or one that will work in all circumstances. This behaviour also applies to the pre-size sections assigned during steel auto-design using 2nd order analysis. Hence the setting can be used to assist when buckling is occurring in the 2nd order analysis that is part of steel auto-design.

Since the setting adds ‘artificial’ stiffness to the model, the engineer should take care to ensure that it does not inappropriately increase the lateral stiffness of the structure as a whole leading to inaccurate results.

Note:

This multiplier is applied to prevent unwanted behaviour in the analysis model. While the analysis results may be affected by this adjustment, the increased minor axis inertia is NOT used in design checks.

Web openings

You cannot currently automatically design sections with web openings, you must perform the design first to get a section size, and then add and check the openings. This gives you complete control of the design process, since you can add appropriate and cost effective levels of stiffening if required, or can choose a different beam with a stronger web in order to reduce or remove any stiffening requirement.

When openings are added they can be defined as rectangular or circular and can be stiffened on one, or on both sides.

Openings cannot be defined from the Properties window, they can only be defined from the Properties dialog box, (by right-clicking on the member and selecting Edit...)

For guidance in relation to a specific head code, see:

Fire check (Eurocode only)

The mechanical resistance of a steel beam in case of fire can be checked in accordance with EN 1993 & national annex for the UK, Ireland, Singapore, Malaysia, Sweden, Norway, Finland or the recommended Eurocode values.

The fire check is activated by selecting Check for fire resistance located under Fire proofing in beam properties, after which the user is required to specify:

  • Load reduction factor for fire
  • Required time of fire exposure
  • Exposure
  • Protected, or unprotected
    • If the protected option is selected, the engineer is then required to specify the fire protection material details.

The time interval for critical temperature iteration is specified in Design Settings, for both 'unprotected' and 'protected' situations.

The check compares the design shear force against the design resistance at the required time of fire exposure.

For the scope of the check and the limitations that apply, see: Fire resistance check (Beams: EC3 Eurocode)

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