32A-048 65 Market Street FirestopViadero Engineering
82 South Plain Rd
Sunderland, MA 01375
Mr. Peter Gelinas
Valley Building Co Inc.
190 Russell Street
Hadley, MA 01035
Subject: Property at 65 Market Street, Northampton, MA
Peter,
As we discussed briefly, below is a recommendation for remediation work to be performed on the central wall at 65 Market Street. Based on these guideline, you should be able to maintain
continuity of the fire rated wall without having to alter joist placement or modify current construction. If you have any additional questions or would like to discuss further please
let me know.
Background
The memorandum below outlines the steps that need to be taken to provide an adequate 1 hour fire resistant rated wall assembly between two dwelling units at the property located at 65
Market Street, Northampton, MA. This has been done to ensure compliance with provisions of section R302.3 of the International Residential Code which requires that:
Dwelling units in two-family dwellings shall be separated from each other by wall and floor assemblies having not less than a 1-hour fire-resistance rating where tested in accordance
with ASTM E119 or UL 263. Fire-resistance-rated floor/ceiling and wall assemblies shall extend to and be tight against the exterior wall, and wall assemblies shall extend from the foundation
to the underside of the roof sheathing.
As currently constructed, the structure at 65 Market Street consist of a 2 story single family home which has been divided though recent renovations into two separate dwelling units,
separated by a central load bearing wall. During renovation and reconstruction, framing for the A and B side dwelling units was constructed in a manner such that floor joists on the
first and second floor both rest on the central, shared, load bearing wall. The direction of the joist, moving longitudinally from inside the A and B side and the subsequent joist bay
located on top of the load bearing wall provides a break in the continuous unobstructed vertical fire resistant plane as required per IRC 302.3. As such, the requirements below shall
be taken to ensure a 1 hour fire rated assembly and ensure that the vertical fire resistant assembly maintains continuity against the ingress of flame and hot gas.
Requirements for 1 hour Fire Resistant Wall Assembly
Current standard practice for the construction or reconstruction of two family homes with 2 dwelling units, is to provide a continuous fire rated assembly running unobstructed either
horizontally or vertically depending on how the dwelling units are divided (by walls or floors). In dividing dwelling units with vertical separating walls, standard industry guidelines
are provided for specific materials to be used
in construction to ensure wall assemblies maintain the requisite 1 hour fire rating. Materials in these configurations have been tested per ASTM E119 and shown to provide adequate 1
hour fire resistance under test conditions. Below is an excerpt from the American Wood Council’s Design for Code Acceptance Part 3 (2009) pertaining to the construction of 1 hour fire
rated assemblies from both sides.
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Based on these requirements, the central interior wall dividing the A and B side dwelling unit of 65 Market Street (constructed of nominal 2x4 lumber), shall be filled with 3.5” mineral
wool batts (Roxul or equal) and shall have minimum 5/8” type X gypsum wallboard attached to both sides to provide fire resistance from both the A and B sides. Where floor joists pass
from inside the A and B side dwelling units onto the central load bearing wall, provisions shall be taken to ensure that combustion of joists from either side will not extend into the
opposite dwelling unit for at least 1 hour.
While the International Residential Code (IRC) part 302.3 offers little guidance on determining 1 hour fire resistance outside of using standard materials tested in accordance with ASTM
E 119 or UL 263, the International Building Code (IBC 2009) provides more broadly applicable guidelines for determining 1 hour fire resistance ratings for structures. Referring to part
7 of the IBC, part 703.3 provides alternative methods for determining fire resistance. This section from the IBC 2009 codebook has been excerpted below.
703.3 Alternative methods for determining fire resistance
The application of any alternative method listed shall be based on the fire exposure and acceptance criteria specified in ASTM E 119 or UL 263. The required fire resistance of a building
element, component or assembly shall be established by any of the following methods or procedures:
Fire resistance deigns documented in sources
Prescriptive design of fire resistance rated building elements, components or assemblies as prescribed in section 720.
Calculations in accordance with section 721
Engineering analysis based on a comparison of building element, components or assemblies designs having fire resistance ratings as determined by the test procedures set forth in ASTM
E 119 or UL 236
Alternative protection methods as allowed by section 104.11
As allowed under part 3 of subpart 703.3, 1 hour fire resistance exposed wood assemblies may be calculated utilizing calculations in accordance with section 721 of the IBC (2009). As
such referenced in section 721.1 “the calculated fire resistance of exposed wood members and wood decking shall be permitted in accordance with chapter 16 of the ANSI/AF&PA National
Design Standard (NDS). As such calculation methodology referenced in the NDS 2015 version has been applied to elements surrounding joists that penetrate the central bearing wall from
the A and B side dwellings.
Referring to chapter 16 of the NDS, wooden structural members, nominal 2 x and larger, may be used as a fire resistant element, similar to noncombustible construction materials such
as type X gypsum wallboard. The superior fire performance of timber can be attributed to the charring effect of wood. As wood members are exposed to fire, an insulating char layer
is formed that protects the core of the section. Thus beams and columns can be designed so that sufficient cross section of wood remains to sustain the design loads for the required
duration of fire exposure.
In chapter 16.2.1 of the NDS, equations for the calculation of fire resistance and char rate of structural timbers is listed. These equations have been listed below.
Equation 16.2-1
Beff= 1.2Bn / t0.187
Where:
Beff = Effective Char Rate
Bn = Nominal Char Rates
t= exposure time
Applying this to standard 2 x dimensional lumber, the nominal 1 hour char rate is approximately 1.8 inches per hour. As such to ensure that the central wall at the property at 65 Market
Street maintains a 1 hour fire resistance rating, wood assemblies not protected in accordance with industry standard practices (such as type X gypsum) must maintain a minimum 1.8” thickness
in any horizontal path that could allow fire to pass from that A to B side or vice versa.
To accomplish the requisite minimum 1.8” char thickness for wood members, 2 x fire blocking in all joist bays from both sides is required. In addition, a secondary layer of nominal 2
x fire blocking will be required in addition to this primary layer of fire blocking. The secondary layer may be attached directly over the primary layer of fire blocking or on top of
type X gypsum wallboard (if this is continued up to the level of the subfloor from the wall below). In so doing, a solid wood barrier 3” thick shall be established. This shall resist
direct failure though the inherent char resistance of the wood, while also preventing hot gas and flame from contact inner portions of the joists and burning back into the wall. A drawing
view has been provided below:
Top View/
In addition to the above noted fire blocking, during installation should any gaps still exist between required blocking and floor joists, gaps shall be filled with fire resistant foam
meeting the requirements of ASTM E814.
Additional Required Measures
In addition to the continuity of the fire rated wall assembly as noted above, any penetrations through the wall or penetrations in the wall membrane shall be protected as required by
IRC section 304.1 and 304.2
If you have any additional questions or would like to discuss further please let me know. Thank you again for the opportunity to provide this guidance.
Jason Viadero
11/25/2017
Massachusetts Professional Engineer # 53372