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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