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Changes in IRC 2009 Wall Bracing

Changes in IRC 2009 Wall Bracing. Jay H. Crandell, PE ARES Consulting. PHRC Housing Conference February 11, 2009. Engineering & Consulting www.aresconsulting.biz. Introduction. CHALLENGES FOR BUILDERS & CODE OFFICIALS:

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Changes in IRC 2009 Wall Bracing

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  1. Changes in IRC 2009 Wall Bracing Jay H. Crandell, PE ARES Consulting PHRC Housing Conference February 11, 2009 Engineering & Consulting www.aresconsulting.biz

  2. Introduction • CHALLENGES FOR BUILDERS & CODE OFFICIALS: • “We don’t build them like we used to.” – conventional wall bracing provisions were developed for 1950’s era housing and had not been upgraded to keep pace with design changes (bigger size, open floor plans, window walls, etc.) • “The IRC wall bracing provisions are broken.” – IRC wall bracing provisions are becoming increasingly complex to keep pace with house design trends and bracing options to address those trends. • There’s still more…

  3. Introduction (cont’d) • CHALLENGES FOR ENGINEERS:“I-I-I know what you’re gonna say son. When two halves is gone there’s nuthin’ left - and you’re right. It’s a little ol’ worm who wasn’t there. Two nuthins is nuthin’. That’s mathematics son. You can argue with me but you can’t argue with figures. Two half nuthins is a whole nuthin’.” (Foghorn Leghorn, Warner Bros.) • Accepted engineering methods are also becoming more complex, yet they do not capture system behavior and can yield costly or inefficient solutions. • “Structural design is much, much more than simply crunching numbers.” (HUD, 2000)

  4. SOLUTIONS ? • Be prepared to efficiently use 2009 IRC changes. • Smart use of value-added engineering alternatives

  5. Outline & Goals • PART 1: Efficient Wall Bracing (2009 IRC) • Quick review of wall bracing basics • Highlight and explain changes in IRC 2009 • Show some “tricks of the trade” • List “self-help” resources • PART 2: Efficient Engineered Solutions • When is engineering required or beneficial? • Typical engineering resources • Using the bracing design approach behind the IRC seismic and wind bracing provisions • Design flexibility using rigid diaphragm & rigid beam analysis and collector design • Innovative bracing and shear wall solutions

  6. PART 1 Efficient Wall Bracing (2009 IRC) Session #1

  7. Braced Wall Line (BWL) • A line through the building plan used to determine amount of bracing required and locate Braced Wall Panels (BWPs).

  8. Braced Wall Panel (BWP) ▼ ▼ • A full-height section of wall constructed in accordance with a bracing method to provide racking strength to a BWL • Spaced no greater than 25 feet o.c. along a braced wall line (18’ max for cripple wall) • Begin no more than 12.5’ from the end of a braced wall line (total for both ends of a BWL – new for 2009 IRC) • WSP bracing permitted to begin 8’ from each end in SDC D0-D2 with special detailing (most other methods must start at each end in these SDCs) – N/A for PA ▼ ▼ ▼ ▼ ▼ ▼

  9. BWL Length Rule • The length of a BWL line is the distance from end to end of the BWL • The end of a BWL shall be taken as either: • Intersection with a perpendicular BWLs • Intersection with perpendicular exterior walls or the projection thereof, or • Chosen such that the maximum length results • What is the “maximum length”? (vague) • NOTE: Lengths of BWLs are only important for seismic bracing in 2009 IRC. Seismic bracing is based on length of a BWL whereas wind bracing is based on the BWL spacing. • Angled corners up to 8’ in length are also permitted to be part of a BWL (over 8’ then treat as a separate BWL)

  10. Using BWL Length & Offset Rules • BWLs can be visualized as a dotted line through the plan representing offset wall portions containing BWPs. • BWPs associated with a BWL can be offset by as much as 4’ with 8’ total out-to-out offset. • BWLs do not need to overlay any actual wall line.

  11. Must provide two separate braced wall lines ( >8’ total offset) Braced wall lines can have 4’ offsets but no more than 8’ of total offsets in a single line. Using the Offset Rulesto Layout BWLs on a Plan Do not use offset rule to place all bracing on one side of the BWL and reduce bracing

  12. BWL Spacing Rules • Wind: • Maximum braced wall line spacing is 60’. • Seismic: • SFD Exempt in SDC A/B/C • SFA Exempt in SDC A/B • Maximum braced wall line spacing is 25’, except that: • Up to 35’ spacing OK in SDC C if bracing amount increased 43% • Up to 35’ spacing OK in SDC D0-D2 if bracing amount increased 43% and diaphragm has 3:1 aspect ratio • Two adjacent lines can be spaced at 35’ in SDC D0-D2 to flank a single 900sf room. Possible interior BWLs to reduce BWL spacing, as required to meet wind or seismic BWL spacing limits (see left) N W E S = Spacing between E-W (Longitudinal) BWLs = Spacing between N-S (Transverse) BWLs Key:

  13. Use of Interior BWLs? • Use when required or to reduce bracing on exterior walls • PRO: More room for doors & windows on exterior walls • CONS: Interior walls need special detailing and must mix bracing if using different bracing methods on the same BWL.

  14. IRC 2009 Rules for Mixing Wall Bracing Methods • Can mix bracing methods from story to story. • Can mix bracing methods from wall line to wall line • Can mix bracing methods within a wall line • Bracing amount based on lowest-capacity method • Limited to low seismic only • Exceptions (limits) for Continuous Sheathing: • Cannot mix intermittent and continuous sheathing in the same BWL • Where wind speed > 100 mph or SDC D0-D2, continuous sheathing must be used on all exterior BWLs of a given story level if it is to be used at all on that story level

  15. IRC 2009 Bracing Amount Rules • Minimum 4’ of bracing required in each BWL • Separate wind and seismic bracing amount tables provided (bracing amounts given in “feet” not “%” of wall length) • For seismic, required bracing lengths for each BWL are tabulated based on the BWL length. • For wind, required bracing lengths are tabulated based on BWL spacing. • Greater of wind or seismic bracing controls • Various adjustment factors must be applied • Single-family detached homes in SDC A/B/C and Townhouses in SDC A/B are exempt from seismic bracing – only wind bracing required.

  16. IRC 2009 Uplift Load Path for BWPs • 2009 IRC will require framing straps at braced wall panels where the net wind uplift load at the top of a BWL exceeds 100 pounds per linear foot per Table R802.11. • Net Uplift = (Table R802.11 load) – (60 plf for each full wall above) • EXCEPTION: Where the basic wind speed does not exceed 90 mph, the wind exposure category is B, the roof pitch is 5:12 or greater, and the roof span is 32 feet or less • ALTNERATIVE: Bracing and fasteners designed in accordance with accepted engineering practice to resist combined uplift and shear forces.

  17. Intermittent vs. Continuous Bracing • The 2009 IRC distinguishes between intermittent and continuous bracing methods. • Both approaches are intended to provide equivalent performance, but with different rules • Each approach has pros & cons. • Structural fiberboard (new) and wood structural panels have continuous sheathing provisions in the IRC. Both of these materials are also used as intermittent bracing when sheathing is not placed above and below wall openings on all “sheathable” wall surfaces.

  18. BWP Minimum Length (Width) – Intermittent Bracing • Minimum length of a BWP for intermittent bracing methods is generally 4-feet (except 8’ for GB one-sided) • “Partial credit” for intermittent BWPs as narrow as 3-feet included in 2009 IRC • PROS: Rules are easier to follow and allows other sheathing products to be used between BWPs. • CONS: Wider BWPs, less room for wall openings, and slightly more bracing length required.

  19. BWP Minimum Length (Width) – Continuous Sheathing • BWP minimum length depends on the maximum clear opening height to either side of a panel (e.g., for typical window 24” BWP length or door 30” BWP length) • PROS: Less bracing and narrower panels permitted (more room for wall openings). • CONS: Must evaluate opening heights next to each BWP and follow special framing requirements for corners (i.e., minimum 2’ panel located on both sides of corners or use an 800# hold-down strap)

  20. Options for Large Openingsand Narrow Panels • 6 options follow that are included in the 2009 IRC • Three methods for use in an intermittent braced wall line • Two methods for use in a continuously-sheathed wood structural panel wall line • All methods have special detailing/framing and fastening requirements except the last method which uses braced wall line offset and BWP location rules to accommodate foyers and great rooms

  21. (1) Method ABW: Alternate Panel (Intermittent) • Supports up to one floor plus roof, replaces one standard 48” wide panel in an intermittent BWL • 32” minimumwidth (but counts as 48”) • Requires useof two hold-down bracketsfor each ABWpanel

  22. (2) Method PFG: Garage Openings (Intermittent) • Portal frame w/o hold-downs in an intermittent braced wall line, supporting up to one floor plus roof. Low seismic only. 24” minimum width (but counts as 36” toward required bracing).

  23. (3) Method PFH: Any Openings (Intermittent) • Portal frame with hold-downs in an intermittent BWL • Bottom story of a one- or two-story house • 16” minimumpanel width forone-story, 24”minimum panelwidth for two-story

  24. (4) Method CS-G: Garage Openings (Continuous) • Supporting roof only, on one wall of garage only • Max 3 psf roof covering weight in high-seismic • 24” minimumpanel width(4:1 length-to-height ratio) • No hold-downs

  25. (5) Method CS-PF: Any Openings • Portal frame w/o hold-downs in a continuously-sheathed BWL • Over concrete/masonry foundation, raised wood floor, or 2nd floor framing • 16” minimumpanel width(6:1 length-to-height ratio)

  26. (6) Foyers and Great Rooms within IRC Offset and BWP Spacing Rules Fasten roof sheathing to a common member (valley rafter) or use a ledger to tie diaphragms together

  27. BWP Connection and Support Requirements • Sole plates at BWP locations must be nailed to wood floor deck per IRC Table R602.3(1) or anchored to foundation per IRC Section R403.1.6 (3-16d at 16”oc or 1/2” anchor bolts @ 6’-0”) • Mudsill anchors, mechanical or adhesive anchors spaced to provide equivalent capacity are permitted • All edges and horizontal joints of sheathing at BWP locations must be attached to blocking or framing • Can omit blocking if bracing amount provided is twice that required, or for panels not used as bracing. Also not required for horizontal gypsum panels, unless fastened at 4”oc for higher capacity • New details provided for masonry piers supporting 48” or narrower BWP widths. Anchor bolts in bond beams and/or vertical rebar in grouted cells (and hooked into footing) required.

  28. Blocking Requirements for FramingAbove & Below BWPs • Full-depth blocking required between floor framing members (joists or trusses) above and below BWPs • In 2009 IRC, blocking between roof framing members is also required above BWP locations. • EXCEPTION: Not required in low wind/seismic when “heel height” is not greater than 9.25” • Where heel height > 9.25” partial height blocking can be used • Structural soffit sheathing detail permitted • Can use blocking panel provided by truss manufacturer/designer for large heel heights. • May be able to use pre-engineered clips in lieu of blocking to provide lateral stability as well as uplift resistance • May be able to use “skip-blocking” (every other bay) along a BWL rather than blocking just at BWP locations

  29. IRC Wall Bracing Resources • ICC Wall Bracing Guide (www.iccsafe.org) • FSC Wall Bracing Guide (www.foamsheathing.org) (FREE download) • The ICC guide is currently based on the 2006 IRC, but notes some concepts that were approved in the 2007 IRC Supplement • The FSC guide is currently based on the 2006 IRC, but includes notes and tips relevant to changes made in the 2009 IRC • Other resources (e.g. Bracing Manufacturers, APA, Fairfax County, VA, etc.)

  30. PART 2 Efficient Engineered Solutions (Session #2) “Two half nuthins is a whole nuthin’.” (Foghorn Leghorn, Warner Bros.)

  31. When Is Engineering Required or Beneficial? • When the house or portion exceeds the various limits of the IRC • R602.10 Wall bracing …Where a building, or portion thereof, does not comply with one or more of the bracing requirements in this section, those portions shall be designed and constructed in accordance with Section R301.1 • When alternate materials or systems are substituted for code-approved materials and construction methods • When alternate and innovative design methods may yield a more cost-effective solution • When it’s the “path of least resistance” to a permit (a half nuthin is better than a whole nuthin)

  32. Combining Prescriptive and Engineered Design • IRC Section R301.1.1 allows portions of a house otherwise designed using prescriptive provisions to be designed using alternate standards • AF&PA’s Wood Frame Construction Manual • AISI’s Standard for Cold Formed Steel Framing—Prescriptive Method (AISI S230) • ICC-400 Design & Construction of Log Homes • IRC Section R301.1.3 requires portions of a house not complying with prescriptive provisions to be engineered • Engineered structural design per the IBC is allowed for any portion of a house

  33. Using Alternate Methods and Materialsfor Construction and Design • IRC Sections R104.10 and R104.11 allow the use of alternate materials and methods of construction (or design), with the building official’s approval • Typically involves using alternate products with a code evaluation report (ICC-ES, COLA, etc.) • Also includes construction methods not fully implemented in the IRC (e.g., balloon framing, post-frame construction, etc.) • Similarly, use of alternate methods of design or analysis by an engineer requires adequate justification and code official approval.

  34. Typical Engineering Resources • Code requirements for design (non-exhaustive): • IBC, 2006 • ASCE 7, 2005 • AF&PA NDS, 2005 (wood design) • AF&PA SPDWS, 2008 (wood lateral design) • Various other industry standards (concrete, steel, etc.) • Product code evaluation reports, manufacturer catalogs and approved data • Recognized texts & principles (“standards of care”): • Residential Structural Design Guide (HUD, 2000) – www.huduser.org (FREE download) • Design of Wood Structures (McGraw-Hill) • Last, but not least….YOUR GOOD JUDGMENT!

  35. Efficient Engineering Alternatives to the IRC • Interior Partition Walls as a Bracing Method • Altering Braced Wall Panel Location Requirements • Allowance for Bracing Transfer • IRC 2009 Braced Wall Design Approach • IRC Equivalent Design Example • IRC Townhouse Design Approach • Alternate Uplift Load Calculation and Uplift Load Path Connections

  36. Interior GWB Finish as Bracing • Interior partition walls with segments at least 48” wide, minimum ½” thick gypsum panels on both wall faces, and IRC Chapter 7 fastening may be counted as a braced wall line • An interior partition wall constructed as above provides about one-half the bracing strength of Method GB panels on “both sides” • IRC 2009 Bracing Method GB (Gypsum Board) with 4” o.c. fasteners in lieu of 7” o.c. is good for a 30% reduction in required bracing length per 2009 IRC • Higher-capacity gypsum products (5/8” thickness, or fiber-reinforced) can also provide a higher capacity than standard ½” gypsum board • NOTE: Gypsum wall panel bracing is not permitted or severely limited for high seismic (SDC D and higher)

  37. Braced Wall Panel Location • Braced wall panels can be designed to begin further than 12.5’ from the ends of a braced wall line and spaced greater than 25’ oc along a braced wall line provided that: • An adequate overall bracing amount is maintained for a braced wall line and • The top plate (collector) is designed to accommodate the additional in-plane tension or compression forces that result from a wider spacing of braced wall panels. • In general, this only affects the number or size of fasteners used in lap-splices of the top plate

  38. Allowance for Bracing Transfer (Rigid Diaphragm Design) • Buildings adequately braced on three sides can be stable against lateral loads (limitations apply) • Racking forces (shear) are redistributed and resisted by torsional (twisting) response of the building

  39. IRC 2009 Braced Wall Design Approach • The wind bracing amounts in the 2009 IRC are based on nominal capacities from the AF&PA SDPWS, assuming gypsum board finish on interior: • Let-in brace (LIB): 400 lbs/foot • Gypsum board both sides (GB): 400 lbs/foot • Wood structural panels (WSP): 700 lbs/foot • Capacity of other non-gypsum panel products (fiberboard, diagonal boards, particleboard, PC stucco, and hardboard siding) are the same as WSP. • Deduct 200 lbs/foot if interior finish not present on interior side (does not apply to GB)

  40. IRC 2009 Braced Wall Design Approach (cont’d) • BWP design values for IRC 2009 wind bracing: BWP design value = (nominal capacity) x 1.2 / 2.0 where 2.0 is the safety factor and 1.2 is a net “calibration” factor • The 1.2 factor comes from a necessary simplification of a very complex problem – whole building system performance. It is associated with two effects or factors:

  41. Justification? “…you can’t argue with figures.” • The “partial restraint factor” accounts for loss of bracing strength when the framing system is not able to provide full restraint against BWP overturning. • The whole building factor accounts for contributions from non-structural components and non-compliant bracing elements which form a whole building:

  42. IRC Braced Wall Design Approach (cont’d) • Sum length of qualifying BWPs (full-height segments) on each BWL and multiply by design value to get bracing strength (lbs) for each BWL: BWL Design Strength (lbs) = (Sum of BWP lengths) x (BWP design value) • Determine wind load (pressure) acting on the vertical projections of the building using ASCE 7 provisions (required by IBC). • Using a tributary area approach or relative-stiffness (rigid diaphragm) approach, distribute total wind load (shear force) to each braced wall line in each story level for each plan direction. • CHECK: BWL Design Strength ≥ Design Load? Yes (done)  No (make adjustments).

  43. 2009 IRC Design by Equivalency Example Low Wind (90/B) Low Seismic (A/B – exempt)

  44. Step 1: Outline plan area for 1 & 2 story portions (1st floor plan shown)

  45. Step 2: Determine Total Wind Bracing Length Required for Each Portion Main Building (2-story Portion, 56’ x 66’) • Front-to-Back Direction: 2* x (19’ WSP)(1.0)(1.1)(0.95) = 40 feet WSP (total required) – based on 56’ BWL spacing • Left-to-Right Direction: 2* x (22’ WSP*)(1.0)(1.1)(0.95) = 46 feet WSP (total required) – based on 66’ BWL spacing NOTES: • Bracing amounts shown are based on new 2009 IRC wind bracing provisions • 1.0, 1.1, and 0.95 factors adjust IRC wind bracing amounts to specific plan conditions (wind exposure B, 13’ roof-to-eave height, 9’ story height) • Factor of 2* doubles IRC tabulated bracing to give total bracing required for the building portion for a given wind loading direction. IRC tabulated bracing is based on the bracing amount required for one of two braced wall lines on opposite sides of a building (half the bracing load on each wall line) • The 22’ length for WSP in the left-to-right loading direction is derived by extrapolation beyond 60’ BWL spacing limit of 2009 IRC (OK to do this?).

  46. Step 2: cont’d Conservatory & Suite (1-story Portions, 15’ x 22’ ea.) • Front-to-Back Direction: 2* x (3’ WSP)(1.0)(1.0)(1.0) = 6 feet WSP (total required) • Left-to-Right Direction: 2* x (4.3’ WSP)(1.0)(1.0)(1.0)) = 9 feet WSP (total required) NOT APPLICABLE, the plan portion is in “shadow” of sail area for main building in this wind loading direction.

  47. Step 3: Select Trial BWLs

  48. STEP 4: Evenly distribute total required bracing to each BWL Front-to-Back Wall Lines • Wall Line 1:3’ of WSP bracing required (50% of 6’ WSP total required for conservatory) OK, 4’ provided as CS-WSP (2’ of bracing provided at each end with 2’ corner returns). Could also use CS-PF if necessary to achieve 18” panel widths for a total of exactly 3’ of bracing. • Wall Line 2:3’ WSP (50% of conservatory bracing) + 1/3(40’ WSP, main building) = 16.3 ft WSP required OK, ~16 of WSP bracing provided on exterior wall plus additional 22’ of GWB interior wall not counted. One third of main building bracing is distributed to each of three Wall Lines 2, 3, and 4. • Wall Line 3:1/3 (40’ WSP, main building) = 13.3 ft WSP required OK, 9.5’ WSP + 6’ WSP (2-3.5’ segments at partial credit) = 15.6 feet provided plus additional 20’ of interior wall not counted.

  49. STEP 4: (cont’d) Front-to-Back Wall Lines (cont’d) • Wall Line 4:Same as Wall Line 2 = 16.3 ft WSP required. OK. Through trial and error this BWL only works if WSP and GB bracing on exterior and interior portions are counted together and the GB bracing uses 4”oc fastening permitted by IRC 2009 for extra capacity. Per IRC 2009 mixed bracing rules, the total length of WSP + GB bracing is based on the bracing amount required as if the entire wall was GB (the weaker of the two methods). The GB bracing amount can be selected from the IRC 2009 wind bracing table or calculated from the required WSP amount. Based on WSP (700 plf) and GB, 2-sided (400 plf), the equivalent amount of GB, 2-sided required on the BWL is (700/400)x16.3’ = (1.75)x16.3’ = 28.5’. Because the GB is fastened at 4”oc in lieu of standard 7”oc, the amount required is 0.7 x 28.5’ = 20’ using an adjustment factor of 0.7 provided in the 2009 IRC. The total length of bracing provided (GB + WSP) is 5’ WSP + 19’ GB (4”oc 2-sided) = 24’ > 20’ required. Note that the 5’ WSP panel is more than 12.5’ from outside end of garage portion of BWL, therefore top plate along this portion of the BWL needs to be designed as a collector (simply increase top plate lap splice nailing or use straps at joints) • Wall Line 5: Same as Wall Line 1 = 3’ of WSP required. OK. Use two 4-foot WSP panels at corners. 8’ WSP provided.

  50. Step 4 (cont’d) – End of Example Left-to-Right Wall Lines • Evenly distribute total bracing length required (46’ WSP or equivalent) to the five L-R wall lines as follows: Wall Line A: 8% x 46’WSP = 3.7 feet WSP Wall Line B: 17% x 46’WSP = 7.8 feet WSP Wall Line C: 50% x 46’WSP = 23 feet WSP Wall Line D: 17% x 46’WSP = 7.8 feet WSP Wall Line E: 8% x 46’WSP = 3.7 feet WSP 100% x 46’ WSP = 46’ WSP • As with the Front-Back BWLs, verify each wall line meets or exceeds the above required bracing amounts. On this plan all walls are OK. NOTE: The above distribution can be taken to represent a maximal inward distribution of wall bracing to interior Wall Line C rather than to exterior Wall Lines A, B, D, or E. If the building had fewer interior walls (more interior open space) and less openings on the front and rear facing exterior walls, then more of the bracing could have been distributed toward exterior Wall Lines A, B, D, and E rather than C.

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