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• When looking to maximize the code's current allowances in 3. Calculations in accordance with Section 722 terms of building size for mass timber structures,considering the differences between Type III-A and IV construction is 4. Engineering analysis based on a comparison of buildihr• important. For example'. element,component or assemblies designs having fi. resistance ratings as determined by the test procedure • Type IV does not allow concealed spaces in floor or set forth in ASTM E119 or UL 263 roof assemblies(e.g.,dropped ceilings, soffits, chases, etc.),but 1-hour fire resistance-rated interior partitions 5. Alternative protection methods as allowed by Section are permitted.All other construction types including 104.11 III-A allow concealed spaces. Note that requirements 6. Fire-resistance designs certified by an approved agency for sprinklers and draft stopping/fire blocking apply within concealed spaces per IBC Section 718 and the These alternatives are options when the exact assembly applicable NFPA sprinkler standard. has not been tested per ASTM E 119 and a test report • Except for exterior bearing walls,Type IV does not is therefore not available.They are all founded on ASTM require demonstration of fire-resistance ratings for E 119 testing. structural elements.This is a requirement for all other There are currently limited options for fire resistance-rated construction types including III-A(but only when mass timber assemblies from approved sources(e.g., a fire-resistance rating is required). Gypsum Association GA-600,American Wood Council's • Type IV construction allows the use of CLT in exterior Design for Code Acceptance 3-Fire Resistance-Rated walls;Type III does not. Wood Floor and Wall Assemblies, IDCA 31)or certification agencies(e.g., UL listings). However, an increasing number Table 5 illustrates these differences and others for a group B of assemblies have been tested according to the ASTM occupancy building. E119 standard and are available publicly or on request from The requirements of Type IV construction to have no manufacturers.The number of available tested assemblies concealed spaces in floors or roofs and for all interior partition can be expanded using comparative engineering analysis walls to be solid wood or 1-hour rated can significantly described in Item 4 of IBC Section 703.3. Such an analysis, impact its utility far some applications.The alternative of which seeks to justify the fire-resistance rating of an using Type III construction for Type V where building size assembly or component similar to one that has passed an permits)avoids this limitation;however,the processes for E119 test,can be performed by a fire protection engineer. demonstrating fire-resistance ratings also vary between Type Item 3 of IBC Section 703.3,which permits the use of IV and Types III and V. Methods for meeting fire-resistance calculations in accordance with Section 722, is also frequently rating requirements for mass timber elements in buildings used to demonstrate the fire-resistance rating of exposed other than Type IV construction are the focus of the rest of mass timber. IBC Section 722.1 states: The calculated fire this paper. resistance of exposed wood members and wood decking shall be permitted in accordance with Chapter 16 of ANSI/ Methods to Demonstrate Fire- AWC National Design Specifications for Wood Construction Resistance Ratings of Mass Timber (NDS').Chapter 16 of the NDS can be used to calculate up When a mass timber building element or assembly is to a 2-hour fire-resistance rating for a variety of exposed required to have a tire-resistance rating, IBC Section 703.2 wood members including solid sawn, glulam, SCL,and CLT. requires the rating to be determined by testing in accordance with ASTM E 119(or UL 263)or via one of six alternatives listed in IBC Section 703.3: The required fire resistance of a building element, component or assembly shall be permitted to be established by any of the following methods or procedures: 1. Fire-resistance designs documented in approved sources 2. Prescriptive designs of fire resistance-rated building elements, components or assemblies as prescribed in Section 721 3