L/360: Understanding the L/360 Deflection Limit for Floors and Structures

L/360 is a critical deflection limit used in structural design to ensure serviceability and prevent excessive movement under live loads. This comprehensive guide explains what L/360 means, why it matters, how to calculate it, and how to apply it in structural design.

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What Does L/360 Mean?

Basic Definition

L/360 is a deflection limit expressed as a ratio where:

• L = Span length of the structural member
• 360 = Divisor representing the maximum allowable deflection
• Deflection limit = L/360

Example Calculation:

• Floor span: 20 feet
• L/360 = 20 feet / 360 = 0.0556 feet = 0.67 inches
• Maximum allowable deflection under live load: 0.67 inches

Another Example:

• Floor span: 30 feet
• L/360 = 30 feet / 360 = 0.0833 feet = 1 inch
• Maximum allowable deflection under live load: 1 inch

Metric Example:

• Floor span: 6 meters
• L/360 = 6000 mm / 360 = 16.7 mm
• Maximum allowable deflection under live load: 16.7 mm

Why L/360 is More Stringent

L/360 is more stringent than L/240 because:

Stricter Limit:

• L/360 allows less deflection
• L/240 allows more deflection
• L/360 = 1.5 × more stringent
• Requires larger sections
• Higher cost

Floor Applications:

• Floors are more sensitive to deflection
• Occupants notice movement
• Affects comfort
• Affects perception
• Requires stricter limits

Serviceability:

• Prevents excessive movement
• Maintains appearance
• Prevents damage
• Ensures comfort
• Regulatory requirement

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Understanding Deflection Limits

Common Deflection Limits Hierarchy

Most Stringent:

• L/480: Sensitive equipment, precision machinery
• L/360: Floors, sensitive applications
• L/240: Beams, general applications
• L/180: Beams, less stringent
• L/120: Industrial, relaxed limits

Least Stringent:

• L/90: Temporary structures
• L/60: Very temporary structures

Why Different Limits?

Floors vs. Beams:

Floors:

• L/360 for live load (stringent)
• L/240 for total load
• Occupants sensitive to movement
• Comfort critical
• Serviceability important

Beams:

• L/240 for live load (less stringent)
• L/180 for total load
• Less sensitive to movement
• Structural role primary
• Deflection less critical

Live Load vs. Total Load:

Live Load Only:

• L/360 for floors
• L/240 for beams
• Temporary movement
• Less noticeable
• Larger deflection acceptable

Total Load:

• L/240 for floors
• L/180 for beams
• Includes permanent load
• More noticeable
• Smaller deflection required

Application Type:

Residential:

• L/360 for floors
• L/240 for beams
• Comfort important
• Occupant sensitive
• Stringent limits

Commercial:

• L/360 for floors
• L/240 for beams
• Comfort important
• Professional environment
• Stringent limits

Industrial:

• L/240 for floors
• L/180 for beams
• Comfort less important
• Functional role primary
• Relaxed limits

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Why L/360 Matters for Floors

Serviceability Concerns

Excessive Floor Deflection Causes:

Visual Problems:

• Visible sagging
• Uneven surfaces
• Noticeable slopes
• Aesthetic concerns
• Reduces confidence

Functional Problems:

• Doors and windows jam
• Cracks in finishes
• Plumbing problems
• Equipment misalignment
• Operational issues

Comfort Issues:

• Noticeable movement
• Vibration perception
• Psychological discomfort
• Reduced confidence
• User dissatisfaction

Structural Concerns:

• Stress concentration
• Fatigue potential
• Secondary effects
• Long-term damage
• Reduced service life

Example Problems:

1 inch deflection in 20-foot span:

• Visible sagging
• Noticeable to occupants
• Doors may jam
• Cracks possible
• Unacceptable

0.67 inch deflection in 20-foot span (L/360):

• Barely noticeable
• Occupants comfortable
• No functional problems
• No damage
• Acceptable

Building Code Requirements

International Building Code (IBC):

• Table 1604.3: Deflection limits
• L/360 for live load on floors
• L/240 for total load on floors
• Mandatory compliance
• Minimum standard

American Society of Civil Engineers (ASCE):

• ASCE 7: Minimum Design Loads
• L/360 for live load on floors
• L/240 for total load on floors
• Industry standard
• Professional guidance

American Institute of Steel Construction (AISC):

• Steel Construction Manual
• L/360 for live load on floors
• L/240 for total load on floors
• Industry standard
• Design guidance

American Concrete Institute (ACI):

• ACI 318: Building Code Requirements
• Table 24.2.2: Deflection limits
• L/360 for live load on floors
• L/240 for total load on floors
• Design requirements

American Wood Council (AWC):

• National Design Specification
• L/360 for live load on floors
• L/240 for total load on floors
• Industry standard
• Design guidance

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Calculating L/360 Deflection Limits

Step-by-Step Calculation

Step 1: Determine Span Length

• Measure or estimate span
• Convert to consistent units
• Use clear span or effective span
• Design parameter

Step 2: Calculate L/360 Limit

• Formula: L/360 = Span / 360
• Divide span by 360
• Result is maximum deflection
• Serviceability limit

Step 3: Convert Units

• If span in feet: Multiply by 12 to get inches
• If span in meters: Result in millimeters
• Consistent units
• Clear communication

Step 4: Document Result

• Record span length
• Record L/360 limit
• Record units
• Design documentation

Calculation Examples

Example 1: 20-Foot Span

• Span: 20 feet
• L/360 = 20 / 360 = 0.0556 feet
• Convert: 0.0556 × 12 = 0.667 inches
• Maximum deflection: 0.67 inches
• Typical residential floor

Example 2: 30-Foot Span

• Span: 30 feet
• L/360 = 30 / 360 = 0.0833 feet
• Convert: 0.0833 × 12 = 1.0 inch
• Maximum deflection: 1.0 inch
• Typical commercial floor

Example 3: 40-Foot Span

• Span: 40 feet
• L/360 = 40 / 360 = 0.111 feet
• Convert: 0.111 × 12 = 1.33 inches
• Maximum deflection: 1.33 inches
• Large commercial floor

Example 4: 6-Meter Span (Metric)

• Span: 6 meters = 6000 mm
• L/360 = 6000 / 360 = 16.7 mm
• Maximum deflection: 16.7 mm
• Typical residential floor

Example 5: 9-Meter Span (Metric)

• Span: 9 meters = 9000 mm
• L/360 = 9000 / 360 = 25 mm
• Maximum deflection: 25 mm
• Typical commercial floor

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Deflection Calculation Methods

1. Analytical Method

Formula for Uniformly Distributed Load:

• Deflection = (5 × w × L⁴) / (384 × E × I)
• w = Load per unit length
• L = Span length
• E = Elastic modulus
• I = Moment of inertia

Formula for Point Load at Center:

• Deflection = (P × L³) / (48 × E × I)
• P = Point load
• L = Span length
• E = Elastic modulus
• I = Moment of inertia

Process:

1. Identify load type
2. Select appropriate formula
3. Gather material properties
4. Calculate deflection
5. Compare to L/360 limit

Advantages:

• Accurate
• Precise
• Theoretical basis
• Applicable to any case
• Provides understanding

Disadvantages:

• Requires formulas
• Time-consuming
• Requires calculations
• Complex for irregular loading
• Requires engineering knowledge

2. Table Method

Process:

1. Identify beam type
2. Identify load type
3. Find table entry
4. Read deflection value
5. Compare to L/360 limit

Advantages:

• Quick
• Easy to use
• No calculations needed
• Readily available
• Reduces errors

Disadvantages:

• Limited to standard cases
• Requires interpolation
• Less accurate
• Limited flexibility
• Requires tables

Sources:

• Steel manual tables
• Concrete design guides
• Wood design guides
• Manufacturer data
• Design software

3. Computer Analysis

Process:

1. Create structural model
2. Define loads
3. Run analysis
4. Review results
5. Compare to L/360 limit

Advantages:

• Highly accurate
• Handles complex cases
• Quick analysis
• Detailed results
• Industry standard

Disadvantages:

• Requires software
• Requires training
• Requires validation
• Expensive
• Requires computer

Software:

• SAP2000
• ETABS
• RISA
• Specialized software
• Spreadsheet tools

4. Deflection Calculation Example

Given:

• Floor span: 20 feet
• Live load: 40 psf
• Material: Steel (E = 29,000 ksi)
• Section: W12×26 (I = 204 in⁴)

Step 1: Convert Units

• Span: 20 feet = 240 inches
• Load: 40 psf = 40/144 = 0.278 psi
• Beam width: Assume 12 feet = 144 inches
• Total load: 0.278 × 144 = 40 lbs/inch

Step 2: Calculate Deflection

• Deflection = (5 × 40 × 240⁴) / (384 × 29,000 × 204)
• Deflection = (5 × 40 × 331,776,000) / (2,286,336,000)
• Deflection = 66,355,200,000 / 2,286,336,000
• Deflection = 29 inches

Step 3: Check Against L/360

• L/360 = 240 / 360 = 0.67 inches
• Actual deflection: 29 inches
• Exceeds limit significantly
• Much larger section required

Step 4: Select Larger Section

• Try W18×40 (I = 612 in⁴)
• Deflection = (5 × 40 × 240⁴) / (384 × 29,000 × 612)
• Deflection = 66,355,200,000 / 6,859,008,000
• Deflection = 9.7 inches
• Still exceeds limit

Step 5: Continue Iteration

• Try W24×55 (I = 1,350 in⁴)
• Deflection = (5 × 40 × 240⁴) / (384 × 29,000 × 1,350)
• Deflection = 66,355,200,000 / 15,139,200,000
• Deflection = 4.4 inches
• Still exceeds limit

Step 6: Final Selection

• Try W30×99 (I = 3,310 in⁴)
• Deflection = (5 × 40 × 240⁴) / (384 × 29,000 × 3,310)
• Deflection = 66,355,200,000 / 37,041,600,000
• Deflection = 1.8 inches
• Still exceeds limit

Step 7: Acceptable Section

• Try W36×135 (I = 9,210 in⁴)
• Deflection = (5 × 40 × 240⁴) / (384 × 29,000 × 9,210)
• Deflection = 66,355,200,000 / 102,835,200,000
• Deflection = 0.65 inches
• Acceptable (less than 0.67 inches)

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L/360 vs. Other Deflection Limits

Comparison of Common Limits

L/480 (Most Stringent):

• Sensitive equipment
• Precision machinery
• Optical equipment
• Allows least deflection
• Requires largest sections
• Highest cost

L/360 (Stringent):

• Floors
• Sensitive applications
• Occupant comfort critical
• Moderate deflection allowed
• Moderate section size
• Moderate cost

L/240 (Moderate):

• Beams
• General applications
• Deflection less critical
• More deflection allowed
• Smaller sections possible
• Lower cost

L/180 (Relaxed):

• Industrial applications
• Deflection not critical
• Significant deflection allowed
• Smaller sections possible
• Lower cost

L/120 (Very Relaxed):

• Temporary structures
• Deflection not important
• Large deflection acceptable
• Minimal sections
• Lowest cost

When to Use L/360

Use L/360 for:

• Residential floors
• Commercial office floors
• Retail floors
• Sensitive applications
• Occupant comfort important
• Code requirement for floors

Use L/240 for:

• Beams
• Roof structures
• Industrial floors
• Deflection less critical
• Structural role primary

Use L/480 for:

• Sensitive equipment
• Precision machinery
• Optical equipment
• Very stringent requirements
• Specialized applications

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Factors Affecting Deflection

1. Span Length

Effect:

• Deflection increases with span⁴
• Doubling span increases deflection 16×
• Critical factor
• Dominant effect
• Quadratic relationship

Example:

• 10-foot span: Deflection = 0.04 inches
• 20-foot span: Deflection = 0.67 inches (16× increase)
• 30-foot span: Deflection = 3.4 inches (81× increase)

Design Implication:

• Longer spans require larger sections
• Span reduction reduces deflection significantly
• Support placement critical
• Intermediate supports beneficial
• Span optimization important

2. Load Magnitude

Effect:

• Deflection increases linearly with load
• Doubling load doubles deflection
• Direct relationship
• Proportional effect
• Linear relationship

Example:

• 20 psf load: Deflection = 0.33 inches
• 40 psf load: Deflection = 0.67 inches
• 60 psf load: Deflection = 1.0 inch

Design Implication:

• Load reduction reduces deflection
• Load distribution beneficial
• Multiple supports reduce load
• Load path optimization important
• Efficient design reduces deflection

3. Material Properties

Elastic Modulus (E):

• Deflection inversely proportional to E
• Higher E reduces deflection
• Steel: E = 29,000 ksi
• Concrete: E = 3,000-5,000 ksi
• Wood: E = 1,000-2,000 ksi

Example:

• Steel beam: Deflection = 0.67 inches
• Concrete beam: Deflection = 6.7 inches (10× increase)
• Wood beam: Deflection = 20 inches (30× increase)

Design Implication:

• Material selection affects deflection
• Steel most efficient
• Concrete moderate
• Wood least efficient
• Material choice critical

4. Section Properties

Moment of Inertia (I):

• Deflection inversely proportional to I
• Larger I reduces deflection
• Doubling I halves deflection
• Critical factor
• Section shape important

Example:

• W12×26 (I = 204 in⁴): Deflection = 0.67 inches
• W18×40 (I = 612 in⁴): Deflection = 0.22 inches
• W24×55 (I = 1,350 in⁴): Deflection = 0.10 inches

Design Implication:

• Larger sections reduce deflection
• Section optimization important
• Depth more important than width
• Moment of inertia critical
• Section selection affects cost

5. Support Conditions

Simple Support:

• Maximum deflection
• Baseline condition
• Most common
• Typical limit: L/360 for floors

Fixed Support:

• Reduced deflection
• 1/4 of simple support
• More expensive
• Typical limit: L/480 for floors

Continuous Span:

• Reduced deflection
• 1/2 of simple support
• More economical
• Typical limit: L/240 for floors

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Deflection Control Strategies

1. Increase Section Size

Method:

• Select larger structural section
• Increase moment of inertia
• Reduce deflection proportionally
• Most direct approach
• Common solution

Advantages:

• Simple approach
• Directly reduces deflection
• Proven method
• Easy to implement
• Straightforward design

Disadvantages:

• Increases cost
• Increases weight
• Increases material use
• May require larger supports
• Less economical

Example:

• W12×26 deflects 0.67 inches
• W18×40 deflects 0.22 inches
• W24×55 deflects 0.10 inches
• Larger section reduces deflection

2. Reduce Span Length

Method:

• Add intermediate supports
• Reduce effective span
• Reduce deflection significantly
• Quadratic effect
• Powerful approach

Advantages:

• Significantly reduces deflection
• Reduces section size needed
• More economical
• Reduces material use
• Reduces weight

Disadvantages:

• Requires additional supports
• Affects layout
• May not be feasible
• Increases complexity
• Requires coordination

Example:

• 20-foot span: Deflection = 0.67 inches
• 10-foot span: Deflection = 0.04 inches (16× reduction)
• 15-foot span: Deflection = 0.17 inches (4× reduction)

3. Use Higher Strength Material

Method:

• Select material with higher elastic modulus
• Steel vs. concrete vs. wood
• Reduces deflection proportionally
• Material substitution
• Effective approach

Advantages:

• Reduces section size needed
• More economical
• Reduces weight
• Reduces material use
• Proven method

Disadvantages:

• May increase cost
• Requires different design
• May affect appearance
• Requires different skills
• May not be feasible

Example:

• Wood beam: Deflection = 2.0 inches
• Concrete beam: Deflection = 0.2 inches
• Steel beam: Deflection = 0.067 inches

4. Optimize Section Shape

Method:

• Select section with higher moment of inertia
• Increase depth
• Optimize width
• Efficient design
• Cost-effective approach

Advantages:

• Reduces deflection
• More economical
• Reduces weight
• Reduces material use
• Optimized design

Disadvantages:

• Requires analysis
• May affect appearance
• May affect other aspects
• Requires engineering judgment
• More complex design

Example:

• Rectangular section: I = 100 in⁴
• I-section: I = 300 in⁴
• Box section: I = 400 in⁴
• Shape optimization reduces deflection

5. Use Composite Construction

Method:

• Combine materials
• Steel and concrete composite
• Optimize properties
• Efficient design
• Advanced approach

Advantages:

• Optimized properties
• Reduced deflection
• More economical
• Reduced weight
• Efficient design

Disadvantages:

• More complex design
• Requires specialized knowledge
• More expensive
• Requires coordination
• More complex construction

Example:

• Steel beam alone: Deflection = 0.67 inches
• Composite beam: Deflection = 0.22 inches
• Significant improvement

6. Use Camber

Method:

• Fabricate member with upward curve
• Compensates for deflection
• Appears level when loaded
• Aesthetic improvement
• Common practice

Advantages:

• Improves appearance
• Compensates for deflection
• Maintains level appearance
• Proven method
• Relatively economical

Disadvantages:

• Requires fabrication
• Increases cost
• Requires accurate prediction
• May not fully compensate
• Requires coordination

Example:

• Predicted deflection: 0.67 inches
• Camber applied: 0.67 inches upward
• Appears level when loaded
• Improves appearance

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L/360 in Different Applications

Residential Floors

Typical Specifications:

• Span: 15-25 feet
• Load: 40 psf (live)
• Material: Steel or wood
• L/360 limit: 0.5-0.83 inches
• Typical deflection: 0.4-0.7 inches

Design Considerations:

• Occupant comfort important
• Appearance critical
• Stringent limits required
• Moderate section size
• Reasonable cost

Common Sections:

• Steel: W12×26 to W18×40
• Wood: 2×12 to 2×16
• Concrete: 8-12 inches deep
• Varies by span and load

Commercial Office Floors

Typical Specifications:

• Span: 20-35 feet
• Load: 50 psf (live)
• Material: Steel or concrete
• L/360 limit: 0.67-1.17 inches
• Typical deflection: 0.6-1.0 inch

Design Considerations:

• Occupant comfort important
• Professional environment
• Stringent limits required
• Moderate to large sections
• Moderate cost

Common Sections:

• Steel: W16×40 to W24×68
• Concrete: 10-14 inches deep
• Composite: Steel with concrete
• Varies by span and load

Retail Floors

Typical Specifications:

• Span: 25-40 feet
• Load: 100 psf (live)
• Material: Steel or concrete
• L/360 limit: 0.83-1.33 inches
• Typical deflection: 0.7-1.2 inches

Design Considerations:

• Occupant comfort important
• Appearance critical
• Stringent limits required
• Large sections needed
• Higher cost

Common Sections:

• Steel: W18×55 to W30×99
• Concrete: 12-16 inches deep
• Composite: Steel with concrete
• Varies by span and load

Warehouse Floors

Typical Specifications:

• Span: 30-50 feet
• Load: 150-250 psf (live)
• Material: Steel or concrete
• L/360 limit: 1.0-1.67 inches
• Typical deflection: 0.9-1.5 inches

Design Considerations:

• Occupant comfort less important
• Functional role primary
• Stringent limits still required
• Large sections needed
• Higher cost

Common Sections:

• Steel: W24×84 to W36×150
• Concrete: 14-18 inches deep
• Composite: Steel with concrete
• Varies by span and load

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Deflection Verification Process

Step-by-Step Verification

Step 1: Determine Span

• Measure or estimate span
• Convert to consistent units
• Use clear span
• Design parameter

Step 2: Calculate L/360 Limit

• Formula: L/360 = Span / 360
• Divide span by 360
• Result is maximum deflection
• Serviceability limit

Step 3: Calculate Actual Deflection

• Use formula or table
• Calculate for live load
• Determine maximum
• Design parameter

Step 4: Compare

• Compare actual to limit
• Verify acceptability
• Document results
• Compliance verification

Step 5: Adjust if Needed

• If exceeds limit: Select larger section
• If significantly under: Consider smaller section
• Optimize for cost
• Final design

Documentation

Required Information:

• Span length
• Load magnitude
• Material properties
• Section properties
• Calculated deflection
• L/360 limit
• Verification statement
• Designer signature
• Date

Example Documentation:

• Floor beam: W18×40
• Span: 20 feet
• Live load: 40 psf
• Total load: 60 psf
• Deflection (live): 0.65 inches
• Deflection (total): 0.98 inches
• L/360 limit: 0.67 inches
• L/240 limit: 1.0 inch
• Status: Live load acceptable, total load exceeds limit
• Action: Select W21×44

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Common L/360 Design Issues

1. Exceeding L/360 Limit

Causes:

• Undersized section
• Long span
• High load
• Low material strength
• Inadequate support

Symptoms:

• Calculated deflection exceeds 0.67 inches (for 20-foot span)
• Design fails verification
• Section inadequate
• Larger section required

Solutions:

• Increase section size
• Reduce span
• Add intermediate supports
• Use higher strength material
• Improve support conditions

2. Differential Deflection

Definition:

• Different deflection at different locations
• Creates slopes and tilts
• Affects appearance
• Affects functionality
• Causes problems

Causes:

• Non-uniform loading
• Different section sizes
• Different support conditions
• Unequal spans
• Unequal loads

Solutions:

• Uniform section sizing
• Uniform loading
• Uniform support conditions
• Careful design
• Detailed analysis

3. Creep Deflection

Definition:

• Additional deflection over time
• Occurs in concrete and wood
• Increases with time
• Can be significant
• Long-term effect

Causes:

• Material properties
• Sustained loading
• Moisture changes
• Temperature changes
• Long-term behavior

Solutions:

• Account for creep in design
• Use creep factors
• Increase section size
• Use less creep-prone materials
• Detailed analysis

4. Vibration and Oscillation

Definition:

• Excessive movement from dynamic loads
• Causes discomfort
• Affects perception
• Can cause damage
• Serviceability issue

Causes:

• Low natural frequency
• Dynamic loads
• Resonance
• Inadequate damping
• Flexible structure

Solutions:

• Increase stiffness
• Increase mass
• Add damping
• Avoid resonance
• Detailed analysis

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Conclusion

L/360 is a critical deflection limit for floors ensuring structural serviceability and occupant comfort. Understanding L/360, calculation methods, and control strategies is essential for proper structural design.

Key Takeaways:

• L/360 means maximum deflection equals span divided by 360
• L/360 is more stringent than L/240
• L/360 applies to floors under live load
• L/240 applies to floors under total load
• Deflection increases with span⁴
• Deflection increases linearly with load
• Material properties significantly affect deflection
• Section size is critical to deflection control
• Multiple strategies available to control deflection
• Verification is essential in design
• Proper design ensures serviceability
• Building codes specify minimum requirements

Need help calculating deflection for your floor design? Consult with structural engineers to ensure proper analysis and design for your specific needs.

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Frequently Asked Questions

What does L/360 mean exactly?

L/360 means the maximum allowable deflection equals the span length divided by 360. For a 20-foot span, L/360 = 20/360 = 0.0556 feet = 0.67 inches.

Why is L/360 used for floors but L/240 for beams?

Floors are more sensitive to deflection because occupants notice movement. Beams are less sensitive. Stricter limit (L/360) for floors ensures better comfort and appearance.

How do I calculate deflection for L/360?

Use the formula: Deflection = (5 × w × L⁴) / (384 × E × I), where w is load per unit length, L is span, E is elastic modulus, and I is moment of inertia.

What if my floor exceeds L/360?

Select a larger section with higher moment of inertia, reduce the span with additional supports, use a higher strength material, or optimize the section shape.

Does camber eliminate deflection?

Camber compensates for deflection, making the floor appear level when loaded. It doesn’t eliminate deflection but improves appearance.

Why is L/360 important?

Excessive deflection causes visual problems, functional issues, comfort concerns, and potential structural damage. L/360 limits ensure serviceability.

Can I ignore L/360 if strength is adequate?

No. Strength and deflection are separate requirements. A floor can be strong enough but deflect excessively, causing serviceability problems.

What is the difference between L/360 and L/240?

L/360 is more stringent (allows less deflection). L/240 is less stringent (allows more deflection). L/360 is typically used for floors, L/240 for beams.