AS1684 SS N2 L Shaped House Tie-Down Example Nov 07

Tie Down ExampleTie Down Example

• Wind classification - N2• Sheet roof• Single storey• ‘L’- shaped• Gable & hip roofs

Understanding

AS1684Residential Timber

Framed Construction

Tie - Down Design ProcessTie - Down Design Process (Clause 9.3) (Clause 9.3)

1. Determine if specific tie-down required Table 9.2

2. Determine the wind uplift load width Clause 9.6.2 and Fig 9.5

3. Determine the uplift pressure Table 9.5

4. Calculate the wind uplift force Clause 9.6.4

5. Determine the Joint Group Table 9.15, Fig 9.6

6. Enter the appropriate design strength Tables 9.16 to 9.25

& select a suitable tie-down connection

7. Determine shear force Clauses 9.7.5 & 9.7.6

8. Enter the appropriate design strength Tables 9.27 to 9.30

& select a suitable shear connection

a) Use Tables 9.2 (uplift) and 9.3(shear) to determine if Nominal and additional Specific fixings are required.

AS1684.2 p166AS1684.2 p166

1. Determine if specific tie-down required

Specific connections

required

AS1684.2 p165AS1684.2 p165

No specific connections

required

1. Determine if specific tie-down required Cont.

b) Determine Nominal Fixing requirements. (Clause 9.5 & Table 9.4)

AS1684.2 p167/168AS1684.2 p167/168

1. Determine if specific tie-down required Cont.

The area to be held down via roof battens is calculated by multiplying the:

Batten Spacing x Tie - Down Spacing

resulting in the area of roof acting on the Tie - Down connection at that point.

c) Determine Specific Tie-Down Requirements

1. Determine if specific tie-down required Cont.(Roof battens to rafters/trusses)

Wind uplift area =

Batten Spacing x Tie-Down Spacing

Wind uplift area of roof acting on batten tie-down connection:

= 0.9m x 0.9m = 0.81m2

900

900

2. Determine the wind uplift load width/area(Roof battens to rafters/trusses)

NOTE: Whilst in this example the roof batten spacing has nominally been specified as 900mm, in high wind areas, manufacturers specifications for the sheet roofing may require closer batten spacing to ensure satisfactory performance of the sheeting.

Check manufacturers requirements

Roof battens fall into 2 categories -

1. Within 1200 mm of edges

2. General area

A different Net Uplift Pressure is applied to each category - Table 9.5 [pg 170]

3. Determine the uplift pressure(Roof battens to rafters/trusses)

3. Determine the uplift pressure(Roof battens – General areas and Edge areas)

3. Determine the uplift pressure(Roof battens to rafters/trusses)

Roof battens to rafters/trusses -

Within 1200 mm of edges.

Net Uplift Force =

Wind Uplift Area of roof x Net Uplift Pressure

0.81m2 x 1.84 kPa (kN/m2)

= 1.49 kN

4. Calculate the wind uplift force(Roof battens to rafters/trusses)

Roof battens to rafters/trusses -

General Area.

Net Uplift Force =

Wind Uplift Area of roof x Net Uplift Pressure

0.81m2 x 0.98 kPa (kN/m2)

= 0.79 kN

4. Calculate the wind uplift force(Roof battens to rafters/trusses)

Select the appropriate Joint Group -

38 x 75 unseasoned batten to seasoned pine truss.

Joint Group = JD4 (truss specification)

AS1684.2 p180/181AS1684.2 p180/181

5. Determine the joint group(Roof battens to rafters/trusses)

Select a Tie-down connection - [Table 9.25]

Batten to truss (within 1200mm of edges)

Uplift Capacity = 1.49 kN

Adopt - 1/75mm long, No.14 Type 17 screw

AS1684.2 p207AS1684.2 p207

1/90 mmlong 7.4 5.5 3.2 6.0 4.7 3.6

2/75 mmlong 11 8.4 4.8 9.0 7.2 5.4

Diagram (d)

38 x 75 or 38 50 mm batten

No14 Type17 screws as per table

Two screws shall be used only with 75 mm widebatten

2/90 mmlong 15 11 6.4 12 9.4 7.2

1/75 mmlong 5.7 4.2 2.4 4.5 3.6 2.7

J2 J3 J4 JD4 JD5 JD6

6. Select a suitable tie-down connection(Roof battens to rafters/trusses)

Screws(length)

Select a Tie-down connection - [Table 9.25]

Batten to truss (General Area)

Uplift Capacity = 0.79 kN

Adopt - 2/75 x 3.05 deformed shank nails

AS1684.2 p207AS1684.2 p207

Diagram (b)

38 x 75 or 38 x 50 mm batten

Nailed as per table

mm wideTwo nails shall be used only with 75batten

1/75 3.05 1.2 1.0 0.90 0.65 0.43 0.30

2/75 3.05 2.5 2.1 1.8 1.3 0.86 0.60

2/75 3.75 2.8 2.5 2.2 1.7 1.0 0.72

Deformed shank J2 J3 J4 JD4 JD5 JD6

Connection can be used throughout the roof area

6. Select a suitable tie-down connection Cont.

(Roof battens to rafters/trusses)

For all other connections (truss to wall frame, wall frame to floor), the following inputs are required...

i) Uplift Load Width

ii) Uplift Area

iii) Wind Uplift Force

iv) Joint Group

d) Determine specific tie-down requirements where appropriate.


i) Uplift Load Width. This is used to determine the tie-down requirements for each structural joint, excluding roof battens.

(Clause 9.6.2 and Figure 9.5)

AS1684.2 p 168/169AS1684.2 p 168/169


AS1684.2 p169AS1684.2 p169FIGURE 9.5 ROOF UPLIFT LOAD WIDTH ULW

U LW

U LW

U LW

U LW

U LW

U LW

(c) Roof truss construction

ULW for roof and wall frames

ULW for floor frames


Indicates Tie-down

connectionlocations

The Uplift Load Width (ULW) will be taken across the lounge/dining area.

This is the “worst case” and will be applied throughout the structure for convenience.

The width is 8.6m (including overhangs) and therefore the truss ULW is 4.3m.


(Roof trusses to wall frame)

ii) Uplift Load Area.

This is calculated by multiplying -

ULW x Tie - Down Spacing = ULA



ULW =43000 mm

Fixing Spacing

(Rafter/Truss Spacing)

= 900 mm

ULA = 4.3m x 0.9m

= 3.87 m2



iii) Wind Uplift Force. We now require a pressure to use in designing the connection.

This can be determined by multiplying the -

Uplift Load Area x Net Uplift Pressure (Table 9.5)

for the joint in question.

(Clause 9.6.4 and Figure 9.5)

AS1684.2 p 170AS1684.2 p 170





AS1684.2 p 170AS1684.2 p 170

Wind Uplift Force =

3.87 m2x 0.74kN/m2 (kPa) = 2.86 kN

Wind Uplift Force:

= Uplift Load Area x Net Uplift Pressure

Rafters/Trusses to wall frames - Top and bottom plates to studs.



iv) Joint Group. This is the rating assigned to a piece or parcel of timber to indicate the design capacity appropriate to that timber for a range of connectors. (Clause 9.6.5 & Table 9.15)

AS1684.2 p 180AS1684.2 p 180


AS1684.2 p 180AS1684.2 p 180

Where a timber joint is comprised of two or more different species, the joint group allocated to that joint shall, in general, be that appropriate to the weakest material in that joint (Clause 9.6.5)


design strength is controlled by the shank of the nail or screw in withdrawal

Joint group (J, JD rating) shallbe based on this member as

(a) Joint type 1

Joint group (J, JD rating) shallbe based on the weakest ofeither member as

(b) Joint type 2

Joint group (J, JD rating) shallbe based on this member as

(c) Joint type 3

design strength is controlled by the nails working in shear

design strength is controlled by shear or bearing of the bolt in both members

AS1684.2 p 181AS1684.2 p 181

Large arrows indicate direction of load.

Joint group (J, JD rating) shallbe based on the weakest ofeither member as the designstrength is controlled by thenails or screws in shear in bothmembers

(d) Joint type 4

Joint group (J, JD rating) shallbe based on the weakest ofeither member as the designstrength is controlled by thenails working in shear in bothmembers

(e) Joint type 2

AS1684.2 p 181AS1684.2 p 181

d) Select tie-down connections.


(see Table 9.21)

Required uplift capacity = 2.86 kN

Joint Group = JD4 (softwood)

Adopt - 1 Framing Anchor with 4/2.8 ø nails to each end. (3.5 kN)

AS1684.2 p 195AS1684.2 p 195

No. of anchors

1 4.9 3.5 2.5 3.5 2.9 2.2

(b)

Framing anchor as pertable, 4/2.8 mm nailsto each end

2 8.3 5.9 4.2 5.9 4.9 3.7


AS1684.2 p 198AS1684.2 p 198



(see Table 9.19)


Joint Group = JD4 (softwood)

Adopt - 30 x 0.8 G.I strap with 2/2.8 ø nails at each end. (3.5 kN)

No. of nails

2 4.9 3.5 2.5 3.5 2.9 2.2

3 6.5 4.7 3.3 4.7 3.8 2.9

4 8.3 5.9 4.2 5.9 4.9 3.7

6 8.5 8.4 5.9 8.4 6.9 5.2

(d)

a

a

3 0 0 .8 G . I . s t ra p 2 .8 n a i l s e a c h e n d a s n o te d

NOTE: a = 100 mm or longer to prevent splittingfor number of nails used.



(Wall studs to top and bottom plates)

AS1684.2 p 190AS1684.2 p 190

Note: A Strap connection could be used to eliminate the need for a framing anchor by being nailed to one side of the stud, looped over the truss and finally nailed to the other side of the stud.


(Wall studs to top and bottom plates)

Strap with 2 / 2.8 ø nails at each end.


(Bottom Plates to Floor Frame or Slab)

= ULW x tie-down spacing (joist spacing)

x Net Uplift Pressure

4.3 m x 0.45 m x 0.42 kN/m2 (kPa) = 0.81 kN

Wind Uplift Force:

= Uplift Load Area x Net Uplift Pressure

Single or upper storey bottom plates to floor frame or slab.




Joint Group = JD4 (softwood bottom plates)

Adopt - 3/90 x 3.05 ø Nails (Min. 40 mm Penetration)

AS1684.2 p 188AS1684.2 p 188



NOTE: Similar connection details can also be “manipulated” to provide additional connections that can be used to achieve the required tie-down.



2 nails each end of strap 4 nails each end of strap

Equivalent capacity





For this connection, the maximum tie-down spacing can also be determined as follows:

Wind Uplift Force =

ULW x tie-down spacing x Net Uplift Pressure

Therefore, 1.0 kN (masonry anchor capacity to resist force)

= 4.3 m x tie-down spacing m x 0.42 kN/m2

Tie-down spacing = 1.0 / (4.3 x 0.42) x 1000 = 550mm

NOTE: AS1684.2 provides numerous connection details that can be used to achieve the required tie-down.

There are also new connectors available and manufacturers should be approached to provide the relevant connector capacities – refer Table 9.18 (e)

AS1684.2 p 189AS1684.2 p 189



Determine specific shear force fixing requirements, where appropriate.

Refer to clauses 9.7.1 - 9.7.6 (pages 209 - 213) Based on examination of Table 9.3, pg.166

7. Determine shear force

No specific connections

required

No specific shear connections required for this example.

8. Select shear connection

AcknowledgementAcknowledgement

This educational resource has been prepared as part of the Forest & Wood Products Australia

Technical Resources Program – Supporting Timber Education & Training

Prepared and reviewed by: TPC Solutions Pty Ltd (Vic)

Timber Queensland Place Designs (Qld)

Documents

AS1684 SS N2 L Shaped House Tie-Down Example Nov 07