60 BUILD August/September RESEARCH. NZS review preparation. The current technical review of our timber-framed building standard is limited by. period, both NZS and NZS will be Acceptable Solutions. -Under this licence use of both the PDF file and your single permitted printed. BRANZ Engineering Basis of NZS ACKNOWLEDGEMENTS Writer Roger Shelton Design and layout Paul Brittain-Morby ISBN (pbk).
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An introduction to using NZS Timber-framed buildings for structural design on 'good ground'. Download a free copy [PDF]. Superseding NZS Page 2. COMMITTEE REPRESENTATION. This Standard was prepared under the supervision of the P technical committee . BRANZ Engineering Basis of NZS 1 INTRODUCTION. 8. Objective of this book. 9. Background. 9. Status of NZS in the building control.
The value stated is load per metre of balustrade length. Proprietary tested subfloor bracing elements are also provided for. A safety factor of 5 is therefore recommended to obtain a safe working load when using the Hiley formula for pile sizes covered by NZS Prentice Hall. Loads Construction Load kPa Light roof including framing and ceiling 0.
This method defines two types of engineered foundations — shallow and deep foundations. Back to top. NZS type foundations This standard provides prescriptive details for constructing seismically resilient foundations. Topics Seismic science and site influences Resilient design Superstructure Foundations Overview Foundation performance Residential foundations NZS type foundations Engineered foundations Shallow foundations - residential Deep foundations - residential Commercial foundations Building envelope Strengthening strategies Non-structural systems.
The standard provides prescriptive details for constructing seismically resilient slab-on-ground foundations and pile foundations based on: Any soil or rock capable of permanently withstanding an ultimate bearing capacity of kPa … but excludes: Testing for good ground As a simple guide, signs that may indicate the presence of good ground include: Relevant resources NZS Seismic science and site influences Resilient design Superstructure Foundations.
Building envelope Strengthening strategies Non-structural systems. Seismic science and site influences Resilient design Superstructure Foundations Building envelope Strengthening strategies Non-structural systems. The coverage of the demand tables takes these permutations into account. For Table 5. Practical subfloor and storey heights and building widths will limit roof heights and pitches for buildings that are at their maximum permitted height.
For Tables 5. The building width on which to base the roof pitch and therefore its Cpe value for wind across the ridge has been taken as 7. The equations used are tabulated opposite.
Kl and Kp were all taken as 1. The governing equation for seismic base shear is equation 6. The NZS Demand tables were only produced for zone 3. Zone Hazard factor Z 1 0. NZS has a range for Z between 0. To accommodate this greater range of hazard.
Soil types NZS provided for three soil categories — rock. Zones were allocated numbers rather than letters as previously to avoid confusion with the soil classes provided in NZS The adjustment factors were combined with the soil class adjustment factors see below.
These classes result in a difference in demand for low-rise timber-framed buildings of about 1. This value should be used where equivalent specific designs are being undertaken for timber-framed buildings outside the scope of NZS Horizontal design action coefficient The horizontal design action coefficient Cd T1 is given as: The decision made was to produce demand tables for soil class E along with a table of reduction factors for other classes as an incentive for users who took the trouble to identify soil class.
It was expected that. Using the parameters above. For simplicity. Vertical distribution between levels used the provisions of NZS Based on the floor area of m2.
Degree of participation is influenced by lack of connection with the structure. For the purpose of deriving the bracing demand. Some resistance to accidental eccentricity and torsion of the building as a whole is provided by the limits on distribution of bracing lines and bracing elements in clauses 5. These values are designed to produce good structural performance without unnecessarily compromising architectural planning options. Minimum capacities for internal and external brace lines are set by clauses 5.
As architectural styles moved towards open planning. At the same time. They recognise that typical timber-framed ceilings and floors cannot provide the rigid diaphragms normally assumed in structural engineering design.
In addition. Anchor piles Anchor piles began as short. The maximum cantilever height above ground for anchor piles has been increased to mm to give more flexibility with floor and ground levels. The cantilever strength of the shallow founded option is limited because of the minimal size of the footing.
Cantilever piles Cantilever piles were initially intended to be either shallow piles bedded in a concrete footing or timber piles driven to a specified set to provide adequate vertical loadbearing through softer ground. In This is addressed in the standard for timber piles and poles.
Tests on typical brace assemblies under cyclic loading Wood et al. The necessity to describe details more and more precisely for a wider variety of applications increased the demands of this section. The option of extending them up so they could be directly connected to the bearer was first introduced in The resulting bending moment demand on an anchor pile has meant that they are loaded beyond the capacity of visually graded timber and are required to be proof tested.
Braced piles Braced piles started out as diagonal braces that could be attached to a wide variety of elements at each end a pile. The rating for braced piles was then derived from that basic 12 kN load. The committee increased the ratings for these elements accordingly. Ultimate pile top deflections of between 30 mm and 50 mm were recorded in these tests. Elastically responding structures. Lateral load tests were undertaken on anchor piles.
F2 was based on the spectral coefficient from Table 4. For the revision of NZS onwards. The resulting parameters are tabulated as follows: This gives: The P21 test has enabled the rating of many proprietary bracing systems and also revealed the inefficiency of many of the generic systems. This prevents the use of elements whose applied load would be greater than the tested load. While dynamic considerations would indicate that overturning stability of foundation walls will never be an issue.
The longer the wall becomes. BRANZ research Thurston indicates that there may be shortcomings in the current provisions for timber plate fixings. The provisions for subfloor ventilation and access crawl space encapsulate typical good practice and have remained essentially unchanged since Beyond these heights.
In practice. Section 5 also covers the arrangement and disposition of bracing elements so as to match the applied loads. This ranges from mm for anchor piles to a maximum of 3 m in the case of timber ordinary piles.
The complete subfloor structure is specifically required to resist vertical loads gravity and horizontal loads from wind and earthquake actions. Several of the foundation elements have also been allocated a bracing resistance. Details are provided in this section for a variety of vertical support elements piles.
Proprietary tested subfloor bracing elements are also provided for. The loads were applied as a uniformly distributed load over the tributary area given by the product of the span of bearers and joists in Table 6. The height limits for anchor and cantilever piles match the proof loading provisions of NZS It was assumed that approximately half the live load in a house is furniture and therefore should be considered as a permanent load with a safety factor of 3.
This equates to an ultimate bearing capacity of kPa using a safety factor of 3. Because the remaining portion of the live load is considered to be of short duration.
The upper limit of 3 m for timber piles is to prevent excessive slenderness. An independent check by ultimate strength methods on the foundation sizes done for the revision did not justify changing the table. The table values are based on working stress design WSD methods assuming that the soil has a safe bearing capacity of kPa. The lower height limits for timber piles in clause 6. The piles were loaded vertically. Set per blow mm Ultimate load tonnes 25 6.
Dead load G Live load Q Light roof cladding 0. The piles were 1. Loads Loads used to obtain pile spacings in Table 6. Chapter 9. A safety factor of 5 is therefore recommended to obtain a safe working load when using the Hiley formula for pile sizes covered by NZS Roof loads were based on an eaves width of 0.
It should be noted that the Hiley dynamic pile-driving formula provided ultimate loads well in excess of those measured. Because there is no lining attached to a jack stud.
The details may be found in Cocks et al. For the revision Amendment 2. The selection table for jack studs Table 6. Load cases considered: No further changes were made to the revision. These provisions were not calculated but represent common sensible practice.
No restraint was assumed to each end of the member. P stud height P Figure 2. Lateral support may also be provided by soil pressure at wall steps. Dry timber properties were used. The maximum length of the jack stud was calculated iteratively.
Figure 6. The stiffening effect of the wall above. New tables were developed for the revision Amendment 2. The reason for this change is unknown. Loadbearing walls above either parallel or perpendicular to the bearer were not taken into account. Clause 6. Loads Dead load of floor G 0. The bearer tables include loading from the ground floor only.
Structural model used for bearers Load was applied to both spans as a uniformly distributed load perpendicular to the member. No allowance was made for load sharing. The load duration factor k2 was taken as 2.
Loads Loads were as given above for strength design. Floors designed for 2 kPa loading are most likely to be used for decks and exposed to the weather. Dry timber properties from NZS were used for 1. Load cases considered were: Load was applied to both spans together as a uniformly distributed load. Timber properties are set out in section 2. Deflection calculation The average modulus of elasticity E was used to calculate bearer deflection.
Table 3. Test specimens Bolts of the range of diameters suitable for stringers were cast in to ready-mixed concrete of nominal strength Results A summary of the test results is given in Table 3 of this book.
Ten specimens were tested for each bolt size and direction of load in relation to the grain direction. Moisture content and density were measured on the timber components of the test specimens.
Green radiata pine stringer sections were attached to the concrete via the bolts using 50 mm square washers and the nuts tightened finger tight. Test method The concrete blocks were attached to the platen of a universal testing machine.
The range of specimens tested is given in Table 3 of this book. The specimens were then set aside and allowed to dry out. The density of the timber components is of the order of the density to be expected of structural timber for light-framed construction. None of the concrete blocks failed at these deformations.
At 5 mm. Table 6. Hence it was recommended that the allowable loads be the average of the test loads at 2 mm deflection. A check of timber bending under the ULS was done for Amendment 2 using the timber properties of section 2. A ground floor may be a concrete slab on grade or of timber joist construction supported on a subfloor structure.
As far as the floor plate itself is concerned the flooring and the joists and associated framing members. The main structural function of a suspended floor is to resist gravity loads perpendicular to its own plane. The subfloor structure. These aspects are discussed in detail in section 4 of this book. Where bracing lines below the floor are spaced within 6 m centres. Timber ground floors may be up to 3 m above the ground.
In the balcony situation Figure 4a of this book. The alternative of notching or ripping the joists to achieve this invalidates the timber grading and compromises the H3. It is clear that buildings coming within several of the occupancies included in NZS for example. Secondary members such as blocking are required to provide lateral stability to the joists and support edges of floor sheeting. Floor live loads are set out as part of the overall scope of the document in Table 1.
Floor joists may also be required to support ceiling linings either directly or via ceiling battens and interior walls running either parallel or perpendicular to the joist direction. Cantilever joist sizes smaller than x 45 mm SG8 provide insufficient strength to resist these forces so are identified in the notes to Table 7.
Table 7. With ready-made solutions in NZS available for floors and other building elements. Bearers and below are considered part of the subfloor structure and are covered in section 4 of this book. Cantilever floor joists may be used in three situations. Configurations for cantilevered floor joists Structural models used for strength The structural models used were a simply supported.
The point load per joist depends on joist spacing. Case Simply supported joists Cantilevered joists 1 1. Back span length was taken as 1. The value stated is load per metre of balustrade length. Loads on the back span were taken as zero for ULS considerations. Loads The loads used in the derivation of the tables were: Internal floors External decks and balconies Superstructure enclosed Dead load 0.
Timber decking or plywood with a fibre-cement soffit comes within this limit. Stresses as used are summarised in Table 4 of this book.
Duration of load factors: Load case k1 1 0. Characteristic stresses Dry stresses from NZS were used for 1. Table 4. For the deeper joists x 45 mm at maximum span. No 1 framing dry 1. Loads Loads were the same as used for strength design. If the floor is specifically designated as a diaphragm. This has the effect of reducing the free-end deflection of the cantilever. Concentrated loads. Under some loading situations.
The back span joists were assumed to be the same timber section as the cantilevered portion of the joists in internal situations columns 3—8 of Table 7. For beams continuously restrained by flooring. It is expected to be very conservative and restrictive for joists over x 50 mm in size. For cantilever joists. Floor joists supporting walls clause 7.
To transfer lateral loads from the floor acting as a diaphragm into bracing elements below the floor. Right angles to joists The basis of the mm positional restriction for bearing walls at right angles to the joists clause 7.
The provisions are a simplified version of those contained in NZS Short-term and long-term load factors: To provide lateral stability to the floor joists acting as beams clause 7. Static deflection calculation Floor joists were considered to be constrained to similar deflections, as noted in commentary clause C2.
The duration of load factor k2 was taken as 2. An absolute value was not used. Vibration considerations Vibration criteria were introduced with the version of NZS as a result of submissions by Forest Research Bier , which concluded with a set of amended spans. These were adopted for joist spacings of mm and sizes x 50 mm and smaller. Forest Research analysed a simply supported, uniformly loaded timber joist floor system with a width of 4.
Formulae for estimating natural frequency and RMS acceleration were based on equations from Chui and Smith Criteria chosen were that natural frequency should be above 12 Hz and RMS acceleration should be less than 0.
This work was based on No 1 framing timber, so to adjust the joist spans for the other timber grades for Amendment 2 in , equation 2 of Chui and Smith was used to keep acceleration Ar constant:. So to keep Ar constant, for two similar floors differing only in span and E: This expression was then used to adjust all spans where vibration was the limiting case.
Where bracing lines below the floor under consideration are spaced at greater than 6 m, the floor is specifically designated a structural floor diaphragm and must comply with the details of clause 7. This situation may occur, for example, where an upper floor spans a large open space or where a ground floor is supported on unbraced piles but surrounded by perimeter foundation walls.
The additional requirements are fairly modest and, in practical terms, merely restrict the diaphragm span and aspect ratio and prohibit the use of strip flooring. Also, there are slightly more onerous requirements for lateral support of the floor joists and bracing details around the diaphragm perimeter in the storey below. However, there are no specific provisions to resist the diaphragm chord forces.
Joints between joists, as provided for in clause 7. Alternative load paths utilising other structural members for example, wall plates are tenuous.
Considering these limitations on potential diaphragm performance, the committee arbitrarily reduced the maximum diaphragm dimension from 15 metres to 12 metres for the revision of the standard. This was done in recognition that a 15 m open space is very large for a non-specifically designed timber-framed building and would probably require an engineer-designed floor support system anyway floor beam or EWP joists.
Floor diaphragm to cover entire floor area see 7. Diaphragm size see 7. Single storey maximum length must not be greater than 2. Double storey maximum length must not be greater than 2.
See section 4 for durability requirements. Clause 7. The details were a mixture of standard practice and common sense, and the scope and contents of Appendix E remained largely unchanged until Amendment 11 makes several changes to the provisions for floor slabs in NZS To avoid design issues with movement at control joints reflecting through tiled flooring. Control joint spacing is required at 3 m centres for unreinforced slabs option removed by B1. These changes cover the whole of New Zealand.
The reason for the changes is to provide more robust and ductile details into concrete floor slabs. For the revision of NZS The basis is not known but is probably based on a rule of thumb. The revision further clarified and expanded these anchor details. The aspect ratio limits are intended to prevent long narrow panels. These provisions remained unchanged through to the revision.
This limit is to avoid unintended cracking across the corner. Details of the slab thickening under loadbearing internal walls were taken from NZPCA for wall loads of up to 2. Slabs for two-storey construction required reinforcing by mesh or reinforcing bars. Details of these changes should be read from Amendment 11 to clause B1 and are not covered in detail in this book. The option of bent dowels has been removed.
Bracing applications are specifically excluded from these values because the demand on the anchor in that application can only be quantified by specific testing on the proprietary bracing element being evaluated. On request. The importance of avoiding random cracking in a residential situation is more critical when the slab is to be exposed or covered with vinyl or ceramic tiles.
In the revision of NZS The resulting strain is relieved by cracking induced at the control joints. The rationale was that. To establish a benchmark strength for a minimum level of general robustness. A free joint is defined as: The experimental part of the BRANZ study highlighted the poor performance of bolts installed in slabs whose edges are formed by masonry header blocks. Slabs longer than 24 m are now allowed. These values were slightly less than those in the version and were adopted.
The use of steel reinforcement is intended to limit the crack widths at the joints and distribute the cracking evenly between joints. In a typical residential floor slab. These loads are all provided for in the selection tables and detailed provisions for individual wall members. This nomenclature follows Australian practice. Walls that are specifically designed to contribute resistance to lateral racking loads are defined as wall bracing elements in clause 1.
External walls will be subjected to wind pressure loading. All walls will be required to resist face loads at some time in their life. Walls resisting vertical gravity loads are defined as loadbearing walls. No 1 framing Studs are required to carry vertical gravity loads from the supported roof and floor s and also to transfer wall face loading to the top and bottom plates.
Additional studs provided for the purpose of supporting the ends of a lintel are called doubling studs more simply called prop studs in Australia and are considered to contribute to the strength or stiffness of the trimmer stud if they extend up to within mm of the full wall height clause 8. Where studs are less than full wall height for example. Studs each side of an opening supporting a lintel or sill trimmer are referred to as trimmer studs and are of greater thickness to provide enhanced strength and stiffness.
Under wind loading. In the rare situation where a proprietary bracing system may not be available or desired. Trimmer studs at the sides of openings are also required to resist the concentrated bending loads resulting from the reactions at the ends of lintels and sill trimmers under face loading. The floor dead and live loads were based on contributory loading from an assumed joist span of 5. Both interior and exterior studs were considered.
The tables were originally developed on the basis of providing the most economical timber size that met both the strength and serviceability criteria for the specific loading conditions established by the table. Tables 8. Loads Gravity axial loading on studs Loads were calculated per metre of wall length and then multiplied by the required stud spacing to arrive at load per stud. A face load of 0. Restraints at mm were assumed for in-plane action. Partial fixity was accounted for by using an effective length factor k10 from NZS of 0.
Wind loading on the roof was not considered in conjunction with face loading on the wall so did not contribute to axial loading on the studs. This restraint is provided by either dwangs or lining fixings. Structural models used for strength design of studs The model is a beam column with partial end fixity due to square ends.
Structural model used for strength The models used for strength are shown in Figure 7 of this book. For uniformly distributed bending caused by wind face loading. Cpi was taken as 1. Structural model used for serviceability of studs The systems effect of the linings and claddings resulting in an increased stiffness of the studs in the wall was allowed for by effectively increasing the stud E value by a factor of 1.
Under dead load only and dead plus live load cases. Live and earthquake loads were not considered for serviceability. Loads Wind face loading on studs: An SG8 stud thus has an effective E for serviceability of 5. The bearing area factor k3 was taken as 1. Structural model used for serviceability The model used for serviceability is shown in Figure 8 of this book. This factor was derived from full-size face loading tests conducted on lined and clad 2.
This may be critical for short studs. Design for serviceability SLS General The deflection of the stud under wind face loading was the critical load case for serviceability. The spacing adjustment factor is 0. Table 8. To ensure that the first condition is met. The maximum spacing of the x 75 mm stud is 0. This is particularly relevant to the situation of studs in raking walls. The reduced stiffness of a smaller stud size may be compensated for by placing these studs closer together in proportion to the ratio of the smaller cross-section moment of inertia to the larger using equation 6.
What spacing must be used with a x 75 mm stud? The basis of the recommendations is as follows: T he stud size is determined by bending stiffness requirements only. They are supported by either a doubling stud or a check-out in the trimmer stud. Selection tables are provided for solid timber lintels only in section 8 of the standard. Wind loads Only the extra high wind zone was considered. Design for safety included consideration of the ULS in bending. Loads Gravity loads Component light.
External pressure coefficients Cpe: Composite lintels are covered in section In deriving the lintel selection tables. Above this. Structural model used for strength The model used for strength is shown in Figure 9 of this book. Load sharing by other members such as roof and wall framing was not considered.
W lintel span Figure 9. Load was applied to the member as a uniformly distributed load. A more robust nail strap connection between lintel and trimmer stud and between bottom of trimmer stud and floor.
Load transfer is provided either by the check into the trimmer stud or by the prescribed end nailing to the lintel through the trimmer stud. There is no specific attachment between trimmer stud and floor — a load path through lining or cladding was assumed.
S tandard fixings for lintel to trimmer stud as provided in the nail schedule in Table 8. Loads Gravity and wind loads are as given above for strength design. Connection capacity Provisions for securing of lintels against uplift were provided for two levels of connection capacity: The capacity of this system was assessed at 2 kN. The capacity of this system was initially assessed at 5 kN in WSD values.
Because the lintel loaded dimension is measured horizontally between these points. Only dry timber was considered. Overturning forces on a steep roof The steep roof multipliers of clause 8. The second effect is illustrated in Figure 10 of this book: Loads Construction Load kPa Light roof including framing and ceiling 0. Top and bottom plates are primarily designed to transfer and distribute vertical loads between rafters or joists and the supporting studs.
They support and distribute loads from walls. For this reason. There are no provisions for the support of trimmer joists or roof girder trusses.
Although linings or claddings are usually directly fixed to plates and can transfer loads directly to or from them. Face loads on the wall originating from wind pressure and earthquake inertial forces are applied to top and bottom plates as a series of reactions from the ends of the studs.
Load case k1 Dead 0. The next step was to derive the concentrated load to reach that capacity for each stud spacing and structural model. It was assumed that these loads are transferred to the orthogonal bracing walls by ceiling linings acting as diaphragms or by framing members spaced at 2. Plates are also required to transfer horizontal loads between the wall studs and roof or floor framing members.
Member capacity Member capacities in bending and shear were calculated for the plate configurations shown in Figure 11 of this book. Structural model used for strength The structural model used for the strength check was a two-span continuous beam with a concentrated load in two alternative positions within one span as shown in Figure 12 of this book.
No load sharing between rafters or floor joists was assumed. Structural models used for plates Loads The concentrated load P was derived from roof rafter loads or floor joist loads for top plates and stud loads for bottom plates. The resulting shear capacity was calculated from: Bending moments. Compression perpendicular to the grain was not considered. Shear strength fs was taken as 3.
This was used in earlier versions of the standard and was retained for Amendment 2.
Capacities of the combined top plate member. The values of gravity loads used are given in Table 7 of this book. According to Keenan Ground snow load sg was taken as 2. Refer to section 11 of this book for background on snow loading. The plate loaded dimension LD was then determined by subtracting the constant load components floor.
Wind loads were based on a design wind pressure p of 1. These provisions are based on good practice rather than rational engineering analysis. Deflection calculation Limiting plate loads were calculated using the lower bound modulus of elasticity Elb from NZS and are set out in Table 6 of this book.
Structural model used for serviceability The structural model used for serviceability was the same as used for strength Figure 12 of this book. Loads Dead and live loads used for the serviceability check were the same as for strength and are set out in Table 7 of this book. If a plate is connected to a diaphragm complying with clause 5. The same approach was used as for strength design.
They are designed to tie the walls together and provide rational load paths to bracing elements. If the adjacent ceiling is low density softboard. Only timber in the dry condition was considered.
Deflection was calculated including both bending and shear effects. To ensure a continuous load path down the wall. Any contribution from the lining fixings was ignored. Deflection criteria The maximum allowable total mid-span deflection including bending and shear was limited to 5 mm under long-term or short-term loading.