SAMPLE SNOW DESIGN ANALYSIS FOR DETERMINING THE CAUSE OF ROOF FAILURE IN BUFFALO, NEW YORK

SNOW DESIGN ANALYSIS FOR DETERMINING THE CAUSE OF ROOF FAILURE IN BUFFALO, NEW YORK

This blog and an associated blog posted in this website provides information regarding the application of the Residential Code of New York State (RCNYS) as it pertains to design snow loads. It discusses two acceptable methods for determining roof design loads while emphasizing the need to consider unbalanced snow loads in engineered design.

List of Abbreviations

BCNYS; Building Code of New York State

CEO; Code Enforcement Official

GSL; Ground Snow Load

PSF; Pounds Per Square Foot

RCNYS; Residential Code of New York State

The determination of design snow loads under the Residential Code of New York State (RCNYS) is often confused with the requirements of the Building Code of New York State (BCNYS). The RCNYS uses a prescriptive approach to meet the design requirements for snow loads whereas the BCNYS uses an analytical approach which includes the use of an equation and considers various roof configurations and design conditions such as exposure, thermal and importance factors, warm roofs vs. cold roofs, partial loading, unbalanced loads and drifting and sliding snow. That which is often misunderstood in the RCNYS is the need to include unbalanced snow loads for common hip and gable roofs.

An adequately designed roof structure should be capable of handling a snow load of 30 psf. In western New York (where Buffalo is located), the design code requires a minimum of 50 pounds per square foot (psf) snow load. While the weight of snow varies and changes with temperature it has been determined through various studies that heavy wet snow can weigh from 12 pounds per cubic foot to 20 pounds per cubic foot. This would suggest that a properly designed roof should be capable of handling a snow pack of 50” (4.2’) of light snow (12 pounds/cubic foot) and 30” (2.5’) of heavy snow (at 20 pounds/cubic foot being the weight of the heavy snow). Assuming that the entire snow pack from all the Lake-effect snowstorms remained on the roof (no snow melt) there would have been 50” or less of snow accumulation. This analysis does not include drifting snow (i.e., an unbalanced snow load, as per the NYS regulations), that could create even greater snow accumulations.

However due to the fluctuating water content of snow the actual weight is computed on the water equivalent of the snow. Based on climatological data these storms produced an equivalent of approximately 4.0” of liquid precipitation in the area where this failed structure was located which would equate to an approximate total weight of less than 20.8 pounds per square foot (water weighs approximately 62 pounds per cubic foot or approximately 5.17 pounds per square foot per inch of water height), which is well under the design capacity of a structurally sound roof and roof framing and therefore the weight of snow was not the root cause of the failure. Inspection of the structural members showed that the downward displacement/deflection of the ridge beam of the right roof is the result of a long term progressive condition also referred to as “creep deformation” that also resulted in the outward rotation and movement of the laterally unsupported right wall.

Of course the above analysis is quite different where these storms produced more than an equivalent of 4.0” of liquid precipitation. There are areas shown in the NOAA map above where the accumulation exceeded 6.0" of liquid precipitation in the area where this failed structure was located. This would create a load of greater than 50 psf, thus causing severe stress on the structure and perhaps failure, either partial or complete failure. With the upcoming additional snow storm and rainstorm, these roofs are at significant risk of failure.

After a loss such as a roof collapse, it is in the best interests of all parties involved to formulate and develop a plan to consider reviewing any changes, additions or conditions to building(s) that could affect the structural stability of the building’s roof system. A structural engineer can provide this valuable information and more. An additional item to consider in the prevention of a possible roof collapse is an immediate and thorough inspection of the entire roofing system before and after an expected heavy snow and ice event. These inspections are recommended to visually check and compare all structural framing members and connections, both inside and outside the building. A structural engineer should be called in to perform these inspections and help observe and pinpoint any damage or problems associated with or causing excessive deflection, extreme cracking or other structural failures. The inspection should also check the roof drains and gutters for debris clogging material where ponded water and/or ice can form and lead to a roof collapse. Also inspect the building walls for verticality and soundness, the windows and doors for proper opening and closing operations and the foundations for heaving and cracking due to frost penetration and settlement from heavy snow and ice loading. After a heavy snow or ice accumulation event and prior to any inspections, develop a plan to safely remove the snow and ice accumulations so that the situation does not further become unsafe.

There are several questions an adjuster should ask when investigating a roof collapse:

• What were the specific circumstances (weather event) leading to the collapsed roof?
• What loading criteria and code year were used to design the roof system?
• Did the roof system as designed meet the required loading criteria?
• Were there any new building additions constructed with varying roof slopes and elevations?
• Were there signs of drifting snow areas which increase the loading to the roof structure?
• Was a new roof system, a layered reroofing system, or added weight from roof-mounted signs or HVAC equipment installed from what was originally provided?
• What, if any, improvements may be needed to prevent further or possible future roof collapse?

All of these questions lead to load bearing analyses that can change the roof loading design where allowances that were once anticipated for snow loads may now be compromised. In addition, particular attention should also be given to older structures where past design criteria may have been revised or updated based on code changes or the accumulated local historical data. Special attention should be focused on drifting snow conditions that are extremely dangerous and are most likely the cause attributed to roof collapses. Based on the answers to these questions, an adjuster may now want to consider engaging in the services of a structural engineer to determine the cause or causes of a loss and the feasibility and cost of repair.

Removal of Sand, Silt and Gravel from Cayuga Creek

BUFFALO, N.Y. (WIVB) – More than $1 million will flow into the villages of Depew and Lancaster as part of an effort to reduce the flood threat from Cayuga Creek.

Crews were back at work Monday on a the Lancaster Federal Flood Control Project, which will remove silt, sand and gravel from 1.5 miles of the creek to better protect the nearby villages to future storms.

The $1.7 million project is funded by NY Works. It will also realign and widen the channel, create earth levees and enhancements, a floodwall, bank protection, two pump stations and multiple drainage structures. It’s aim is to protect the nearly 140 properties along that stretch of Cayuga Creek.

NY Works began in July and is removing 60,000-cubic yards of accumulated shoal material from the engineered channel of the creek from Lake Avenue in Lancaster to Penora Street in Depew. Removing the shoal deposits should restore the creek’s capacity to contain flood waters.

Around half of the shoal deposits are contaminated with Japanese Knotweed, an invasive species that can form large thickets. Though the shoal removal project should wrap up by the end of October, it will take longer to dispose of the contaminated shoal in accordance with proper procedures.

Flood Hydrographs at Creeks and Streams in the Snow-Impacted Area of Buffalo, NY

CAYUGA CREEK AT LANCASTER

CAZENOVIA CREEK AT EBENEZER

BUFFALO CREEK AT GARDENVILLE

TONAWANDA CREEK AT BATAVIA

ELLICOTT CREEK BELOW WILLIAMSVILLE

APPLICATION OF THE RESIDENTIAL CODE OF NEW YORK STATE (RCNYS) AS IT PERTAINS TO DESIGN SNOW LOADS.

This blog provides information regarding the application of the Residential Code of New York State (RCNYS) as it pertains to design snow loads. It discusses two acceptable methods for determining roof design loads while emphasizing the need to consider unbalanced snow loads in engineered design.

List of Abbreviations

BCNYS; Building Code of New York State

CEO; Code Enforcement Official

GSL; Ground Snow Load

PSF; Pounds Per Square Foot

RCNYS; Residential Code of New York State

The determination of design snow loads under the Residential Code of New York State (RCNYS) is often confused with the requirements of the Building Code of New York State (BCNYS). The RCNYS uses a prescriptive approach to meet the design requirements for snow loads whereas the BCNYS uses an analytical approach which includes the use of an equation and considers various roof configurations and design conditions such as exposure, thermal and importance factors, warm roofs vs. cold roofs, partial loading, unbalanced loads and drifting and sliding snow. That which is often misunderstood in the RCNYS is the need to include unbalanced snow loads for common hip and gable roofs.

Snow Loads by Prescriptive Design

The RCNYS requires ground snow loads to be determined from Figure R301.2(5), based on the geographical location of a building. The Ground Snow Load (GSL) and other climatic and geographic design criteria, must be established by the local jurisdiction as set forth in Table R301.2(1). Section R301.5 requires roofs to be designed for the minimum snow load indicated in Table R301.2(1). Since the snow load in Table R301.2(1) is the GSL, it follows that the roof must be designed for the GSL without any adjustments when using the prescriptive method and not a value determined from an engineered approach. One exception is found in section R301.2.3 which requires buildings in regions with GSLs greater than 70 psf to be designed in accordance with accepted engineering practice and ASCE 7-98. (“R301.2.3 Snow loads. Wood framed construction, cold- formed steel framed construction and masonry and concrete construction in regions with ground snow loads 70 pounds per square foot (3.35 kPa) or less, shall be in accordance with Chapters 5, 6 and 8. Buildings in regions with ground snow loads greater than 70 pounds per square foot (3.35 kPa) shall be designed in accordance with accepted engineering practice.”)

Therefore, under the RCNYS a house located in an area where the GSL is 55 pounds per square foot (psf), must have a roof designed as fully loaded for a minimum snow load of 55 psf. In the alternative, section R301.1.2 allows for an engineered design for structural elements not conforming to the RCNYS. Therefore, a roof structure may be designed for either the full GSL by the use of the RCNYS prescriptive provisions or engineered in accordance with the BCNYS.

Snow Loads by Engineered Design

Snow loads for roofs engineered in accordance with the BCNYS is provided for in section 1608. Section 1608.1 of the BCNYS requires design snow loads to be determined in accordance with section 7 of ASCE 7-98, entitled,“Minimum Design Loads for Buildings and Other Structures”. This design option allows for adjustments to the GSL but is more complicated than the simplified prescriptive approach offered in the RCNYS. Section 1608 as well as ASCE 7 requires other factors to be applied in the design of a roof. Those most often associated with residential construction include the exposure factor (Ce), thermal factor (Ct), and importance factor (I). These values are used to determine the flat roof snow load (pf) in equation 7-1 as follows:

pf = 0.7 Ce CtIpg

where pg = is the ground snow load determined from BCNYS Figure 1608.2 or RCNYS Figure R301.2(5).

The values for Ce, Ct, and I are obtained from Tables 1608.3.1, 1608.3.2, and 1604.5 respectively. For residential construction, these values are typically 1.0. Therefore, in most cases the flat roof snow load is the product obtained by multiplying the GSL by 0.7 or 70% of the ground snow load. As an example, the flat roof snow load for a roof located in a 55 psf snow zone (such as in many buildings in the Buffalo area) is typically 70 % of 55 or 38.5 psf. In some cases this load may be reduced for a sloped roof to account for sliding snow and improved drainage of meltwater. However, for roofs having a non-slippery surface such as conventional asphalt shingles, live load reduction for roof slope is not introduced until the slope exceeds a 7 on 12 pitch pursuant to Figure 7-2 of ASCE 7. Therefore, the sloped-roof snow load (ps) would most often be equal to the flat-roof snow load (pf).

In most cases for residential buildings, the design snow load is substantially less than the prescriptive GSL determined from RCNYS Table R301.2(1). However, the engineered roof design is subject to other snow loading conditions identified in ASCE 7-98. One such condition that is most often overlooked and has a substantial impact on a roof design is accounting for unbalanced snow loads.

Unbalanced Snow Loads

Unbalanced roof snow loading occurs as a result of wind. Winds carry snow from the windward side to the leeward side. Section 7.6.1 of ASCE 7-98 requires a roof with an eave to ridge distance of 20 feet or less to be designed to resist an unbalanced uniform snow load on the leeward side equal to 1.5 ps/Ce. Since the exposure factor Ce is typically 1.0, the leeward side of a sloped roof in most cases must be designed for a uniform load of 1.5ps or 50 % more than the load determined from equation 7-1. Since it is not possible to determine wind direction, each side of the roof should be considered. It should be noted that the windward side is considered not to be covered with snow.

This is illustrated in Figure 7-5 of ASCE 7-98 and in the diagrams below:

Therefore, the roof of a house located in an area where the GSL is 55 psf and the flat or sloped-roof snow load is 38.5 psf, would have to be designed for both balanced and unbalanced conditions with a load of 57.8 psf [1.5x38.5] applied on the leeward side of the roof.

Inspection Recommendations

Rafters can be checked using the rafter span tables R802.5.1(1) through R802.5.1(8). These tables only list live loads of 20, 50, and 70 psf whereas actual GSLs for New York State include 45, 50, 55, 65, 70 and 85 psf. Where a GSL does not equate to a live load given in the tables, it is necessary to use a table with the next highest load. This may result in a conservative roof design. Rafters for other design loads, spacings, species and grades, and spans not found in the tables, may be designed as fully loaded with the GSL or designed using equation 7-1 and all loading conditions prescribed in ASCE 7-98. Construction drawings should always identify the GSL for the roof. If the design is based on an engineering analysis, the drawings should also reference compliance with the BCNYS and ASCE 7-98 and include the flat-roof snow load (pf), snow exposure factor (Ce), snow load importance factor (I), and the thermal factor (Ct). The unbalanced snow load should also be identified to ensure that it has been considered in the design.

For wood trusses, section R802.10.1 requires design drawings to be provided to the code enforcement official. Such drawings should be stamped and sealed by a professional engineer or registered architect licensed to practice in New York State and must provide sufficient information to allow a determination by the code enforcement official that the truss has been designed to comply with the RCNYS. It is important to verify whether the design load is for a live load equal to the GSL or determined in accordance with ASCE 7-98. Drawings for trusses designed in accordance with ASCE 7-98 should include the following information:

1. An indication that the design is based on reference standard ASCE 7-98.

2. A statement that the design has been analyzed separately for both balanced and unbalanced load conditions.

3. The flat-roof snow load (pf), snow exposure factor (Ce), snow load importance factor (I), and the thermal factor (Ct).

4. An indication that the unbalanced snow load factor is 1.5 or 1.5/Ce.

It should also be noted that RCNYS section R802.10.1 further requires truss design drawings to include at a minimum the following additional information:

1. Slope or depth, span and spacing.

2. Location of all joints.

3. Required bearing widths.

4. Design loads as applicable.

4.1. Top chord live load (including snow loads).

4.2. Top chord dead load.

4.3. Bottom chord live load.

4.4. Bottom chord dead load.

4.5. Concentrated loads and their points of application.

4.6. Controlling wind and earthquake loads.

5. Adjustments to lumber and joint connector design values for conditions of use.

6. Each reaction force and direction.

7. Joint connector type and description (e.g., size, thickness or gage) and the dimensioned location of each joint connector except where symmetrically located relative to the joint interface.

8. Lumber size, species and grade for each member.

9. Connection requirements for:

9.1. Truss to truss girder.

9.2. Truss ply to ply.

9.3. Field splices.

10. Calculated deflection ratio and/or maximum deflection for live and total load.

11. Maximum axial compression forces in the truss members to enable the building designer to design the size, connections and anchorage of the permanent continuous lateral bracing. Forces shall be shown on the truss design drawing or on supplemental documents.

12. Required permanent truss member bracing location.

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