CIVL6264 Chap.5 Composite Slabs and Floor Systems
Composite Slabs and Floor Systems
Profiled steel sheeting changes from temporary formwork to part of the completed slab system. This chapter checks deck spanning and support during concrete placement, then examines positive bending, longitudinal and vertical shear, negative support regions, crack control and fire detailing.
It treats manufacturer tables as bounded evidence and shows why openings, unequal spans, concentrated construction loads and altered propping require explicit review. The placement-stage check begins with sheet thickness, span, continuity, bearing, concrete depth, construction action and temporary support.
Product tables are efficient screening tools only while the actual case remains inside their geometry, load and support assumptions. A concentrated pump-line load, unusual span ratio or changed prop position can leave that scope. In the completed slab, positive bending may use the sheet in tension, but longitudinal shear can produce slip before full yielding and concrete crushing.
Vertical shear and negative bending at internal supports require separate mechanisms. Top reinforcement must cross and anchor through negative regions, while openings can remove deck area, interrupt bars and alter the fire path. Joints, end laps, embedded services and concrete-placement tolerances also affect the load path.
The chapter links these local details to floor-system selection because a rapid, shallow solution is valuable only when it can be propped, poured, reinforced, penetrated and inspected within the project constraints. Its field scenarios require reconstruction of the actual sequence whenever prop removal or site cutting departs from the approved plan.
What this chapter covers
- 01
Deck formwork stage
- 02
Composite slab action
- 03
Positive and negative bending
- 04
Longitudinal shear
- 05
Openings and trimming
- 06
Propping and support details
Worked example · free
Audit a deck-table selection
- +1Compare actual span ratio with the table scope.
- +1Treat the pump support as a concentrated action.
- +1Trace the opening through deck and reinforcement.
- +1Recheck placement-stage strength and deflection.
- +1Recheck completed-slab bending, shear and detailing.
Key terms
- Profiled deck
- Cold-formed steel sheeting that can act as formwork and participate in a composite slab.
- Longitudinal shear
- Interface action that can produce slip between profiled sheeting and concrete.
- Negative region
- A support region where the slab top is commonly in tension under hogging bending.
- Temporary prop
- A removable support controlling construction-stage span, stability or deflection.
- Trimming
- Local framing or reinforcement that redirects force around an opening.
Composite Slabs and Floor Systems FAQ
Why does a composite slab need two stage checks?
Before curing, the profiled sheet acts as formwork for wet concrete and construction loading. After curing, sheet-concrete interaction can provide composite resistance. The load, stiffness, supports and failure mechanisms differ, so one completed-stage capacity cannot prove placement safety.
Why are openings especially important near supports?
Support regions can carry negative bending, high shear and essential top reinforcement. An opening can remove sheet area, interrupt bars, alter local fire performance and create a new load path. Its location and trimming should be designed before cutting.
When can a manufacturer table be used?
Use it when deck profile, thickness, slab geometry, span arrangement, support, concrete, loading and detailing remain inside the published scope. A departure does not automatically ban the product, but it requires current-standard analysis or validated software for the actual case.
Exam move
Sketch one slab bay through placement and service stages. Add support widths, deck span, props, negative reinforcement and an opening. Practise auditing a table by matching every assumption to the project. Finish with a site checklist for bearing, fastening, reinforcement, concrete placement and prop removal. Draw a typical bay in plan and section.
Add deck span direction, support width, props, slab depth, positive sheet action, negative reinforcement and a large opening. For the placement stage, list the sheet and support assumptions beside the relevant manufacturer table and circle every project departure. For the completed stage, write separate lines for positive bending, negative bending, longitudinal shear, vertical shear, cracking and fire.
Move the opening toward an internal support and identify which checks change. Rehearse a prop-removal investigation using concrete test records, loads present, measured levels and the approved sequence. End by writing a hold-point checklist for bearing, fastening, reinforcement, penetrations, concrete placement and prop release. Read a manufacturer table from its notes upward rather than from a desirable span downward.
Copy the stated concrete, loading, continuity, support and deflection assumptions beside the selected cell. Mark any departure in the project model and assign it to calculation, validated software or a different tested configuration. Practise an opening review at midspan and near an internal support so the change in moment region becomes obvious. Add fire stopping and services coordination to the structural response.
This habit prevents an apparently small architectural move from bypassing deck, reinforcement and compartmentation checks. When reviewing a floor plan, circle every location where the usual one-way slab strip is interrupted. Supports, joints, edges and openings deserve local sections because the standard midspan mechanism may not pass through them unchanged.