Ad Hoc Committee Tall Wood Buildings STRUCTURAL WORK GROUPS ISSUES

Transcription

1 Ad Hoc Committee Tall Wood Buildings STRUCTURAL WORK GROUPS ISSUES Date: 8/10/2016 Revised 8/12/2016 based on WG meeting Revised 9/9/2016 based on WG Chair meeting Revised 10/17/2016 based on Fire/Structure WG meeting Revised 10/28/2016 for Nov committee meeting STRUCTURAL MAIN TOPICS: 1) Seismic design values (R, Cd, Omega) Description: Seismic design values are not available for CLT shear walls. Here is a tentative schedule for the FEMA P-695 R-value research being conducted by Jon Van De Lindt: FPL P695 CLT shear wall project schedule: Present to Dec December 2016 March 2017 June 2017 team. July - November 2017 December 2017 Design methodology, experimental study, and analytical study in progress. Experimental studies complete. Analytical studies complete. Project team report complete. Submitted final report to project peer review Address peer review team comments. Final report complete. There are also efforts underway to study rocking shear walls and wood buckling-restrained braced frames (BRBF), but it s too early to really develop a schedule and unclear whether these systems will be standardized or proprietary systems. Action: Does not preclude use of mass timber as gravity framing. Track for information purposes. Draft report due June 2017 contact authors to access draft report. Affected IBC Section: n/a (ASCE) 2) CLT diaphragm design values Description: CLT panels do not have a listed in-plane (diaphragm) shear value in building codes. Copyright 2016 International Code Council, Inc. Page 1

2 On the standardization front, last week [late July 2016] AWC s Wood Design Standards Committee agreed to begin the revision process for the 2015 Special Design Provisions for Wind & Seismic (SDPWS). Identified issues for the 2021 SDPWS include deflection of stacked shear walls, diaphragm deflection calculations, and CLT diaphragms and shear walls. Also, as BJ Yeh mentioned, APA has begun the revision process for PRG-320 with a schedule for completion by 11/30/17. 10/17/16: In-plane shear values for Structurlam panels are provided in ICC ESR-3631 with diaphragm component use clarified in ICC AC455. In-plane shear values for Nordic -Lam are provided in APA PR-L306. Action: Lack of codified CLT diaphragm values does not preclude use of mass timber as gravity framing when used in conjunction with plywood or concrete diaphragm. CLT in-plane shear values are available for diaphragm design based on testing done by manufacturers. Affected IBC Section: n/a 3) Connection design Description: Connections need to maintain structural integrity for determined time during fire event. Connections need to exhibit deformation compatibility to be able to withstand expected inter-story drift during a wind or seismic event. This is not an issue specific to tall timber buildings, although the need for effective fire resistance may be more pronounced in a tall timber building where fire department access is more difficult. Action: Depends on fire protection approach char layer used similar to heavy timber or concealed with fire-resistive material (gypsum, intumescent, etc.). Track Fire Work Group issues and decisions. IBC Section addresses deformation compatibility (geometric compatibility). Deformation compatibility requirements are given in ASCE 7 Section for seismic design categories D through F. No provisions are provided in ASCE 7 for deformation compatibility wind loads. Affected IBC Section: ) Long term service concerns Copyright 2016 International Code Council, Inc. Page 2

3 Description: How does short and long term deflection and creep affect building design using mass timber? Addressed by ASCE 7-10 Section Self-Straining Forces: Provision shall be made for anticipated self-straining forces arising from differential settlements of foundations and from restrained dimensional changes due to temperature, moisture, shrinkage, creep, and similar effects To my knowledge, there hasn t been any evidence there is structural degradation of timber of a long time period. There are load duration effects that are addressed in the design using the NDS design provisions. Load duration effects in wood have been studied for over a century and are addressed by reducing the long-term strength of wood based on that research. References are in the NDS on the subject. Most of the historic research was conducted by the U.S. Forest Products Laboratory (FPL) during the 1930 s-1990 s, but research continues at various institutions. If you want more information than what is provided in the NDS and NDS Commentary on load duration, we can download papers from FPL s website. Note that structural degradation of wood is generally due to excessive heat, excessive moisture, or excessive chemical exposure all of which are addressed by design provisions in the NDS. Action: None. Affected IBC Section: n/a 5) Test and Inspections Description: How are we verifying the inspections of these panels as they are installed and connected, are there special inspections we need to add? Addressed by ASCE 7-10 Section Self-Straining Forces: Provision shall be made for anticipated self-straining forces arising from differential settlements of foundations and from restrained dimensional changes due to temperature, moisture, shrinkage, creep, and similar effects Affected IBC Section: Ch 17 Action: Proposed code changes/additions (DRAFT): Self-tapping screws. Special inspections of self-tapping screws shall be performed in accordance with Table TABLE REQUIRED SPECIAL INSPECTIONS OF SELF-TAPPING SCREWS Copyright 2016 International Code Council, Inc. Page 3

4 TYPE 1. Verify use of proper installation equipment in accordance with manufacturer s instructions. 2. Verify use of pre-drilled holes where required in accordance with manufacturer s instructions. 3. Inspect screws, including diameter, length, head type, spacing, and installation angle. 4. Verify screw heads are flush with side members and not over-driven. 5. For specialty elements, perform additional inspections as determined by the registered design professional in responsible charge. CONTINUOUS SPECIAL INSPECTION PERIODIC SPECIAL INSPECTION -- --?? Comments and questions: Format: Text versus table? I see examples of both in IBC Chapter 17. I m guessing once you get over two or three items, a table is clearer. Phrasing and terminology: I m sure I m getting some of this wrong, for example, inspect versus verify. I m sure others on the committee can help straighten it out. Table item 1: As far as I know, all manufacturers require the use of low rpm, high-torque drills and prohibit the use of impact drills. Table item 3: o Not all screws have the diameter and length indicated on the screw head, so you can t always verify these items after installation. Is the only way to deal with this to require continuous special inspection? Seems onerous. Also, is there an effective way to verify the angle of the screw after installation? o Include verification that manufacturer s minimum required spacing, edge distance, and end distance are met? o Do we need to say in accordance with contract documents, or is that understood? Table item 5: Is this necessary? Do we need to include a cross-reference to the wind and seismic inspection provisions ( and )? Mass Timber Products. Special inspections of Mass Timber Products (Cross Laminated Timer, Nail Laminated Timer, Dowel Laminated Timber) in buildings, structures, or portions thereof of Type 4A, 4B, 4C construction shall be in accordance with Table Note: Mass Timber Products and the elements listing in parenthesis are a placeholders for a description of the systems. This may not be the final description of the product through the committee. I used the Type of Construction that is currently being proposed for the tall wood structures, with the intent to leave out the current Type 4 H.T. construction. TABLE REQUIRED SPECIAL INSPECTIONS OF MASS TIMBER PRODUCTS Type Continuous Special Inspection Periodic Special Inspection Comments 1. Inspection of anchorage and Potentially, anchorage to a foundation of Mass Copyright 2016 International Code Council, Inc. Page 4

5 connections of Mass Timber Products not included in Table Inspect erection of Mass Timber Products in accordance with approved erection/placement drawings: a. Location of panel installation. b. Product labelling and fabricator verification. c. Grade of panel. d. Layup dimension (number of plies) and orientation. e. Dimensions of panels (width, depth, and length). f. Shape, including straightness and squareness. 3. Where minor variations exist in the Mass Timber Panels, these variations are confirmed to be compliant for structural and/or fire protection use through the RDP. 4. Inspection of connections where installation methods are required to meet design loads (pre-stressed connections, applied torque, depth of connector in parent material, etc.). 5. Site constructed or site-repaired Mass Timber products. 6. Where gypsum board products applied to Mass Timber products to enhance fire protection. 7. Inspection and testing of spray applied fireproofing elements or intumescent products applied to Mass Timber products to enhance fire protection. Timber Products could be in something besides concrete (driven wood piles). This would also include all other connections not listed in #4. This is intended for the special inspector to verify that the panels delivered to the jobsite are installed in the correct location. Several of the specifics are taken from PRG-320 In some cases, variations may exist; either from manufacturing, transportation, or installation; the RDP should confirm acceptance of variations. Connections where design loads are dependent on installation methods. Work done on the job-site would need oversight by special inspector. Application of additional layers of gypsum board to panels. Intumescent application on products and application methods, although it could be argued this is already in the code. Unless, this requires a unique installation/inspection process(es). ASSIGNMENT OF ISSUES TO WORK GROUPS RESPONSES: 1. Structural issues, post tension uses, lateral load unknowns. [Response: See Main Topic #1 above.] 2. Do structural issues arise over time as wood ages? Copyright 2016 International Code Council, Inc. Page 5

6 [Response: See Main Topic #4 above. No WG members could find examples of strength degradation over time, but we recommend finding or recommending research on wood aging effects on structural properties.] 3. Do structural issues arise during a fire event that may expose responders to building collapse? [Response: See Main Topic #3 above.] 4. Structural concerns, are there issues with all-wood buildings, or do we need to focus on composite construction? Issues with lateral strength, lots of unknowns. [Response: If composite construction means pairing a timber gravity framing system with a lateral framing system of a different material, this is currently the only path for a code-compliant timber building (above 6 stories) since seismic design values are not in ASCE 7 for mass timber walls. See Main Topic #1 above.] 5. How does a charred building assembly act in terms of continued operation? If a fire occurs that visibly chars a structural assembly, does that assembly need to be repaired prior to occupancy of the building? What is the char depth that is permitted before calling a building unsafe? [Response: If the Heavy Timber approach to fire resistance is taken for structural framing, the framing will be oversized to provide the required char depth and still maintain the required structural strength and connectivity. The assembly would need to be repaired prior to operation of the building to restore fire resistance capability. This issue is not specific to tall timber buildings.] 6. Current status of using mass timber walls as vertical lateral force resisting systems? [Response: See Main Topic #1 above.] 7. Current status of using mass timber walls as horizontal lateral force resisting systems (eg diaphragm load values?) [Response: See Main Topic #2 above.] 8. How are all the lateral design factors established? (Note that there are more than just R. ) This may be dependent on establishing a system that includes standardized connections. [Response: See Main Topic #1 above.] 9. Connections How do they get designed for structural capacity, structural ductility (related to lateral load design factors), and fire protection? [Response: See Main Topic #3 above.] 10. Testing of connections and develop provisions on embedment of steel connections [Response: This is an industry concern and should be addressed by connection manufacturers and material codes. See Main Topic #3 above.] 11. Addition of a section within Chapter 7 of IBC for acceptable methods of protection of connections for mass timber. Metal connectors and adequate means of embedment/encasement/proprietary protection to achieve specific ratings. [Response: See Main Topic #3 above.] Copyright 2016 International Code Council, Inc. Page 6

7 12. Details for protection at penetrations and assembly connections, methods of installation. [Response: Fire Work Group Issue.] 13. Connections/ Penetrations/Barrier Junctions [Response: Fire Work Group Issue.] 14. Review and comment on proposed test scenarios developed for Fire WG. [Response: We need to start collecting typical framing plans and connections used in tall timber buildings. Is typical construction post, beam, and plank (Framework?) What about post and plank (Brock Commons?) Whatever information you can provide on projects you ve worked on or projects you ve seen would be helpful. Consider it a survey of known (and expected future) structural approaches to inform large scale fire testing.] 15. How are we verifying the inspections of these panels as they are installed and connected, are there special inspections we need to add? [Response: See Main Topic #5 above.] 16. Need for Structural Peer Review? [Response: In our previous meetings it was discussed that there is adequate information provided by existing codes and standards to guide a design professional to design a tall timber building, same as for a low timber building. Therefore, the flags that require peer review of high rise timber buildings should be the same as the flags for buildings constructed of other materials, such as height. (This response might be too direct; let me know if there should be more explanation.)] 17. How amenable are CLT and other engineered panels to being modified during building alterations? If one cuts a hole through a panel (open up two spaces, add HVAC, add electrical, add a door, etc.) what kinds of effects can we expect? [Response: From what we ve seen, CLT and other engineered timber panels are modifiable during alterations. It seems their repetitive construction would provide redundancy that makes most alterations low risk. As is the case with all materials, alterations affecting building structure should be reviewed by a professional engineer. This issue is not specific to tall timber construction.] 18. How is CLT constructed, what are the weak components? When CLT fails during fire/load tests, what are the failure points of the product? [Response: The weak points of CLT construction seems to be at connections and not the panels themselves. Structural WG will work with Fire WG to determine a test assembly including expected connection.] 19. Not all species of wood are the same, if there a list of acceptable wood species? Differences between hard and soft woods? Are there minimum characteristics such as shear strength, density, hygroscopy, potential heat content, flame spread, char formation and resulting insulation, any others, that can differentiate an optimum wood versus a poor wood choice? [Response: This seems to be covered by PRG 320.] 20. Impact of wood aging? [Response: See Issue #2 response.] Copyright 2016 International Code Council, Inc. Page 7

8 FURTHER STUDY: 1) Keep track of fire testing to make sure assembly is based on probable structural assemblies in tall buildings: CLT ceiling exposed, connection to columns 2) Recommend testing to determine three-hour char rates of heavy timber and mass timber assemblies for inclusion in CH 16 of NDS. Anticipated need for these values based on fire protection requirements of mass timber framing to meet enhanced fire resistance requirements. Copyright 2016 International Code Council, Inc. Page 8