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Standardized Load Tables Characterizing
Residential Solar Thermal and Solar Electric
Installations For Residential Structures in
Minnesota
Jason S. Hanlon, PE, MLSE
Braun Intertec Corporation
September 30, 2013
Overview
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

Establishing Analysis/Review Criteria
Research and Analysis

Pre-Manufactured Trusses

Roof Rafters
Rafter Tables
Establishing Analysis/Review Criteria



Closely match IRC content
Consider conditions common to MN
Consider Types of Roof Mounted Solar
Technologies
Establishing Analysis/Review Criteria

Closely match IRC content
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Spans presented try to closely match
that shown in IRC Tables
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Wood Species

Roof Pitches
Establishing Analysis/Review Criteria


Closely match IRC content
Consider conditions common to MN

Wood Species --- Old Doug-Fir
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Design Loading Criteria
»
»

Based on 2007 MSBC, 2006
IRC/IBC and ASCE 7-05
Wind Load Exposure B, C, and D
Dead Weight
»
»
One Layer ½-inch OSB/Plywood or
¾-inch plank board
One layer asphalt shingles
Establishing Analysis/Review Criteria



Closely match IRC content
Consider conditions common to MN
Consider Types of Roof Mounted Solar
Technologies

Photovoltaic Panels supplementing
electrical services

Solar Thermal Panels supplementing
heating services
Research and Analysis
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
Pre-Manufactured Wood Trusses
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Challenges

Things to consider
Roof Rafters
Research and Analysis – Pre-Manufactured
Wood Trusses

Challenges

Common design layout hard to define

Wood trusses are “efficiently”
designed.
»
Controlling component will be the
nailer plates
Research and Analysis – Pre-Manufactured
Wood Trusses

What to consider

Common design throughout the
industry:
»
»

Top Chord DL = 7 psf
Bottom Chord DL = 10 psf
Changes in Code prescribed Snow
Load
Research and Analysis – Pre-Manufactured
Wood Trusses
1998-Previous MSBC
2003-Current MSBC
Research and Analysis – Pre-Manufactured
Wood Trusses
- Roof snow load decreased by 5 psf
- Roof snow load increased by 5 psf
- Roof snow load increased by 12 psf
- Roof snow load increased by 2 psf
Summary of Design Load Adjustments
Research and Analysis – Roof Rafters
(Hand-Framed Construction)
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Framing Conditions
Snow Loading
Wind Loading
Material Selfweight
Allowable Stress Design
Research and Analysis – Roof Rafters
(Hand-Framed Construction)

Framing Conditions

Use of Ridge Nailer Plate (not a ridge
beam)

Outward thrust is resolved at exterior
wall bearing
Research and Analysis – Roof Rafters
(Hand-Framed Construction)
Research and Analysis – Roof Rafters
(Hand-Framed Construction)

Framing Conditions

Use of Ridge Nailer Plate (not a ridge
beam)

Outward thrust is resolved at exterior
wall bearing

Allowable stress values taken from
NDS (National Design Standard) for
Wood Construction and Historical
Douglas Fir Use Book
Research and Analysis – Roof Rafters
(Hand-Framed Construction)


Framing Conditions
Snow Loading

Exposure Rating B, C and D

Design for Snow Drifting not required
per IRC

Limit installation angle of panel so
drifting is limited to design depth of
snow
»
Tables presented for different
lengths between PV and Solar
Panels
Research and Analysis – Roof Rafters
(Hand-Framed Construction)
Table 1 - Maximum Allowable PV Panel Installation Angle
Ground Snow Load = 50 psf, Flat Roof Snow Load = 35 psf
Maximum Allowable PV Panel Installation Angle (deg)
Roof Pitch
PV Panel
Length
3:12
4:12
5:12
7:12
9:12
12:12
(in)
36
37.0
40.9
44.4
38
35.8
39.7
43.2
40
34.6
38.5
42.2
42
33.6
37.6
41.2
44
32.7
36.7
40.3
46
31.8
35.8
39.5
48
31.1
35.1
38.8
50
30.4
34.4
38.1
44.8
52
29.7
33.8
37.5
44.2
54
29.1
33.2
37.0
43.7
56
28.6
32.7
36.4
43.2
58
28.1
32.2
36.0
42.7
60
27.6
31.7
35.5
42.3
Note: dash indicates that the solar panel may be installed at any angle up to 45
degrees from horizontal
Research and Analysis – Roof Rafters
(Hand-Framed Construction)



Framing Conditions
Snow Loading
Wind Loading

Structures 30-ft. or less in height
(average)

Not located near isolated hills or
ridges or other abrupt changes in
topography

Followed ASCE 7-05. Slight changes
in ASCE 7-10 (decrease)
Research and Analysis – Roof Rafters
(Hand-Framed Construction)



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Framing Conditions
Snow Loading
Wind Loading
Material Selfweight

Member DL based on wood species,
size, and spacing
Research and Analysis – Roof Rafters
(Hand-Framed Construction)





Framing Conditions
Snow Loading
Wind Loading
Material Selfweight
Allowable Stress Design

Shear

Bending/Compression

Deflection
Rafter Tables

Info needed to determine which table to
use

Wood Species
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Grade of Wood
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Spacing
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Ground Snow Design Requirement
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Exposure Classification
Rafter Tables

Info needed to determine which table to
use
Rafter Tables

Info needed to use the table
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Rafter Pitch
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Rafter Size
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Projected Rafter Span
Rafter Tables
Rafter Tables
Rafter Tables
Rafter Tables

Reinforced Rafter Tables
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Similar Analysis approach
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Uses a sistered 2x4 member of equal
or greater species and grade
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Installed as follows:
Rafter Tables
Questions???
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