HomeMy WebLinkAboutENGINEER LETTER - 18-00641 - 1011 Westwood Dr - Solar Array
55 WEST 500 SOUTH, HEBER CITY, UT 84032
T 925.787.3067 U RIGHTANGLEENG.COM
November 2, 2018
CR Solar
380 King Street
Layton, UT 84041
RE: Structural Roof Evaluation for the Bowman Residence: 1011 Westwood Drive,
Rexburg, Idaho
As per your request, we have evaluated the roof structure under the proposed solar panel array. The
information used to evaluate this structure was gathered during a field visit by CR Solar on behalf of
Right Angle Engineering. The roof structure consists of 2x4 rafters spaced at 24” on center. The roof
material consists of asphalt shingles. The design criteria used to analyze this structure are included with
this letter. The adopted building codes in this jurisdiction are: the 2015 International Building Code, the
2015 International Existing Building Code, and ASCE 7-10.
International Existing Building Code (IEBC) 2015 section 807.4 indicates that alterations to an existing
building that results in less than a 5% increase in the total stress may be performed without a structural
evaluation of the existing building. As demonstrated in the attached calculations, the additional weight
of the solar panels will be less than 5% increase in the gravity loading and the stress on the existing roof
framing.
Based on our assessment we have determined that the existing roof framing will safely and adequately
support the additional loads imposed by the solar panels. In order for the loads to be evenly distributed,
the roof attachments should be staggered and spread evenly throughout the panel array. Attachment
points vary from 24”-74”. The racking system should be installed per the manufacture’s specifications.
There should be a minimum of 30 attachment points to the roof. Each attachment should have a 5/16”
or 18/8 SS lag screw with 2.5” minimum penetration centered on each truss top chord. Waterproofing
around the roof penetrations is the responsibility of others. Right Angle Engineering assumes no
responsibility for improper installation of the solar panels.
Regards,
Robert D Smythe, P.E.
Right Angle Engineering
11/2/18
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Design Criteria: Design Wind Speed (3 second gust) 120 mph Exposure Category C Risk Category 2
Mean Roof Height 30 ft
Roof Type Gable Roof
Building Type enclosed
Roof Slope 22 degrees
Number of panels 15
Panel Area 262.5 ft^2
Wind Calculations- ASCE 7-10
GCp Zone 1 -0.9 Figure 30.4-(2A-5B)
GCp Zone 2 -1.7 Figure 30.4-(2A-5B)
GCp Zone 3 -2.6 Figure 30.4-(2A-5B)
Gcpi 0.18 Table 26.11-1
Velocity Pressure (qh) 30.7 psf
qh= .00256KhKhtKdV^2 Equation 30.3-1
Kh 0.98 Table 30.3-1
Kht 1 Equation26.8-1
Kd 0.85 Table 26.6-1
Designed wind pressure (P) psf Equation 30.9-1
P= qh(GCh) - (GChi))
Zone 1 Pressure (P) -33.2 psf
Zone 2 Pressure (P) -57.7 psf
Zone 3 Pressure (P) -85.4 psf
Lag Screw Connection 5/16" Lag Screw Shear Capacity 190 lbs NDS 2015 Table 12K
Shear max tributary area 16.5 ft^2
5/16" lag screw pullout capacity 266 lbs/in
NDS 2015 Table
12.2A
Lag screw embedment 2.5 in Total pullout capacity 665 lbs Pullout max tributary area 11.5 ft^2
Factor of Saftey 1.42
Minimum number of connectins 30
3
Roof Dead Load- ASCE Table C3-1 asphalt shingles 2 psf 7/16" Plywood Sheathing 1.5 psf Roof Framing 4 psf Insulation 3.85 psf Gypsum sheathing 2 psf Solar Panel Array 3 psf Dead Load Without Panels 13.35 psf Dead Load With Solar panels 16.35 psf
Roof Live Load Exisiting Roof Live Load 20 psf ASCE 7-10 Table 4-1
Roof Live Load with Solar Panels 0 psf 2015 IBC 1607.12.5
Roof Snow Load-ASCE 7-10 Ground Snow Load (pg) 50 psf Section 7.2 Exposure Factor (Ce) 0.9 Table 7-2 Thermal Factor (Ct) 1.1 Table 7-3 Importance Factor (Is) 1 Table 1.5-2 Flat Roof Snow Load (Pf) 35 Equation 7.3-1 Slippery surface Slope Factor (Cs) 0.8 Figure 7-2 Nonslippery Surface Slope Factor (Cs) 1 Figure 7-2 Snow Load Without Solar Panels 35 psf Equation 7.4-1 Snow Load With Solar Panels 28 psf Equation 7.4-2
Load Combinations - ASCE 7-10 Section 2.4-1
Without Solar
Panels
With Solar
panels D + Lr 33.4 psf 16.4 psf D + S 48 psf 44.1 psf
Beam Stress IEBC 2015 Section 807.4 Beam Span 13 ft Spacing 2 ft Roof Framing type 2x4 rafters Number of Panels per rafter 2
Without Solar
Panels
With Solar
Panels Percent Increase Bending Moment 2028 ft-lbs 1572 ft-lbs 77.5% Less than 105%
Vertical Reaction (V1) 624 lbs 556.1 lbs 89.1% Less than 105%
Vertical Reaction (V2) 624 lbs 328.6 lbs 52.7% Less than 105%