Bracing - BRANZ Build

3 Wind zones andNZS 3604OFTEN WIND DETERMINES THE BRACING REQUIREMENT FOR TIMBER-FRAMEDBUILDINGS. WE WALK THROUGH HOW TO FIND THE CORRECT WIND ZONE FOR ASITE USING NZS 3604:2011.FOUNDATIONS AND WALLS of timber-framedbuildings must be braced to resist the horizontalforces from earthquakes and wind. When designing bracing, calculations of both earthquake andwind forces (called bracing demand) must bemade and the building constructed to withstandthe stronger of the calculated forces (calledbracing capacity). Although New Zealand lies in aregion of high seismic activity, it is often the horizontal forces imposed by wind that determinethe bracing requirement.The shape, size and level (whether basement,ground or first floor) of the building, as well asits actual location, all affect the wind bracingdemand, but in order to calculate the bracingdemand, the wind zone, rated as low (L) to extrahigh (EH) wind speed, must first be determined.Figure 5Built-up residential areas are generally defined as urban.Six steps to determine wind zoneA means of determining the wind zone for aspecific location is in NZS 3604:2011 Table 5.1 (seeTable 1). This describes a six-step process.Step 1 – Wind regionThe first step is to identify the wind region for thebuilding from NZS 3604:2011 Figure 5.1. This mapdivides the country into two wind regions – A andW – based on wind speed data from the NewZealand MetService.The regions are too general, however, as landformations can modify and create significantshelteredexposedlocalised variations to wind speeds. For example,wind speed will increase as it passes over andbetween hills and decrease when passing overrough ground.8 — Build — BracingFigure 6Sites adjacent to an open space such as a playing field are defined as exposed.The buildings more than two rows back are defined as sheltered.

Step 2 – In a lee zone?Table 1Determine if the site is in a lee zone. These areDetermine the ground roughness from the twoPROCEDURE FORDETERMINATION OFWIND ZONESoptions defined by NZS 3604 paragraph 5.2.3:(NZS 3604:2011 TABLE 5.1, PROVIDED BY STANDARDS NEW ZEALAND UNDER LICENCE 001083.)shown as hatched areas in Figure 5.1. Lee zonesmay have higher wind speeds.Step 3 – Ground roughness Urban terrain – more than 10 obstructionsover 3 m high, such as houses or trees, perSTEPSACTIONREFERENCEVALUES AVAILABLE1Determine wind regionFigure 5.1A, W2Determine if in a leezoneFigure 5.1See Table 5.43Determine groundroughnessParagraph 5.2.3Urban terrainOpen terrain4Determine site exposureParagraph 5.2.4Sheltered, exposed5Determine topographicclassFrom Tables 5.2, 5.3 andFigure 5.2Gentle to steep6Determine wind zoneTable 5.4L, M, H, VH, EHhectare. Open terrain – open areas with only isolatedtrees or shelter, such as adjacent to fields orbeaches and open bodies of water.Generally, any built-up residential area (seeFigure 5) or any forested area will be definedas urban. A site adjacent to farmland or otheropen space will be defined as open terrain.Where a site is within 500 m of the boundarybetween urban and open terrain, it must beconsidered as open terrain.Step 4 – Site exposureDetermine site exposure from the two optionsin paragraph 5.2.4: Sheltered – a site surrounded by at least twoStep 5 – Topographic classas the region beyond a crest where therows of obstructions that are permanent,Determine the topographic class (T1–T4), fromgradient is less than 1 in 20.similar in size and at the same ground level.Table 5.2 and Figure 5.2 (see Figure 7).Exposed – a site that is steep (as defined inTable 5.2) or adjacent to an open space suchas a playing field (see Figure 6) or beach or Next, determine the smoothed gradient fromFigure 5.2. This requires the gradients of theIf not flat ground, determine if the ground is:upper part of the hill to be considered: adjacent to a wind channel that is more than100 m wide. This consists of a number of steps (see Table 5.2): a hill – land rises to a crest or high point The smoothed gradient of the hill is assessedthen falls again on the other sideover the horizontal upwind distance betweenan escarpment – a steep slope or cliffthe crest of the hill and the lesser of three timesComment C5.2.4 states that typical suburbanseparating two relatively level regions ofdevelopments on flat or near-flat ground areground that are at different elevations. Notegenerally classified as sheltered (see Figure 5).that NZS 3604 5.2.5 defines an escarpmentthe height of the hill (H) or 500 m (L). The smoothed gradient is the elevation (h)divided by the relevant distance (L).Build — Bracing— 9

Figure 7Topography (including escarpment conditions). NZS 3604:2011 Figure 5.2.(Provided by Standards New Zealand under licence 001083.)The topographic class (T) must be determinedCalculate wind bracing demandH (height of hill from crest to valley floor) 180 m.from Table 5.3. In example 1, with steepThe wind zone can now be applied to calculateL the lesser of 3H or 500 m, 3H 540, so L isgradient in outer zone, the topographic classthe wind bracing demand from NZS 3604:2011,500 m.is T3; with the low gradient in outer zone, theTables 5.5, 5.6 and 5.7. These tables give windSo, if h (elevation of the site) 100 m, h/L topographic class is T1.bracing demands (BU/m) for the subfloor struc-Example 1: 100/500 0.2 or 1:5. Therefore, the gradient ofIf the site does not fall within an outer or crest zone, itture and the walls of single and upper floors andthe site is ‘steep’ (from Table 5.2).is classified as T1, but there are some exceptions.the lower of two-storeys.Or if h 50 m, h/L 50/500 0.1 or 1:10. There-Step 6 – Now find wind zonefore, the gradient of the site is ‘low’ (from TableIt is now possible to determine the wind zonefor the relevant wind zone is used to calculate the5.2).from Table 5.4 using the information gathered –correct wind bracing demand. Where the zone is not high (H), the multiplierDetermine the location of the site as T1 (valleywind region, ground roughness, topographic classfloor), outer zone or crest zone. In example 1,and site exposure.Table 5.4), the wind zone is SED or specificthe building is located 250 m from the crest ofExample 2:engineering design and is beyond the scope ofthe hill, which is more than H ( 180 m) so it isFrom Table 5.4, a site in region W classified as T4NZS 3604.outside the crest zone. However, it is within the(moderate crest zone), urban and exposed, is in windouter zone ( 500 m).zone EH (extra high wind speed – maximum 55 m/s).10 — Build — BracingWhere wind zone is above extra high (from

Appraisal No.831 [2013]

Topographic zones4A READER ASKS, ‘HOW DO THE NZS 3604:2011 TOPOGRAPHIC ZONES WORK?’. WITHMORE BUILDINGS BEING CONSTRUCTED ON EXPOSED SITES, THIS IS AN IMPORTANTQUESTION TO UNDERSTAND.crestL 3H or 500 m (whichever is less)h change in elevationvalley floorsmoothed gradient Figure 8H heightfrom valleyfloor to cresthLSmoothed gradient.Then smoothed gradientWE ALL KNOW from experience that hilltops (andMost of New Zealand’s hill country is ‘spur/other exposed locations) have higher wind speedsgully’ formation where the land drops away onThe next step is to determine the slope of the hillthan the valley floor, and the topographic classesboth sides of a hilltop, ridge or spur. This is a ‘hillor ‘smoothed gradient’. This is also big pictureT1 to T4 are a measure of just how much higher.shape’ in NZS 3604-speak.stuff, and contours from a typical site survey willHowever, around the coasts or beside largerarely extend far enough. The best source ofStart with shape of groundriver valleys, there are often ‘escarpments’information is a large-scale contour map or anThe first step is to stand back and get anwhere the water has cut away one side of theonline tool such as Google Earth.overall picture of the shape of the groundhill and the other side is relatively flat. Note thatsurrounding the site. Don’t get into too muchif the ground comprises undulations of less thandetail. This is big picture stuff and is best done10 m (height of a 3-storey house) or is flatterby a site visit.than 1:20, the topographic class is T1.12 — Build — BracingThe hill slope is measured over either: a distance from the hill crest of 3 height ofthe crest above the valley floor (H), or 500 m, whichever is less.

crest3H 300 mL 200 m50 mhH 100 mFigure 5.2 of NZS 3604:2011 Timber-framed buildings is misleading here, and an alternative is givenin Figure 8.The smoothed gradient is h/L. Where thedistance L extends from the crest up the next hill,valley floorIn this example, smoothed gradient should be:NOT 50/300 0.17 moderate100/200 0.5 steepas can sometimes happen in steeper country (seeFigure 9), take L as the distance to the valley floor.Figure 9Determining topographic zone in steep hillside.Position of buildingNext consider the position of the building site inrelation to the crest of the hill (or escarpment): T1If it is within distance H (or 2H downwind foran escarpment), it is in the ‘crest zone’ where2HHcrest zoneouter zoneHHouter zoneT12Hwind acceleration is a maximum. If it is between 1H and 3H from the crestcrestHill shape(or between 2H and 6H downwind for anescarpment), it is within the ‘outer zone’. If it is more than 3H (or 6H for an escarpment),it is T1 because wind acceleration is notsignificant.Figure 10Building sites adjacent to a crest.See Figures 10 and 11.Note that row 4 in NZS 3604 Table 5.2 isirrelevant for topographic class and should beignored – it fits into Table 5.4.T1Note also that the entry for ‘steep’ in Table 5.22Hshould have no upper limit.crest zoneouter zoneH2Houter zoneT14HHNow the topographic classcrestFinally, the topographic class T1 to T4 is deter-Escarpmentmined from Table 5.3 using the informationdetermined above.Figure 11Building sites adjacent to an escarpment.Build — Bracing— 13

Subfloor bracing5NEXT WE WORK THROUGH THE BRACING CALCULATIONS FOR A SUBFLOOR EXAMPLE.5.600upper floor level4.8006.50030 3001.9007.1004.2001.80030 lower floor levelFigure 12Elevation of example house.THE HOUSE BEING USED in this example has aGross floor plan area for:Table 5.5), H 7.1 m, so round up to 8 m, andsecond storey on part of the house (see Figures2-storey 10.6 5 53 m²h 1.8 (round down to 1 m, this is a higher BU12–13).1-storey 8.1 9.3 75.3 m² (for simplicity, therequirement).area has not been reduced for the entry porch).Data for this exampleOnce the demand is established, the overlap ofSingle-storey to apex H 4.8 m, h 1.9 m.Roof type and building dimensionRefer to pages 4–6 for how to establish these values.the 2-storey will be deducted from the 1-storey.The 2-storey has a gable roof with 300 mm sof-Wind zone: MediumSoil type: Rockfit/verge.Earthquake zone: 2Weight of claddings: Light subfloor, lower storey,Floor plan areaupper storey and roofon the 2-storey part of the building, use theThis example has a mixture of single and doubleRoof pitch: 30 degrees, so choose 25–45 degreesoverall dimensions of the roof for the width andstoreys. Because these have different wind andBuilding shape: Subfloor has no wings or blockslength.earthquake demands, two calculations are re-Heights for buildingquired – one for the subfloor area of the 2-storey2-storey to apex H 7.1 m, roof height aboveLength 10.6 0.300 0.300 11.2 mportion and one for the subfloor area of theeaves h 1.8 m.Width 5.0 0.300 0.300 5.6 m.single-storey (shown in Figure 14). The slab floorNote: Where heights don’t exactly match theAs the roof is over 25 , when considering windSo, 2-storey section building dimensions are:Single-storey dimensions are:in the garage has no subfloor so does not formtable, use the next highest bracing unit (BU).Length 9.3 m (no soffit to lower level)part of the calculation.For example, in the subfloor structure (usingWidth 8.1 m (no soffit to lower level).14 — Build — Bracing

M8.1000N7.040Transfer these values to the calculation sheets(Figures 15 and 17).garageNote that, because this is a hip roof shape,wind demand in both the along and across6.200directions is the same, so choice of length andwidth is not critical.Bracing calculation sheetsThe above data is then entered into bracingwww.branz.co.nz.11.200loaded from the Toolbox on the BRANZ websiteroof line(lower level)2-storey sectionconcreteslab floor3.100(see Figures 15 and 17). Sheets can be down-13.700calculation sheets to obtain the bracing demand3.100Using the calculation sheets (see Figure 15),bracing demand for the 2-storey section is: 1176 BUs for wind across the ridge 627 BUs for wind along the ridge 636 BUs for earthquake.extent of upper levelroof line(upper level)5.0005.600Use 1176 BUs for wind across and 636 for bothwind along and earthquake.Single-storey sectionBracing demand results for the single-storey areaFigure 13Floor plan of example house.(see Figure 17) are: 521 BUs for wind across 454 BUs for wind along 603 BUs for earthquake.MN0extent of concrete slab floorAUse 603 BUs for along and across as it is thehigher value in both directions.Choose bracing elementThe subfloor is 600 mm or less high. Anchor pileshave been chosen as the subfloor bracing ele-dualpile area(singleand 2storey)Bment as they are rated as 160 BUs for wind and120 BUs for earthquake.CMoving to the bracing lineslower levelsuspended floorFor this example, the exterior walls will be usedas bracing lines in each direction along with theDanchor pile: single storeycommon wall between the garage and the house.These are within the 5 m rule and provide an evenupper levelsuspendedfloordistribution of bracing throughout the building.We now need to calculate the minimumbracing needed in each line and check the bracingdistribution complies with the requirements ofoutline of lowerfloor levelEanchor pile: two storeyanchor pile: for dual pile area(single and two storey)ordinary pileNZS 3604:2011 clause 5.5: maximum spacing of bracing lines in thesubfloor 5 mFigure 14Foundation plan.Build — Bracing— 15

Figure 15 Calculation sheet for demand – 2-storey section ofsubfloor.Figure 16Calculation sheet for bracing achieved – 2-storeysection of subfloor.minimum capacity of subfloor bracing lines isMinimum bracing for single-storey sectionplate on the lowest floor to the top of the roof).the greater of:Using the calculation sheet (see Figure 18) gives:In this example, width 5 m 1.7 8.5 m, so this 100 BUs 1080 BUs for earthquake bracing acrossdesign is OK as the height is 6.5 m from underside 15 BU/m of bracing line 1080 BUs for earthquake bracing along.of bottom plate to top of roof. 50% of the total bracing demand, dividedThis meets the minimum demand requirementsby the number of bracing lines in thefrom the calculation sheet (see Figure 17) andbraces (NZS 3604:2011 clause 5.5.6) – adirection being considered.NZS 3604 clause 5.5.2.minimum of four braced or anchor piles placedSee Table 2 where this has been worked through.The piles in brace line N are staggered toThere is also a minimum number of subfloorin each direction symmetrically around theMinimum bracing for 2-storey sectioncomply with the requirement that braced or load-perimeter. Wherever practical, they should beUsing the calculation sheet (see Figure 16)bearing walls are within 200 mm of the pile line.placed near a corner. This design has five pilesgives:in the across direction and nine in the along 1280 BUs for wind acrossMore to check 960 BUs for earthquake and along.Buildings where the height exceeds 1.7 timesdirection so is OK.NoteHaving trouble reading FiguresThis meets the minimum demand requirementsthe width must be on a continuous foundation15–18? You can download these with thisfrom the calculation sheet (see Figure 15) andwall (NZS 3604:2011 clause 5.4.3.2). Height isarticle from www.buildmagazine.co.nz thenNZS 3604:2011 clause 5.5.2.measured from the underside of the bottomThe Right Stuff.16 — Build — Bracing

Figure 17Calculation sheet for demand – single-storey sectionof subfloor.Figure 18Calculation sheet for bracing achieved – single-storeysection of subfloor.Table 2MINIMUM BRACING NEEDED IN EACH LINE2-STOREY SECTIONSINGLE-STOREY SECTIONWIND ACROSS RIDGEBracing linesB, C, D and E 5 m longA, B, C, D 8.1 m longBracing demand per line(greatest value)100 BUs or75 BUs (5.0 x 15 BUs) or147 BUs (1176 BUs divided by 2 588 divided by 4 lines)100 BUs or122 BUs (8.1 x 15) or76 BUs (603 BUs divided by 2 301.5 divided by 4 lines)Minimum BUs per line147 BUs122 BUsMinimum anchor piles per line1 anchor pile 160 BUs (wind)2 anchor piles 240 BUs(120 each for earthquake)Bracing linesM and N 10.6 m longM, N, O 9.3 m longBracing demand per line(greater value)100 BUs or159 BUs (10.6 x 15) or159 BUs (636 BUs (for earthquake) divided by 2 318 divided by 2 lines)100 BUs or140 BUs (9.3 x 15) or100 BUs (603 BUs divided by 2 301.5 divided by 3lines)Minimum BUs per line159 BUs140 BUsMinimum piles per line2 anchor piles 240 BUs (120 each for earthquake)2 anchor piles 240 BUs (120 each for earthquake)WIND ALONG RIDGEBuild — Bracing— 17

6Bracing forsuspended floorsHERE ARE A FEW POINTERS FOR INTERPRETING NZS 3604:2011 BRACING PROVISIONSFOR BUILDINGS WITH SUSPENDED SUBFLOOR STRUCTURES.DESIGNERS WILL HAVE NOTICED that there is asubstantial increase in bracing demand frombrick veneer claddingbuildings on slabs to those on suspended floors.This ranges from about double the demandfor walls of single-storey buildings to about a30% increase in demand for walls of 2-storeybuildings.This increase is due to the additional seismicweight of the suspended floor and its contentsfloor joists(people, furniture and so on), and the greatereffect of earthquake ground movements onsuspended floors.Experience from Christchurchbearer on DPCObservations in Christchurch after the earth-concrete half pilequakes clearly showed that piled buildings with aperimeter foundation wall of concrete or concretereinforced concrete footingmasonry performed very well, even when therewas ground disturbance due to liquefaction andlateral spreading.Figure 19This is because of the bracing effect of theSuspended floor structure with semi-detached or half pile.perimeter foundation wall, together with thefloor acting as a diaphragm.Bracing design advice if the suspended floor structure is notAfter discussions with practitioners, BRANZconnected to the perimeter foundationGap in NZS 3604advises:(for example, the semi-detached pile inNZS 3604:2011 provides two sets of tables for earthquake bracing demand: Table 5.10 for buildings built on a concrete slab. Tables 5.8 and 5.9 for buildings on asuspended floor structure. if the building is on a slab, use NZS 3604Figure 19 – a common construction detailTable 5.10for older timber-framed buildings), thenif the building is all piled, use NZS 3604 Tableconservatively Table 5.8 or 5.9 should be5.8 (single-storey) or 5.9 (two-storey)used. Structural engineers experiencedif the suspended floor structure is wellin timber-framed constructi

Bracing for steps in floors and ceilings 21 8. Wall bracing 26 9. Walls at angles to bracing lines 27 10. Roof bracing 30 11. Bracing ratings . magazine looks at the bracing requirements for . floors or ceilings