CIVL6264 Chap.4 Serviceability and Long-Term Response
Serviceability and Long-Term Response
Composite floors remain useful only when their movement and vibration suit the building. This chapter distinguishes effective width from tributary width, builds short-term and long-term transformed properties, and separates construction-stage, immediate, creep, shrinkage and connection-slip deflection.
It also explains why static sag, incremental movement and dynamic response use different criteria and require explicit load histories. Effective slab width represents longitudinal stress participation and should not be confused with the tributary width used to allocate vertical load.
The transformed section then expresses concrete on a steel reference basis through a modular ratio, with distinct treatment for immediate and sustained response. A serviceability ledger associates each action with the structural stage that receives it. Wet-concrete loading can create permanent steel-beam deflection before composite action exists.
Later imposed load uses the cured section, while creep and shrinkage add time-dependent curvature and partial interaction permits extra slip. Total sag and movement after finishes are installed therefore have different datums and consequences. Camber changes initial geometry but does not erase stress or solve dynamic response.
Floor vibration adds a separate evidence set based on mode shape, forcing, mass, stiffness, damping and acceleration. The chapter insists on matching each result to its criterion: appearance, partition damage, ponding, comfort or another stated limit.
Surveyed levels can provide a useful comparison only when measurement dates and load conditions are recorded, because an unexplained elevation cannot be assigned to one part of the history.
What this chapter covers
- 01
Effective slab width
- 02
Modular ratio
- 03
Transformed centroid and stiffness
- 04
Creep and shrinkage
- 05
Partial-interaction deflection
- 06
Floor vibration
Worked example · free
Build a deflection history
- +1List all compatible downward components.
- +1Add total movement to obtain 18.0 mm.
- +1Set the fitout datum after the first two terms.
- +1Add 3.0 and 1.5 to obtain 4.5 mm post-fitout movement.
- +1Compare each result with the criterion matching its consequence.
Key terms
- Modular ratio
- A ratio used to express one material area on the stiffness basis of another.
- Effective width
- The slab width credited after accounting for nonuniform longitudinal stress.
- Creep
- Increasing concrete strain under sustained stress with time.
- Shrinkage
- Concrete contraction with time that does not require external loading.
- Incremental deflection
- Movement occurring after a selected installation or reference time.
Serviceability and Long-Term Response FAQ
Why is tributary width not always effective width?
Tributary width allocates vertical floor load, while effective width represents the slab participating in longitudinal composite stress. Shear lag makes that stress nonuniform. Using the same full width for both purposes can overstate composite stiffness and resistance.
Why do short-term and long-term properties differ?
Concrete stiffness changes under sustained compression because of creep, while shrinkage creates additional strain. A long-term transformed section therefore uses the governing standard's treatment of time effects rather than silently retaining the immediate elastic modular ratio.
Does passing a deflection limit prove vibration comfort?
No. Static deflection is a useful stiffness indicator, but dynamic comfort depends on natural frequencies, mode shapes, forcing, modal mass, damping and response criteria. A floor can pass static sag and still respond noticeably to rhythmic activity.
Exam move
Draw a time axis and place every load and installation event on it. Assign the steel-only or composite stiffness that exists at each event, then separate immediate, long-term and slip-related increments. Rework the ledger example and explain which movement affects structure, finishes and occupants without using one limit for all three.
Construct a timeline from fabrication camber through erection, concrete placement, curing, prop removal, fitout and long-term occupancy. Place every load component on that line and identify the section stiffness available when it arrives. Calculate immediate and long-term transformed properties separately, using the current standard rather than importing the older numerical ratios in supporting readings.
Keep a signed deflection ledger and choose a datum for total and incremental checks. For partial interaction, explain why movement trends toward the steel-only response as connection reduces. Then take a floor that passes static sag and list the additional information needed for a vibration decision.
A strong revision answer names the affected finish or occupant, applies the corresponding criterion and identifies the member or mechanism that controls the response. Compare two design reports by asking whether their deflection numbers share the same reference state. One may include construction sag and another may report only movement after fitout.
Reconcile stage, sign, load duration and section stiffness before comparing them. Create a separate vibration card listing occupancy, forcing, mode, modal mass, damping and response criterion. If a proposed remedy adds concrete, predict its competing stiffness and mass effects rather than assuming improvement.
The goal is to explain which component and time interval control performance, not to collect the greatest number of deflection values. Use the final line of every serviceability answer to name the affected consequence. Total sag, post-installation movement and occupant vibration are different performance questions, so a result without its datum and criterion remains incomplete.