Confidential
North Star Group, Inc.
19901 Quail Circle
Fairhope AL 36532
701-770-9118
michaelh@nsgia.com
Joist Report
Executive Summary
Confidential
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This revised analysis examines three SIP panel configurations:
1. Single 2x8 dimensional lumber joists (original design)
2. Double 2x8 dimensional lumber joists (new design with side-by-side joists)
3. Open-web steel joists (with corrected calculations)
All calculations use standard beam theory formulas and account for composite action between
the joists and OSB facings. The results show that all configurations meet deflection requirements,
with the double joist and steel joist designs offering substantial performance improvements.
Analysis Methodology
The analysis was performed using:
● Transformed section method for composite properties
● Standard beam deflection equations from engineering mechanics
● Uniform load of 50 psf across a 15-foot span
● L/360 deflection criteria per building codes
Test Specimens
Configuration 1: Single Dimensional Lumber Joists
● Joist dimensions: 2x8 (actual 1.5" × 7.25")
● Panel width: 24 inches
● Total depth: 8.75 inches
● Composite moment of inertia: 218.22 in⁴
Configuration 2: Double Dimensional Lumber Joists
● Joist dimensions: Two 2x8s side-by-side (actual 3.0" × 7.25")
● Panel width: 24 inches
● Total depth: 8.75 inches
© North Star Group, Inc.
19901 Quail Circle
Fairhope AL 36532
701-770-9118
michaelh@nsgia.com
1
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● Composite moment of inertia: 384.11 in⁴
Configuration 3: Open-Web Steel Joists
● Joist depth: 6.00 inches
● Panel width: 24 inches
● Total depth: 7.50 inches
● Composite moment of inertia: 451.75 in⁴
Results
Parameter
Single Joist
Double Joist
Steel Joist
Composite I (in⁴)
218.22
384.11
451.75
Theoretical Shear (lbs)
375.0
375.0
375.0
Theoretical Moment (lb-in)
16,875.0
16,875.0
16,875.0
Calculated Deflection (in)
0.201
0.114
0.097*
L/360 Limit (in)
0.500
0.500
0.500
Deflection Utilization (%)
40.2%
22.8%
19.4%*
Deflection Improvement Analysis
© North Star Group, Inc.
19901 Quail Circle
Fairhope AL 36532
701-770-9118
michaelh@nsgia.com
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The double joist configuration provides a 43.2% reduction in deflection compared to the single
joist design. The steel joist design (with corrected calculations) offers a 51.7% reduction in
deflection compared to the single joist design.
Stress Analysis
For the double joist configuration:
● Maximum bending stress in joists: 642.1 psi
● Maximum shear stress in joists: 25.9 psi
These stress levels are well below typical allowable values for structural lumber.
Conclusions
1. All three configurations easily meet L/360 deflection criteria.
2. The double joist configuration offers a substantial improvement (43.2%) over the single
joist design, with:
○ Reasonable material increase (double the lumber)
○ Simple manufacturing process
○ Predictable performance characteristics
○ Better redundancy and resilience
3. The steel joist configuration has theoretical advantages but requires verification:
○ Potentially higher cost and more complex manufacturing
○ Slightly better performance than double wood joists (when correctly calculated)
○ Potential concerns with thermal bridging
○ Different connection details required
Manufacturing Considerations
The double joist configuration maintains the simplicity of the original design while significantly
improving performance. Key manufacturing considerations include:
© North Star Group, Inc.
19901 Quail Circle
Fairhope AL 36532
701-770-9118
michaelh@nsgia.com
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1. Precision cutting of foam channels to accommodate the wider joists
2. Additional mechanical fasteners to ensure proper connection between joists and OSB
facings
3. Potential for reduced joist spacing due to improved load capacity
Limitations
This analysis has several limitations:
1. Calculations are based on simplified beam theory
2. Connection details are not fully addressed
3. Long-term effects (creep, thermal movement) are not considered
4. Actual performance should be verified through physical testing
Recommendations
1. Consider adopting the double joist configuration for improved performance
2. Verify calculations through independent structural engineering review
3. Develop and test physical prototypes to validate computational results
4. Evaluate manufacturing process efficiency with the wider joists
© North Star Group, Inc.
19901 Quail Circle
Fairhope AL 36532
701-770-9118
michaelh@nsgia.com
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