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AC 428

Acceptance Criteria for Modular Framing Systems Used to Support Photovoltaic (PV) ModulesSubject AC428-1010-R1 (YM/RK)

Todd Ganshaw – Unirac, Inc.

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Topics

• Unirac

• Outset situation –structural regulation - none specific to PV - needs clarity

• Proposed criteria – essential elements

• Staff comment and resolution

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Background

Unirac– Based in Albuquerque, NM

– Design and build custom and standard PV mounting solutions

– In business for over 11 years

– 20 degreed engineers, including 5 registered professional engineers

– Residential, Commercial and Utility segments

– Trailblazer in application of code to PV structures

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Flush Mount Modular Framing System

Unirac Modular Framing System

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PV Industry Young and Growing Rapidly

The North American market is driven by declining module prices, state renewable portfolio standards, federal incentives and FITs in Ontario

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Unirac’s Observations

Variation in design / test of PV modular framing systems:

– Analysis of system for wind only

– Assessment of system for wind as roof-top equipment & other methods

– Analysis of beam in strong axis only

– Neglect use of IBC load combinations

– Design systems based on rules of thumb (4 or 6 ft spans based on experience)

– No test of components in multiple dimensions & directions

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Unirac’s Observations

Basis for “engineering letter”• Mechanical Test intended for modules. (UL 1703)• Single test in each direction.• Hold for 30 minutes• Not loaded to failure• Factor of Safety of 1.5. – Some don’t include this factor of Safety.• Engineering letter stating “Support System” (Modular Framing System) will

support the Code applied loads.

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Unirac’s Observations

Proposed AC requires:

- System to be within roof envelope

- ESR to specify array limitations / location on roof or wall

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Unirac’s Observations

Proposed AC:

- Requires engineered system

- Does not permit system evaluation for roof overhang

- Requires ESR to state properly flashed penetrations.

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Unirac’s Observations

Proposed AC:

• Roof overhang outside scope of AC

• Requires ESR to state that roof penetrations must be properly flashed

• Requires ESR to state access for fire personnel to be established by local jurisdiction

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Why the Need for an AC 428

Design loads not specifically defined within code for PV systems

• Live• Wind • Snow• Seismic

Testing is needed to define capacity of connections

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Scope of AC 428

What’s in AC 428:– Requirements based on 2006 & 2009 IBC & IRC– Flush mounted systems for roofs & walls & ground mounted

systems – Clarification of design load requirements– Definition of mechanical test requirements

What’s not in AC 428:– Tilted roof mount systems (systems at an angle to roof)– Attachments between system and roof

» ESR’s 1999, 2835

– Foundations for ground mounted systems» Requires site specific engineering

– Electrical safety / electrical grounding– PV modules / strength of PV modules

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Consultants

Dr Tim Reinhold, Institute for Business & Home Safety,– ASCE Subcommittee on Wind Loads– NBSIR 81-2199 Wind Loads on Flat-Plate Solar

Collectors

John Silva, Hilti Corporation– Chair, ASCE/SEI 7 Seismic Task Committee on

Nonstructural Components– Consultation on ASCE/SEI 7 Chapters 13 and 15

Supports AC

TJC and Associates, Inc.– Structural Engineering

Others

In conclusion, I believe that the acceptance criteria is credible with respect to wind loads and makes appropriate use of the latest ASCE 7 wind load provisions. Dr Tim Reinhold

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Dr Tim Reinhold Letter on use of modules less than 4 x 8 feet

While the ICC ES correctly notes that the Virginia Tech tests used a basic panel size of 4-ft by 8-ft with an area of 32 sq. ft., the development of the criteria in the NIST report basically related the Virginia Tech data back to pressure coefficients in ASCE 7 for roof zones. As such, the roof-zone pressure coefficients in ASCE 7 do provide a sound basis for calculating loads on different sizes of panels or even arrays of panels since the coefficients change depending on the effective wind area. These curves have been in use for quite some time. I have checked them in the course of several projects where we had opportunities to install enough pressure taps to allow simultaneous combination of the pressures and map out the reduction in peak pressures as the area increased. My experience is that these area reduction factors which vary in magnitude depending on the zone are reasonable and conservative. Consequently, it is my opinion that ASCE 7 contains all of the information and procedures necessary to calculate design loads on various panel sizes and arrays of panels.

I hope this helps.

Regards,

Timothy A. Reinhold, Ph.D., PE (Colorado)Senior Vice President for Research, Chief EngineerInstitute for Business & Home Safety4775 E. Fowler AvenueTampa, FL 33617

tel: 813-675-1042fax: 813-286-9960switchboard: 813-286-3400

“As such, the roof-zone pressure coefficients in ASCE 7 do provide a sound basis for calculating loads on different sizes of panels…”

“Consequently, it is my opinion that ASCE 7 contains all of the information and procedures necessary to calculate design loads on various panel sizes …”

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Increase in Design Wind Pressure for Reduced Effective Wind Areas

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Conclusion

• All Staff comments have been resolved

• Recommend adoption of criteria as published

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Clarify Design Loads – Dead, Snow, Seismic

– Dead Load • Well defined

– Snow Loads• Used requirements in code for roofs

– Specified systems to be treated as unheated roof

– Seismic• Roof Mounted

– ASCE 7 chapter 13 Nonstructural Components» Rp=1.5 and ap=1.0 in compliance with “Other mechanical or

electrical components » elastic

• Ground Mounted – ASCE 7 chapter 15

» Seismic Design Requirements for Non-Building Structures with Rp=1.25 Ω=2 and Cd=2.5

» elastic

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Clarify Design Loads - Wind

• Wind Loads– Method 2 (Analytical Procedure for Low-Rise Buildings and Building for Heights Less

Than 60 feet) – chapter 6 of ASCE 7

– All elements designed for Component and Cladding (C&C) pressures defined within Chapter 6 of ASCE 7

– System height limited to 10 inches off roof

– System not permitted within 10 inches of the edge of a roof.

– Research NBSIR 81-2199 and letter from Dr Tim Reinhold supports use of ASCE 7 C&C methodology

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Clarify Design Loads - Live

• Live Load– Agreeable to the proposal put forth by ES– Code does not have a position on live loads on modules– International Fire Code (IFC 2012):

• Access, and pathways Roof access, pathways, and spacing requirements shall be provided in order to ensure access to the roof; provide pathways to specific areas of the roof; provide for smoke ventilation operations; and to provide emergency egress from the roof.

• Smoke Ventilation. Panels/modules shall be located no higher than 3 feet (914 mm) below the ridge in order to allow for fire department smoke ventilation operations.

• Residential buildings with hip roof layouts. Panels /modules shall be located in a manner that provides a 3 foot (914 mm) wide clear access pathway from the eave to the ridge on each roof slope where panels/modules are located. The access pathway shall be located at a structurally strong location on the building capable of supporting the live load of fire fighters accessing the roof.

• Marking content. The marking shall contain the words “WARNING: PHOTOVOLTAIC POWER SOURCE”

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Clarify Design Loads – Friction Clips

• Friction Clips– ES Staff position per Memo to Committee, item 4:

• According to the Commentary to Section 13.4.6 of ASCE/SEI 7-10, friction clips should not be used to resist sustained or gravity loads. This language seems to not be entirely consistent with Section 13.4.6 of ASCE 7-10, which states that friction clips shall not be used to support sustained loads in addition to resisting seismic forces for Seismic Design Categories D, E, and F. ASCE 7-10 implies that friction clips can be used to resist sustained loads in Seismic Design Categories A, B, and C. Since the criteria is based on the 2009 IBC, which references ASCE 7-05, not ASCE 7-10, it is staff position that at this time, the criteria needs to not be applicable to the use of friction clips to resist sustained or gravity loads in any seismic design category. …

– Unirac Position• ASCE 7-05 Section 13.1.4 (nonstructural components exempt from reqt’s):

– 13.1.4, item 2: Mechanical and electrical components in Seismic Design Category (SDC) B

– 13.1.4, item 3: Mechanical and electrical components in SDC C provided that the component importance factor, Ip, is equal to 1.0.

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AC 428 and Memo to Committee

• Unirac is agreeable to criteria as modified by ES Staff and presented to committee with the following exceptions related to Staff Memo to the Committee:

• ES Memo to Committee– Memo items:

1. Acceptable to Unirac2. Live Load

1. No code defined for PV2. Access3. Marking

3. Acceptable to Unirac4. Friction Clips

1. 7-05 Exemptions5. Acceptable to Unirac6. Acceptable to Unirac7. Acceptable to Unirac8. Acceptable to Unirac9. Acceptable to Unirac

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Why the Need for an ESR

• Why an ESR for Modular framing systems?

– To provide what code officials are asking for• Primarily an industry dominated by electrical contractors / governed by

electrical officials• Helps code officials deal with structural concerns

– Will require a number of basic and important requirements

– Ease of permitting, inspection

– Provide a recognized tool for designers

– Easier to conduct business with PV designers, distributors, installers & code officials