Composite beam design

Tekla Structural Designer
2022
Tekla Structural Designer

Composite beam design

 

Overview

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

The beams must be simply supported, single span unpropped structural steel beams.

The following are beyond scope:

  • continuous or fixed ended composite beams,
  • composite sections formed from hollow rolled sections,
  • composite sections where the concrete slab bears on the bottom flange,
  • the use of fiber reinforcement.

Beams are designed for gravity loads acting through the web only. Minor axis bending and axial loads are not considered.

Note: If either minor axis bending or axial loads exist which exceed a limit below which they can be ignored, a warning is given in the beam design summary.

Profiled steel sheeting can be perpendicular, parallel and at any angle in between relative to the supporting beam web.

Tekla Structural Designer will determine the size of beam which:

  • acting alone is able to carry the forces and moments resulting from the Construction Stage,
  • acting compositely with the slab using profile steel decking (with full or partial interaction) is able to carry the forces and moments at Composite Stage,
  • acting compositely with the slab using profile steel decking (with full or partial interaction) is able to provide acceptable deflections, service stresses and natural frequency results.

Alternatively you may give the size of a beam and Tekla Structural Designer will then determine whether it is able to carry the previously mentioned forces and moments and satisfy the Serviceability requirements.

An auto-layout feature can be used for stud placement which caters for both uniform and non-uniform layouts.

Construction stage loading

You define these loads into one or more loadcases as required.

The loadcase defined for construction stage slab wet concrete has a Slab wet loadcase type specifically reserved it. Clicking the Calc Automatically checkbox enables this to be automatically calculated based on the wet density of concrete and the area of slab supported. An allowance for ponding can optionally be included by specifying it directly in the composite slab properties.

If you uncheck Automatic Loading, this loadcase is initially empty - it is therefore important that you edit this loadcase and define directly the load in the beam due to the self weight of the wet concrete. If you do not do this then you effectively would be designing the beam on the assumption that it is propped at construction stage.

It is usual to define a loadcase for Live construction loads in order to account for heaping of the wet concrete etc.

Having created the loadcases to be used at construction stage, you then include them, together with the appropriate factors in the dedicated Construction stage design combination. You can include or exclude the self-weight of the beam from this combination and you can define the load factors that apply to the self weight and to each loadcase in the combination.

Note: You should include the construction stage slab wet concrete loadcase in the Construction stage combination, it cannot be placed in any other combination since it’s loads relate to the slab in its wet state. Conversely, you cannot include the Slab self weight loadcase in the Construction stage combination, since it’s loads relate to the slab in its dry state. The loads in the Construction stage combination should relate to the slab in its wet state and any other loads that may be imposed during construction.
Note: TIP: If you give any additional construction stage loadcases a suitable title you will be able to identify them easily when you are creating the Construction stage combination.

Composite stage loading

You define the composite stage loads into one or more loadcases which you then include, together with the appropriate factors in the design combinations you create. You can include or exclude the self-weight of the steel beam from any combination and you can define the load factors that apply to the beam self weight and to each loadcase in the combination.

The Slab self weight loadcase is reserved for the self weight of the dry concrete in the slab. Clicking the Automatic Loading checkbox enables this to be automatically calculated based on the dry density of concrete and the area of slab supported. An allowance for ponding can optionally be included by specifying it directly in the composite slab properties.

If you uncheck Automatic Loading, the Slab self weight loadcase is initially empty - it is therefore important that you edit this loadcase and define directly the load in the beam due to the self weight of the dry concrete. For each other loadcase you create you specify the type of loads it contains – Dead, Live or Wind.

For each load that you add to an Live loadcase you can specify the percentage of the load which is to be considered as acting long-term (and by inference that which acts only on a short-term basis).

All loads in Dead loadcases are considered to be entirely long-term while those in Wind loadcases are considered entirely short-term.

Allow non-composite design

The following controls which are located under the General heading in the beam properties can be selected to enable non-composite design in specific circumstances.
  • Attempt non-composite if auto composite solution fails
  • Try composite section size as non-composite

Attempt non-composite if auto composite solution fails

Typically, at the outset you will know which beams are to be non-composite and which are to be composite and you will have specified the construction type accordingly. However, circumstances can arise in which a beam initially intended to be composite proves to be ineffective and the auto-design process fails to find a suitable composite beam solution. Examples might be:

  • very small beams,
  • beams with a significant point load close to a support,
  • beams where the deck is at a shallow angle to the beam, hence the stud spacing is impractical,
  • beams where, for a variety of reasons, it is not possible to provide an adequate number of studs

Where Tekla Structural Designer is unable to find a section size which works compositely, you can ask for a non-composite design for the same loading. To do this simply select Attempt non-composite if auto composite solution fails in the beam properties. You may find that this facility is particularly useful when you right-click a key beam in the model in order to perform an individual member design.

Try composite section size as non-composite

Typically, at the outset you will know which beams are to be non-composite and which are to be composite and you will have specified the construction type accordingly. However, there can be instances where the composite beam is sufficiently under utilized that the same size beam would be adequate without the studs i.e. non-compositely. Irrespective of whether a suitable composite design solution exists, selecting this option will try the same section size non-compositely and if successful will retain this non-composite solution.

Tip: If working to the US code (AISC 360), ensure that the Beam design section stepback option in Design Settings > Autodesign is set to a value greater than zero. A value of 3 is recommended.

Fabrication

 

The Fabrication type is specified in the beam properties.

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

Refer to the following table for details.

 
Regional 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 N/A - Design of composite beams to IS codes is currently beyond scope
AS N/A - Design of composite beams to AS codes is currently beyond scope

For guidance in relation to design of plated composite beams to Eurocodes or the BS regional code, see: Plated steel beams

Restraints

You can independently set both the top and bottom flanges of a composite beam as continuously restrained in the Properties window.

When the beam is initially created the decking direction is unknown until the beam is actually placed and the floor slab and direction are also created. Hence defaults are provided for each eventuality.

The defaults are:

  • for perpendicular decks the deck restrains the beam top flange
  • for parallel decks the deck does NOT restrain the beam top flange
  • for precast decks the deck restrains the beam top flange
  • for all decks the deck does NOT restrain the beam bottom flange

By setting the top flange as continuously restrained and/or the bottom flange as continuously restrained the relevant buckling checks are not performed during the design process.

When not continuously restrained, LTB and Compression restraints are determined from the incoming members described within the Tekla Structural Designer model. The buckling checks are based on these.

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 continuously restrained sub-beams and also edit length factors.

For composite beams the buckling checks are only performed at construction stage as at composite stage they are always assumed to be fully restrained.

Tip: Restraint settings for composite beams can be easily reviewed and adjusted graphically via Review View > Show/ Alter State. For more details see: Review and modify restraints

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.

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.

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

Whether a natural frequency calculation is performed or not is controlled by the Calculate natural frequency setting in the beam properties.

For EC and BS regional codes the composite beam natural frequency is always calculated and checked against a minimum frequency (with results displayed in Results Viewer).

For the US regional code the natural frequency calculation can optionally be requested, and when requested can optionally be checked against a minimum frequency if required. Note that the US regional code has no requirement for this check.

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:

Floor construction

Deck Type, Angle and Condition

The deck type and angle used in the beam design are determined from the properties of an adjacent slab item, (provided the slab item type has been set to Composite slab). If there are multiple adjacent slab items with different properties, it is the users responsibility to indicate which one governs.

Note the following with regard to the slab item properties:

  • You will find that a wide range of profiled metal decks have been included for manufacturers from many countries. Precast concrete planks are also available, but only for the EC regional code.
  • The slab item’s rotation angle relative to the global X axis is used to set the profiled metal deck as spanning at any angle between 0° (parallel) and 90° (perpendicular) to the direction of span of the steel beam.
  • The concrete type can be set to either normal or lightweight, however design using lightweight slabs is only available for the Eurocode.
  • When designing to the Eurocode, warnings are issued in the design if the concrete slab does not comply with the following constraints:
    • Normal weight concrete range C20/25 - C60/75 - See EN 1994-1-1:2004 Clause 3.1(2),
    • Lightweight concrete range LC20/22 - LC60/66 - See EN 1994-1-1:2004 Clause 3.1(2),
    • Minimum density for lightweight concrete 1750 kg/m3 - see EN 1994-1-1:2004 Clause 6.6.3.1(1).

The beam’s “condition” is:

  • restricted to internal if it has composite slabs attached along its full length on both sides,
  • restricted to edge if it has no composite slabs on one side,
  • defaulted to edge (but editable) if it has composite slabs on both sides but not along the full length.

Shear connector type

The shear connection between the concrete slab and the steel beam is achieved by using shear studs.

Note: The use of channel connectors or Hilti™ connectors is currently beyond the program scope.

Regional Code: US

3/4 in diameter studs with 3 3/8, 3 7/8, 4 3/8, 4 7/8, 5 3/8, 5 7/8 nominal height are offered.

Regional Code: Eurocode
  • 19mm diameter studs with 100, 125, and 150mm nominal height (95, 120 and 145mm as welded height).
  • 22mm diameter studs are also offered with 125 and 150 nominal height. These can only be used for precast plank decks and profiled metal decks with 'closed' ends. (e.g. Sigmat).

Regional Code: BS

19mm diameter studs with 100 and 125 nominal height (95 and 120 as welded height) are offered. 22mm diameter studs are also offered but only for precast plank decks. Studs do not have a given capacity as their resistance is derived.

Effective width

The effective width is used in strength and serviceability calculations. It can be entered manually, or be calculated automatically, either as a one time process or each time a design is performed.

To manually enter the effective width:

  1. Specify the Floor construction > Effective width value you wish to use.
    Note: If any amendments are made to the geometry of the model then you would need to amend the effective width value as necessary.

    The Calculate button shown below the effective width property can be used as a helper to determine the code based values.

To calculate the effective width automatically as a one time process:

  1. First select Calculate effective width in the properties.
  2. Then click the [...] box that appears next to Calculate.
  3. The calculated value is reported in the effective width property.

Although the program calculates the effective width, it is your responsibility to accept the calculated figure or alternatively to adjust it. Engineering judgment may sometimes be required.

Example:

Consider the beam highlighted below:

The program calculates the effective width as the sum of:

  • to the right of the beam, be(right) = beam span/8
  • to the left of the beam, be(left) = one half of the shortest distance to the center line of the adjacent diagonal beam

To the left of the beam, some engineers might prefer to use one half of the mean distance to the adjacent beam. To do so you would need to manually adjust the calculated value via the Floor construction page of the Beam Properties.

Note: If any amendments are made to the geometry of the model then you would need to use Calculate again or use the procedure below.

To automatically calculate the effective width each time a design is performed:

To combat the possibility of model edits and the need to recalculate the effective width for individual beams it is possible to automatically perform the Calculate process discussed above automatically to all beams prior to a Design command.

  1. Ensure Update effective width prior to design check is checked on in Design > Settings > Steel > Composite beam
  2. The floor construction properties will then display an Override effective width option.
    1. With this unchecked, the effective width will be determined when a Design command is performed and the value is reported to you in the Effective width property which is not editable.
    2. With this checked, you can override the effective width value and the value you input into the effective width property is used for design. The Calculate button can still be used to advise the code based values.

Regional code based values: US

The Calculate button determines the effective width of the compression flange, be for each composite beam as per Section I3.1a (360-05/-10).

For each side of the beam, it is taken as the smaller of:

  • beam span/8 – span taken as the center to center of supports
  • one half of the distance to the center line of the adjacent beam
  • the distance to the edge of the slab

Regional code based values: EC

The Calculate button determines the effective width of the compression flange, beff, for each composite beam as per Clause 5.4.1.2 of EC4.

Unless hollowcore units are used, it is taken as the smaller of:

  • Secondary beams: the spacing of the beams, or beam span/4
  • Primary beams (conservatively): 80% of the spacing of beams, or beam span/4
  • Edge beams: half of above values, as appropriate, plus any projection of the slab beyond the centerline of the beam.

For hollowcore precast plank units only, we calculate the effective width for each side of the beam as the minimum of:

  • Assumed concrete fill (500 mm) + recommended gap
  • beam span/8 – span taken as the center to center of supports
  • one half of the distance to the center line of the adjacent beam
  • the distance to the edge of the slab

Regional code based values: BS

The Calculate button determines the effective width of the compression flange, be, for each composite beam as per section 4.6 of BS 5950 : Part 3 : Section 3.1 : 1990.

For each side of the beam, it is taken as the smaller of:

  • beam span/8 – span taken as the centre to centre of supports
  • one half of the distance to the centre line of the adjacent beam (for slabs spanning perpendicular)
  • 40% of the distance to the centre line of the adjacent beam (for slabs spanning parallel)
  • the distance to the edge of the slab

To override the effective width in a Review View:

The Show/Alter State Override Effective Width attribute allows you to graphically set the composite beams to which an effective width override applies. For details, see: Override effective width

Metal deck

Minimum lap distance

The position and attachment of the decking is taken into account in the longitudinal shear resistance calculations.

The applied longitudinal shear force is calculated at the center-line of the beam, and at the position of the lap (if known). If the position of the lap is not known, then the default value of 0mm should be used (that is the lap is at the center-line of the beam) as this is the worst case scenario.

 

Stud strength

The stud properties you can choose from are appropriate to the stud source. For additional Eurocode specific information, see: Stud connector strength (Composite beams: EC4 Eurocode)

All types of stud may be positioned in a range of patterns.

Stud groups - Regional Code ACI/AISC

You can allow group sizes of 1, 2, 3 or 4 studs - any group sizes that you don't want to be considered can be excluded.

For example, if you do not set up groups with 3 or 4 studs, then in auto-design the program will only try to achieve a successful design with a maximum of 2 studs in a group.

For each group that you allow you must enter the 'Set distance emid to' - as either >= 2in or < 2in and you also specify the pattern to be adopted (e.g. along the beam, across the beam, or staggered).

Stud groups - Regional Code Eurocode or BS

You can allow group sizes of 1 or 2 studs - any group sizes that you don't want to be considered can be excluded.

For example, if you do not set up groups with 2 studs, then in auto-design the program will only try to achieve a successful design with a maximum of 1 stud in a group.

For groups with 2 studs you must specify the pattern to be adopted (e.g. along the beam, across the beam, or staggered).

Note: It is up to you to check that a particular pattern fits within the confines of the rib and beam flange since Tekla Structural Designer will draw it (and use it in design) anyway.

Optimize shear interaction

If you choose the option to optimize the shear interaction, then Tekla Structural Designer will progressively reduce the number of studs either until the minimum number of studs to resist the applied moment is found, until the minimum allowable interaction ratio is reached or until the minimum spacing requirements are reached. This results in partial shear connection.

Connector layout

When running in Auto-design mode you may not want to specify the stud layout at the start of the design process. To work in this way check Auto-layout to have the program automatically control how the stud design will proceed. When the beam is subsequently designed Auto-layout invokes an automatic calculation of the required number of studs, which is optimized to provide an efficient design.

Note: 'Auto layout' can actually be checked regardless of whether you are auto designing the beam size or checking it. The combination of 'Check' design with 'Auto layout' of studs can be used to assist you to rationalize your designs e.g. to force a beam to be the same size as others in the building but have Tekla Structural Designer determine the most efficient layout of studs.

You may choose to perform the initial design with Auto-layout checked and then refine the spacing with Auto-layout cleared if the spacing is not exactly as you require. This may arise if for instance the theoretical design needs to be marginally adjusted for practical reasons on site.

Auto-layout for perpendicular decks

For perpendicular decks, the Auto-layout dialog provides two options for laying out the studs:

  • Uniform
  • Non-uniform
  • Near uniform (US design only)

Uniform

The Uniform option forces placement in ribs at the same uniform spacing along the whole length of the beam.

Whether the stud groups are placed in every rib (as shown above), alternate ribs, or every third rib etc. can be controlled by adjusting the limits you set for Minimum group spacing ( ) x rib and Maximum group spacing ( ) x rib.

The number of studs in each group will be the same along the whole length of the beam, this number can be controlled by adjusting the limits you set on the Stud strength page.

Example:

If you set Minimum group spacing 2 x rib and Maximum group spacing 3 x rib, then the program will only attempt to achieve a solution with studs placed in alternate ribs, or studs placed in every third rib. It will not consider a solution in which studs are placed in every rib.

Additionally, if on the Studs - Strength page, you have allowed groups of 1 stud and 2 studs; then if 1 stud per group proves to be insufficient the program will then consider 2 studs per group.

Non-uniform

If optimization has been checked (see Optimize shear interaction in the preceding Stud Strength topic) studs are placed at suitable rib intervals (every rib, alternate ribs, every third rib etc.), in order to achieve sufficient interaction without falling below the minimum allowed by the code.

Note: The optimum and minimum amounts of shear connection are defaulted to 50% and 25% respectively. These can be adjusted if required.

If optimization has not been checked, studs are placed at suitable rib intervals in order to achieve 100% interaction.

Knowing the number of studs necessary to achieve the required level of interaction, it is possible that placement at a given rib interval could result in a shortfall; the program will attempt to accommodate this by working in from the ends, (as shown in the example below). If every rib is occupied and there is still a shortfall, the remainder are 'doubled-up', by working in from the ends once more.

In this example the point of maximum moment occurs one third of the way along the span, this results in an asymmetric layout. If you prefer to avoid such arrangements you can select Adjust layout to ensure symmetrical about center line. A redesign would then result in the symmetric layout shown below.

For both Uniform and Non-uniform layouts, if the minimum level of interaction cannot be achieved this is indicated on the design summary thus: “Not able to design stud layout”.

Near uniform (US design only)

The Near uniform layout option can produce a design which is both more efficient - using less studs - and more practical for construction than Uniform or Non-uniform layouts.

To illustrate, for a case where the optimum solution falls between 1 and 2 studs per rib, the Near uniform auto-layout method will try a [2,1] alternating pattern along the span of the beam saving 25% on studs compared to 2 per rib and producing a simple pattern for construction: 1 stud per rib, then add 1 more to alternate ribs.


Example:

Consider the example above where the bending moment diagram is skewed to one side. The ‘near uniform’ setting requires studs to be placed at the same spacing along the whole length of the beam but the critical spacing is set by the shorter length to the maximum moment, in this case by the left hand portion. This example requires 11 studs to the maximum moment position and there are 8 ribs to this position.

Firstly the ‘base’ layout of studs is calculated as (no. of studs)/(no. of ribs) = 11/8 = 1.375 and thus base_no = 1. That is 1 stud per rib to give 8 studs but 11 are required so ‘remainder’ = 3.

Secondly the ‘remainder’ is evenly spread over the 8 ribs and so some of the ribs must be given an extra stud. In this case this is achieved by doubling up in ribs 1, 3, 5, & 7.

The final layout of studs to the left of the maximum moment is 2,1,2,1,2,1,2,1 = 12 which is > the 11 required and hence OK.

For the other portion of the beam, in this case to the right of the maximum moment, the process starts with rib ‘n’ i.e. the last rib on the beam and then works back towards the maximum moment position using the same procedure viz.

1 stud is added per every rib to give the ‘base’ layout. Then the ‘remainder’ is distributed by adding 1 more to rib ‘n’. Step = 2 as determined by the shorter length so studs are doubled up in rib n-2, then n-4, n-6, n-8, etc.

The final layout of studs to the right of the maximum moment position (starting from end 2) is 2,1,2,1,2,1,2,1,2,1,2,1,2,1,2,1,2,1,2,1,2,1,2 = 35.

Thus the total number of studs = 47 and these are laid out (near) uniform.

For comparison a ‘non-uniform’ layout would give 11 studs to the left of the maximum moment position and 12 to the right (per alternate ribs) = 23 in total. For the ‘uniform’ setting, 1 stud per rib would be inadequate and so 2 per rib would be found to give 62 in total.

Auto-layout for parallel decks

For parallel decks, the Auto-layout again provides Uniform and Non-uniform layout options, but the way these work is slightly different.

Uniform

The Uniform option forces placement at a uniform spacing along the whole length of the beam. The spacing adopted will be within the limits you set for Minimum group spacing distance and Maximum group spacing distance. If the point of maximum moment does not occur at mid span, the resulting layout will still be symmetric.

The number of studs in each group will be the same along the whole length of the beam, this number can be controlled by adjusting the limits you set on the Stud strength page.

Non-uniform

If optimization has been checked (see Optimize shear interaction in the preceding Stud Strength topic) studs are placed at a suitable spacing in order to achieve sufficient interaction without falling below the minimum allowed by the code.

If optimization has not been checked, studs are placed at a suitable spacing in order to achieve 100% interaction.

If the point of maximum moment does not occur at mid span, the resulting non-uniform layout can be asymmetric as shown below.

Note: For US design only, where the Auto-layout option is set to Non-uniform and the beam supports more than five secondary beams, the layout reverts to uniform, producing a more practical design solution.

For both Uniform and Non-uniform layouts, if the minimum level of interaction cannot be achieved this is indicated on the design summary thus: “Not able to design stud layout”.

Manual Stud Layout

You may prefer to manually define/adjust the group spacing along the beam. This can be achieved by unchecking Auto layout.

Note: If you specify the stud spacing manually, then it is most important to note:
  • the resulting design may not be the optimal design possible for the beam, or
  • composite design may not be possible for the stud spacing which you have specified.

To generate groups of studs at regular intervals along the whole beam use the Quick layout facility. Alternatively, if you require to explicitly define the stud layout to be adopted for discrete lengths along the beam use the Layout table.

Manual layout for Perpendicular decks

For perpendicular decks, the dialog for manual layouts is as shown:

To use Quick layout, proceed in one of two ways:

  • Choose to position groups in either every rib, or alternate ribs, then specify the number of studs required in the group and click Generate.
  • Alternatively: specify the total number of studs, then when you generate, if the number specified is greater than the number of ribs, one will be placed in every rib and the remainder will be 'doubled-up' in the ribs at each end starting from the supports. Similarly if the number specified is less than the number of ribs, but greater than the number of alternate ribs, one will be placed in every alternate rib and the remainder will be placed in the empty ribs. Limits of 600mm or 4 x overall slab depth, (whichever is less), are considered.

To use the Layout table:

  • For each segment you should define the following parameters: No. of connectors in length and No. of connectors in group; Group spacing x rib.

Your input for these parameters is used to automatically determine Distance end 2 - this latter parameter cannot be adjusted directly, hence it is dimmed.

  • If required click Insert to divide the beam into additional segments. (Similarly Delete will remove segments). You can then specify a different stud layout for each segment.
  • We would advise that having entered No. of studs in length, group and spacing and ignoring Distance ends 1 and 2 you click Update, this will automatically fill in the missing fields.

Manual layout for Parallel decks

For parallel decks, the dialog for manual layouts is as shown:

 

To use Quick layout, proceed in one of two ways:

  • Choose to position groups at a set repeat distance, then specify the number of studs required in the group and click Generate.
  • Alternatively: specify the total number of studs, then click Generate - the program calculates the repeat distance automatically, subject to the code limits.

To use the Layout table:

  • The preferred method is to choose the option Spacing distance automatic, in which case you can adjust the No. of connectors in length and No. of connectors in group. Alternatively you could choose the option Number in length automatic and then adjust No. of connectors in group and Group spacing dist.
  • If required click Insert to divide the beam into additional segments. (Similarly Delete will remove segments). You can then specify Distance end 1 for each new segment and it’s own stud layout.

Transverse reinforcement

This reinforcement is be provided specifically to resist longitudinal shear.

Since the profile metal decking can be perpendicular, parallel or at any other angle to the supporting beam the following assumptions have been made:

  • if you use single bars they are always assumed to be at 90° to the span of the beam,
  • if you use mesh then it is assumed to be laid so that the main bars are at 90° to the span of the beam.

The reinforcement you specify is assumed to be placed at a position in the depth of the slab where it is able to contribute to the longitudinal shear resistance

Automatic transverse shear reinforcement design

It is possible to automatically design the amount of transverse shear reinforcement for each beam. This is achieved in Tekla Structural Designer by checking the Auto-select option on the Transverse reinforcement tab of the Composite Beam Properties.

Note:

The Auto-select option for designing transverse shear reinforcement is only available when the beam is in auto-design mode.

If you are checking a beam, then you must specify the transverse shear reinforcement that you will provide, and then check out this arrangement.

The auto-selected bars can be tied into the stud group spacing by checking the Bar spacing as a multiple of stud spacing option. Alternatively, the spacing can be controlled directly by the user.

Bar spacing as a multiple of stud spacing

When the option Bar spacing as a multiple of stud spacing is checked, the Transverse Reinforcement tab provides the user with controls on the bar size and the multiples of stud spacing.

These can be used to achieve a selection of say, 12mm diameter bars at 2 times the stud spacing, with a slightly greater area than a less preferable 16mm diameter bars at 4 times the stud spacing.

Controlling the bar spacing directly

When the option Bar spacing as a multiple of stud spacing is not checked, the Transverse Reinforcement tab provides the user with direct control on the bar size and the bar spacing.

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 beam 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 regional code, see:

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