Wood Roof Framing Common Design and Construction Defects Concurrent Session #5.3, Friday April 28, 2017, 9:45 AM-10:30 AM

Transcription

1 Wood Roof Framing Common Design and Construction Defects Concurrent Session #5.3, Friday April 28, 2017, 9:45 AM-10:30 AM William J. Dery, PE Sr. Structural Engineer

2 Outline 1. Wood Roof Framing 101 the Triangle 2. Conventional (Stick) Framing Deficiencies 3. Component Wood Truss Framing Deficiencies 4. Top 5 Questions to help vet your prospective wood framing expert

3 Part 1 of 4 Wood Roof Framing The Triangle

4 Flat ceiling (Triangle) v. Vaulted Flat Ceiling (Triangle) Ridge is not designed as a beam Roof system constructed with top and bottom members Vaulted (Single-Assembly) Ridge is designed as a beam Roof system constructed with top members but not bottom members

5 Triangle v. Vaulted Audience Visual Demo

6 Vaulted (Single Assembly) Roof System Load applied vertically

7 Vaulted (Single Assembly) Roof System Top of walls deflect outward

8 Flat Ceiling (Triangle) Framed Roof Systems Top members (chords) in compression Load applied vertically Bottom member (chord) in tension

9 Component Wood Truss Roof Systems Note the same Triangle compression and tension (red and blue) top and bottom members

10 Deficient Roof System Bottom member (Rafter tie) attached too high for top member (rafter) design

11 Deficient Roof System Top member (Rafter) fails in bending - Allows top of walls to deflect outward

12 Single Assembly Roof System Structural Ridge provides required vertical support for Single Assembly system IBC and IRC R

13 Part 2 of 4 Conventional (Stick) Framing Deficiencies Based on: 2015 International Building Code 2015 International Residential Code 2015 International Existing Building Code

14 Rafter Tie Failures Not enough nails and located too high in attic space Photos courtesy of Photos courtesy of VERTEX

15 Deficient Roof System Top member (Rafter) fails in bending - Allows top of walls to deflect outward

16 Rafter Tie Failures Too high -cracked rafter Too far apart - wall deflection Photos courtesy of VERTEX

17 Deficient Rafter Ties Ceiling joists not parallel/attached to rafters and rafter ties are too high - walls deflected Photo courtesy of VERTEX

18 Rafter Ties ( and R ) Function Resists horizontal roof thrust load at the top of the wall Can also serve as the ceiling joist Not less than 2x4 and no greater than 48 o.c. Also must meet the requirements of Table R (9) or Table Locate where rafter bending stress is not exceeded. Typically in the lower 1/3 of attic space Photo courtesy of VERTEX

19 Ceiling Joists ( and R ) Can also (and typically does) function as a rafter tie Maximum Spans Table (1 or 2) Table R (1 or 2) Connections Top Plate: (4).131 x3 toe-nails Table Table R602.3(1) Rafter and splice: Table Table (9) As an example: IBC requires: (13) 16d nails for 5-pitch roof, 36-ft roof span, 30 psf ground snow load, rafter ties spaced at 24

20 Single Assembly Roof System Structural Ridge provides required vertical support for Single Assembly system IBC and IRC R

21 Ridges ( , and R802.3) Board vs Beam (or Girder) Ridge Board 1x thickness not less in depth than the cut end of the rafter Not a structural element Ridge Beam (or Girder) Structural element & R Photo courtesy of VERTEX

22 Hips ( , and R802.3) A structural beam when roof pitch is less than 3:12 Not less than 2x thickness and not less in depth than the cut end of the rafter Photo courtesy of VERTEX

23 Valleys ( , and R802.3) Always a structural beam (or girder) for any roof pitch R802.3 Requires brace (post) at ridge down to a bearing wall Not less than 2x thickness and not less in depth than the cut end of the rafter Photo courtesy of VERTEX

24 Hip V. Valley Audience Visual Demo

25 Collar Ties (Table and R ) Function Keeps rafters from separating at the ridge during wind event No less than 1x4 and no greater than 48 o.c. Connection (3) 10d common or (4) x3 Table (3) 10d common or (4) x3 or 1.25 x 20 gage ridge strap Table R602.3(1) Located in the upper 1/3 of attic space Against ridge is the optimum location Photo courtesy of VERTEX

26 Collar Tie Failure Photo courtesy of VERTEX

27 Rafter ( and R802.5, R802.6) Spans Tables (1 thru 6) Tables R (1 thru 8) Connections Top Plate: (4).131 x3 toe-nails Table or Table R602.3(1) Ridge/Hip/Valley : (3).131 x 3 end-nails or (4).131 x3 toe-nails Table or Table R602.3(1) Heel (birdsmouth) cuts 1 ½ min. R802.6 Uplift connector per Table or R Photo courtesy of VERTEX

28 Part 3 of 4 Component Wood Truss Framing Deficiencies Design Criteria Based on: 2015 International Building Code 2015 International Residential Code 2015 International Existing Building Code

29 Truss Bracing Photo courtesy of VERTEX

30 Truss Temporary Bracing Building Code Language on Temporary Bracing: Residential Code - R Bracing Trusses shall be braced to prevent rotation lateral stability in accordance with the requirements specified in the construction documents for the building and on the individual truss design drawings. In the absence of specific bracing requirements, trusses shall be braced in accordance with accepted industry practice such as SBCA Building Component Safety Information (BCSI) Guide to Good Practice for Handling, Installing & Bracing of Metal Plate Connected Wood Trusses. Commercial Code Trusses spanning 60 feet or greater The owner or the owner s authorized agent shall contract with any qualified registered design professional for the design of the temporary installation restraint/bracing and permanent individual truss member restraint/bracing for all trusses with clear spans 60 feet or greater

31 Truss Temp. Bracing Cover Page on Truss Package as recommended by BCSI

32 Truss Temporary Bracing Photo courtesy of VERTEX

33 Wood-Framed Building Collapse Photos courtesy of VERTEX

34 Wood-Framed Building Collapse Don t delay assigning an expert and requesting the site inspection Spoliation, biggest concern Expert to determine what permanent and temporary roof bracing was in place Expert to determine what permanent and temporary wall bracing was in place

35 Part 4 of 4 Top 5 Questions to help vet your prospective wood framing expert

36 Top 5 Questions to ask expert? 1. Q: How may stick framed residential structures have you designed and were the engineer of record (EOR)? A: Hopefully Q: What is the difference between a collar tie and a rafter tie? A: Collar ties are for wind uplift, Rafter ties are for the lateral roof load on the walls (Shoelace demo) 3. Q: Is a post almost always required under valley rafters? A: Yes 4. Q: Is there a certain span where engineered plans are required for the temporary bracing of wood trusses? A: Yes 60 feet 5. Q: Are plywood clips required for 1/2 plywood roof decking over framing spaced at 24 inches? Is a gap required between sheets of plywood? A: No to the clips & Yes to the gap (1/8-in.)

37 Questions??? Photos courtesy of VERTEX