Structural Design for Residential Construction ...

1 Structural Design for Residential ConstructionCynthia Chabot, is Residential Construction ? One and two family dwellings Typically wood framed Construction in this part of the worldWhat does a Structural engineer typically do?Drawing by Americad Analyze load paths to ensure they go down to a foundation Connections connections connections Roof, floor, and wall assemblies Beams, columns, headers Lateral load resisting system (diaphragms, shear walls, collectors, struts, anchorage, overturning analysis) Footings/foundationsWhat does a Structural engineer typically not do? Land surveying Geotechnical engineering Layout of rooms Room sizes, ceiling heights Egress, ventilation & lighting Stairway geometry Mechanical, electrical, & plumbing Fire protection Energy efficiency PermittingGray areas Chimneys Moisture protection Termite mitigation DrainageAll you need to know about structure Equal and opposite forces What is up must come down The wind will always blow it overCode Requirements Building Codes: CT: BOCA national Building Code 1996/IRC 2003 MA: State Building Code, 6thEdition (Ch.)

2 36, 1&2 family dwellings) NH: IBC 2000/1&2 family dwellings per town RI: IBC 2003/IRC 2003 VT: BOCA national Building Code Minimum standard Residential code prescriptive vs. engineeredParts of structure Connections, connections, connections Beams, columns, headers Diaphragms, shear walls, collectors, struts, anchorage (lateral force resisting system) Foundations to hold it all up Soil is part of the structure tooWhat we don t use as part of the structure We do not use the plywood as a T beam to increase the capacity of the joists instead the plywood is the diaphragm to transfer lateral loads to shearwalls Interior partitions (excluding center bearing wall) are dead loads only The gypsum board inside is dead load Interior walls not used to resist horizontal forces from Snow LoadsIBC 2003 SLIDING SURCHARGEROOF SNOWDRIFT SURCHARGEANGLENote a 15% increase in the allowable capacity of wood for loads that include snow, which is a short-term loadSnow Loads7/12Cs7 that roofs exceeding an angle of 30 degrees may reduce the ground snow LoadsAbove, Figure 1609, Basic Wind Speed (3-second gust), 33 feet above ground, exposure CIBC 2003 ZoneV30(mph)123708090(Western Mass.

3 (Central Mass.)(Eastern Mass.)Table , Wind velocity fastest mile 30 feet above the ground, exposure CMass. State Code, gustFastest mileReference wind pressuresZonePressure (psf)123121721(Western Mass.)(Central Mass.)(Eastern Mass.)Soil and SurchareUnbalanced fillSeismic??Dead Loads3/4" wood floor/fin psf5/8" plywood psf2x10s @ 16" psfgyp + plaster/paint psfTotal psfFLOOR1/2" gyp. @ 16" " plywood3/4" wood floor5/4" decking psf2x12s @ 16" psfTotal psf2x12s @ 16" " decking5/8" plywood5/8" tile and psf5/8" plywood psf2x10s @ 16" psfgyp + plaster/paint psfTotal psfTILE FLOOR strapping2x10s @ 16" " gyp. " ceramic tile & thinsetwood shingles psffelt paper psf1/2" plywood psf2x6s @ 16" psfbatt insul.

4 Psfgyp + plaster/paint psfTotal psf1/2" gyp. insulation & 2x6s @ 16" " plywoodpainted wood shingles over felt paperEXTERIOR WALL1/2" gyp. @ 16" " plywoodINTERIOR WALLgyp + plaster/paint psf2x4s @ 16" psfgyp + plaster/paint psfTotal psfROOF(unfinished below)shingles psf (1 layer - code allows up to 3)tar paper psf5/8" plywood psf2x12s @ 16" psfTotal psf ( with 3 layers of shingles2x12s @ 16" " plywoodtar paper and shinglesBEAMSS hearBendingNotching and BoringLL/32"d/4 SUPPORT2"L/32" vs UNIFORM LOAD12 feet12 feet2x10 required2x6 requiredConcentrated loads ..more of a challengeUniform loads ..goodLESSON LEARNEDSIMPLY SUPPORTED vsCONTINUOUS OVER SUPPORTSS tress reversal; compression at the top, tension at the bottomHigher shear stress and reaction to column compared to simple span2 simply supported beams1 long beam spanning over center columnShear diagramMoment diagramShear diagramMoment diagramRestraint against twisting & lateral stabilityAspect ratio, d/bbd d/b<2no lateral support required 2 < d/b<4ends held in position 5 < d/b<6laterally restrain ends and at intervals along length of less than 8ft.)

5 And compression edge held in position with sheathing 6 < d/b<7laterally restrain ends both compression and tension sides shall be supported for the entire length. Aspect ratios of common beam UNDER BEARING WALL ABOVEBLOCKING OVER BEARING WALL BELOWRIM BOARD PROVIDES LATERAL STABILITY AT END OF JOISTBLOCK BETWEEN SUPPORTING COLUMNS COLUMN CONTINUING LOAD FROM ABOVE TO FOUNDATIONCOLUMN SUPPORTING BEAM ABOVEB lockingConnections of multiple LVLsNAIL TOGETHER TO PROVIDE STABILITYBOLTING REQUIRED TO TRANSFER LOAD TO ALL BEAMSSIDE LOADING BEAMSUPPORTING GIRDER2"2"TOP LOADING BEAMF ollow the load path due to gravity20 psf30 psf30 psf30 psf40 psf450 plf450 plf150 plf300 plf150 plf225 plf450 plf225 plf225 plf450 plf225 plfTotal = 1050 plf1200 plf1050 plf150 plf450 plf225 plf225 plf2ndfloorAttic floor1stfloorFollow the load path due to gravity10"TOP OF SLAB2X10s @ 16" @ 16" OF

6 SOIL2X12s @ 16" @ 16" simple house framingTTRafter/Ceiling Joist Heel Joint ConnectionTTHcHgDead and Live Loads (psf)DL + LL (plf)RLRR12 Roof SlopeRoof Span (L) MRidge= 0 = T (Hc) + (DL + LL)(L/2)(L/4) - RL(L/2)T = RL(L/2) - (DL + LL)(L/2)(L/4) HcCeiling LoadsRedundancy Unlike bridges, houses have many Structural members. Credit is provided for repetitive members of joists Laterial force resisting system Horizontal Diaphragm (plywood subfloor) Collectors Cords Vertical Diaphragm (exterior wall) Strut Cords The building code provides some information on LFRS see faceWindward sideWest faceWindward sideSouth faceLeeward sideEast faceLeeward sideFollow the load path due to windNorth Wind affect to Horizontal DiaphragmCollector (strut)ChordNorth Wind Horizontal Diaphragm affects to West/East ShearwallsTensionCompressionA closer look at the West ShearwallShear force resisting chord force from attic diaphragmShear force resisting force from shearwall above plus 2ndfloor diaphragmShearwall cord force reaction from attic diaphragm (compression)

7 Shearwall cord force reaction from attic diaphragm (tension)E&O reaction from shearwall above E&O reaction from shearwallabove added to shearwallcord force reaction from 2ndfloor diaphragm in tensionE&O reaction from shearwall above added to shearwall cord force reaction from 2nd floor diaphragm in compressionWest Wind affect to Horizontal DiaphragmCollector (strut)ChordWest Wind Horizontal Diaphragm affects to North/South ShearwallsCompresionTensionA closer look at the North ShearwallWind forces normal to the wallDesigned from top to bottomConstructed from bottom to topSideviewShearwall anchoragePlywood diaphragm details6 spacing at supported edges12 spacing in the fieldPlywood on exterior walls5/8" PLYWOOD3/4" FINISH FLOOR1/2" SHEETROCK OVER 1/2" STRAPPING7'-2"8'-1 1/2"1/2" PLYWOOD2X10s2X10sPlywood installation to exterior wallsCONVENTIONAL LUMBERHORIZONTAL JOINT DETAIL AT FLOOR LEVELALLOW FOR SHRINKAGE WHEN USING CONVENTIONAL LUMBERPLYWOOD SHEATHING1/2" GAPGALV.

8 Z FLASHINGHORIZONTAL JOINT DETAIL WITHIN WALLBLOCK BEHIND HORIZONTAL PANEL JOINTS OF SHEATHING FOR ALL SHEAR WALLSPLYWOOD SHEATHINGGALV. Z FLASHING1/8" GAPF oundation bracing (walk-out basement)STUD KNEEWALLUNBRACED AT TOP OF FOUNDATION - Design AS A RETAINING WALLF oundation drainageWaterproofingFilter fabricAddition on back of houseSliding and drifting snowPotential surcharge on existing foundation wallAdding a shed dormerAdding a second floorClosing in a 3-season porch Consideration of added sail area. May need to reduce size of windows or provide a connection that will not translate at the roof. Don t forget the roof Research at Virginia Tech. University, Department of Wood Science and Forest Products (see resources, Load-Tested Deck Ledger Connection ) Loads on decks consideration of size new codes will require 100 psf for decks over 100 SF.

9 Snow drift & sliding? Firewood? Planters? Long-term loading such as planters more critical than snowPressure Treated Wood The Z-Max is recommended by Simpson Strong-tie Stainless steel may be an option No posted connection capacities Limited available types ~ 4 X $Built-up Column2-2x4 studs fastened together for a column = 1-4x4 column ~ 60% less capacityThey don t build em like that It s against the house framingMortise and tenon cut into 6x85x4 @ 24" ledgerInstall joist hangersMay require additional supportResources (slide presentation location) Massachusetts State Building Code, 6thEdition, 780 CMR version; a copy Wood Frame Construction Manual for One- and two-family dwellings , American Forest & Paper Association & American Wood Council Design of Wood Structures , D.

10 Breyer, K. Fridley, & K. Cobeen Design / Construction guide Diaphragms and Shear Walls , APA The Engineered Wood Association The Journal of Light Construction Load-Tested Deck Ledger Connection , The Journal of Light Construction , March 2004 Fine Homebuilding International Building Code, 2003 International Residential Code, 2003 Chabot, EngineeringMelrose, Massachusetts(781) 665-7110(781) 665-7727