Composite Steel Beam Design
Composite steel beam design is based on steel design, sharing many features, assumptions, parameters, and limits, but is specific to beams with a slab on top.
Creating Composite Beam Groups
Composite Beam design requires member elements to be in a steel design group. Once the steel design group is created, the option to design as composite beam is an available design group parameter in thetab. Because of the way composite shape properties are calculated, you should only group together members that have the same model-shape and length.
Composite Beam Topics:
- Composite Beam Construction Self-Weight Loads
- Composite Beam Deflection Assumptions
- Composite Beam Parameters
- Composite Slab Parameters
- Composite Deck Parameters
- Composite Beam Report
- Composite Beam Check Reports
Composite Beam Construction Self-Weight Loads
In the design of unshored composite beams, there are typically two situations that must be considered. The first is the strength of the steel beam alone, acting under construction loads that include the wet concrete slab weight. The second is the full composite beam acting under design loads typical of all steel members.
Of course, if you choose a shored construction procedure, then construction loads do not apply and no steel-only check is performed.
VisualAnalysis will automatically estimate self-weight construction loads for unshored construction. It uses only the self-weight of the steel and concrete slab as a simple steel-only check. Currently there is no way to specify specific construction loads or load cases for additional steel-only checks. If you have high construction loads, you will need to check the beam manually for these conditions.
Composite Beam Deflections
When designing composite beams there are typically two distinct deflections that are of concern. The first is construction deflection of the steel beam alone, and the second is the deflection of the composite section after construction under service loads.
VisualAnalysis makes no provision for limiting or calculating deflections of the first kind. It is assumed that the deflection values can be readily obtained using construction loads for analysis. Furthermore, the beam is generally cambered to handle this deflection. As with stress calculations, construction deflections play no part if the beam is constructed using a shored procedure.
VisualAnalysis does allow you to limit the service load deflections of the fully composite beam. It also estimates these deflections by scaling the reported deflections by the ratio of the steel moment of inertia to the composite moment of inertia. This check is performed for all serviceability load cases.
Composite Beam Bracing
Bracing for composite beams is assumed to be continuous for the top flange (metal deck or formwork) and the full member length for the bottom flange. Bracing differing from these defaults cannot be specified. If you want a different bottom bracing, you could split the member in the Model View to effectively create shorter elements. See Bracing in VisualAnalysis for a complete description of bracing member elements for design checks.
Composite Beam Parameters
Composite Design Shapes
For composite beam design a reduced shape list is available. For AISC shapes this list is: W, M, S, HP, C, MC, and TS or HSS (square and rectangular). If you are using other database shapes or parametric shapes you will be restricted to those that use the same basic 'property sets' as the AISC shapes listed here.
Composite shapes are generally modeled for analysis using just the steel section. You may choose to model the slab with plate elements in your model, but these elements are not affected by, nor do they directly affect the design software, which will look only at the member element and its forces. There is no built-in way to model composite beams directly in VisualAnalysis by defining slab properties along with the steel members.
Specification: You can choose between ASD and LRFD provisions.
See Steel Design other input design parameters.
Composite Beam Slab Parameters
Some of the following parameters have default values that you may define in preferences to speed up your design work. These are 'per machine' default settings that only apply to new design groups you create.
Shored: This option allows you to specify whether shored or unshored construction will be used. With unshored the construction load cases are enabled so that VisualAnalysis can correctly include the effect of construction loads on the beam (see see construction load section above). With shored construction, no construction loads are necessary as the member is fully supported until it can act as a composite unit.
Strength f'c: Specify the concrete strength, f'c.
Thickness: Specify the slab thickness.
Weight: Choose the proper concrete weight to be used for the slab.
Beam Spacing: Enter the centerline to centerline distance between adjacent beams.
Is Spandrel: If yes, you are required to enter the overhang. Enter the distance from the centerline of the beam to the edge of the slab for the overhang distance.
Effective Width: The effective width of a slab is determined using the beam spacing, slab thickness, distance to a slab edge, and the span length. The effective width the design software calculated and used in the design checks is listed in the design report under "Slab and Deck Parameters".
Composite Beam Deck and Studs
Deck Type: There are three choices for slab construction: no metal deck (solid slab), perpendicular metal deck and parallel metal deck. These are shown in the following sketches.
Rib Height/Rib Width: Deck dimensions for rib height and width must conform to the limits prescribed in AISC. These limits are automatically checked in the inspector. Refer to the diagram above when specifying the rib height and width.
The size and orientation of the deck will determine the effective thickness of the concrete slab, and may also reduce the effectiveness of shear studs. AISC has placed certain restrictions on deck dimensions and the inspector will prevent entry of numbers that are out of the allowable range.
Stud Type: Shear studs may be headed studs or channels.
Stud Length: If you choose a headed stud, the length is interpreted as the height of the stud. For channel studs, the length field is interpreted as the horizontal width. There is a minimum of 0.5" cover recommended. If you have selected a metal deck, then the stud must also extend 1.5" above the deck.
Nr: The Nr parameter is only used for perpendicular metal decks and represents the number of studs per rib. The number must be between one and three (more studs may be placed in the rib, but only three are allowed for calculation purposes). Placing more studs in a rib may reduce the effective capacity of each stud. All of the appropriate stud capacity reductions are used to calculate the member strength.
Composite Beam Report
In the parameters section of the composite beam report there is more information included than simply what was entered in the parameters dialog. Specifically there is a section about stud capacities and another with composite beam properties. Both of these involve additional calculations using the parameters you defined. These sections contain more information if the design group has a valid design shape.
In reporting shear stud requirements, the report contains minimum and maximum stud spacing requirements and the number of studs required between points of maximum and zero moment areas. VisualAnalysis does not produce a detailed stud arrangement schedule, but rather provides enough information for you to layout the studs.
If partial composite action is permitted, information will be shown regarding stud requirements for the minimum partial composite action that provides the required moment capacity. VisualAnalysis does not perform calculations for any level composite action between the full and minimum values reported.
Composite Beam Check Reports
The checks for composite beams are fewer than for general steel design. Axial forces, weak axis shear and bending, and weak axis deflection checks are not made. Forces, stresses, or deflections from these are simply ignored in the software.
There are two types of strong bending checks performed for unshored construction: a Steel Flexure Check (Construction) and a Composite Flexure Check. The first is identical to the strong flexure check appearing in steel reports. The composite check uses a plastic distribution of composite forces. With shored construction only the composite check is performed and construction stresses are assumed negligible. The composite check unity value is equal to the moment demand divided by the fully composite moment capacity. Partial composite action is not reflected in the unity check.
Composite deflections are approximated using the non-composite deflection reported from VisualAnalysis, and scaling them based on the composite moment of inertia for the beam. The scaled deflections are then checked against your specified deflection limits. There is no provision in the software to specify a pre-camber or to ignore deflections from certain loads, other than how you choose your serviceability load cases.