cd contents - part 3 provision of restraint against the rotation of individual timber frame walls

7/28/2019 CD Contents - Part 3 Provision of Restraint Against the Rotation of Individual Timber Frame Walls

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CD3 Provson of restrant aganst the rotaton ofndvdual tmber frame walls

CD3.1 Introducton

There are two ways to ensure that a timber frame building can

resist the overturning moments applied to it by wind forces. Theupper bound approach in Section 10.5.2 of the Manual checksthe overturning resistance of the building as a whole, assuming

that it acts as a rigid box, and where necessary the building

is fastened to the foundation along the base of the windward

wall. The lower bound approach in Section 10.8.1.5 checks the

overturning resistance of the individual walls, and ensures their

stability by requiring tie-downs, where necessary, at the end of

each individual racking wall. If the continuity of floor and ceiling

diaphragms across openings can make the walls act as if they

were continuous across the building to the hinge line, and if all

the connections between the vertical and horizontal diaphragms

are strong enough to mobilise the entire weight of the building in

overturning, then the building will indeed act as a rigid box and

the two approaches will give the same result.

EC0, however, states that the stability of both the whole

building and the individual elements of it should be demonstrated.

In particular, the design method for timber frame walls implies

that individual walls should be restrained against the overturning

forces acting on them. This document therefore shows how the

rotational stability of individual racking walls can be ensured.

As stated in Section 10.8.1.5 of the Manual, a properly

constructed timber frame wall can be restrained against rotation

by means of a vertical restraint force applied to its windward end,

by an equivalent vertical load applied to the top of the wall by

the structure above it, by the connection of its bottom rail to the

foundation or wall beneath it, or by a combination of these.

Any of the following may be utilised.

Co nne c tiono fa ne nd w a llto a na d jac ent ret urnwall.The weight of a limited length of return wall and any

vertical load on it and its attachment to the foundation may all

be utilised to hold down the end of a wall.

C o n n e c t io n o f a n i n te rm e d i a t e w a ll t o a st u d su p p o r t in g t h e lin t e l o v e r a d o o r w a y o r w i n d o w

o p e n i n g . Half the vertical load on the lintel and the weight

of a limited length of wall panel beneath a window and its

attachment to the foundation may all be utilised.

C o nn e c tion o fb o tto m railto th e fo un d a tion o rth e w a llb en ea thit.

Strap san d brac ke ts.Vertical restraint straps or boltedbrackets with adequate strength and stiffness may be specified

to attach the wall to the foundation or the wall beneath.

The designer is at liberty to choose which of the various elements

are utilised.

Where intermediate walls end at openings for doors

or windows, the designer may be able to demonstrate that

the continuity provided by the superstructure (floor or ceiling

diaphragm) will provide the necessary residual constraint against

overturning.

Corner of building attached to racking wall

bfull-height

qreturn, vert, d

Resultant

= Fi,f,Rd greturn, d

h

breturn

Ff,Rdper unit length

Fgure CD3.1 Dimensions for return wall

IStructE/TRADA Manual for the design of timber building structures to Eurocode 5 CDCD 3 Page 1


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CD3.2 Connecton to a return wall panel

The vertical restraint, Fi,f,Rd

, which can be applied to the end of

wall via its connection to a return wall, can be calculated as:

= b ) (kN)Fi,f,Rd return(qreturn,vert,d+ greturn,d+ Ff,Rdh

Where b = min . * (m)return b full heighth = height of wall panel (m)

= actual length of uninterrupted full heightbfull-heightreturn wall adjacent to corner (m)

= design value of uniformly distributedqreturn,vert,dvertical load per unit length onto the

return wall comprising permanent loads

minus the vertical component of any

wind uplift (kN/m)

= design value of dead weight of timbergreturn,d

frame return wall per unit length (kN/m)

= effective fastener resistance per unit lengthFf,Rd

= Ff Rd, 1 0 5. F, (kN/m), f Rd,2+s1 s2=lateral design capacity of one fastenerFf,Rd,1, Ff,Rd,2

in sheathings 1 and 2 respectively in the

return wall (kN)

s1, s

2= fastener spacings in sheathings 1 and 2

respectively in the return wall (m)

Values of Ff,Rd for certain standard wall configurations can be

obtained directly from the Manual, Table 10.9. If the term Ff,Rdisutilised, the fasteners attaching the bottom rail of the return panel

to the substrate should be either ringed shank nails or screws, with

a design withdrawal resistance equal to at least Ff,RdkN/m (see the

Manual, Table 10.8).

The connections between the end of the return wall and

the end of the racking wall should have a design resistance of at

least Fi,f,RdkN/m. Values for some standard fixings are given in

the Manual, Tables 10.5 and 10.6.

CD3.3 Connecton to a stud supportng an openng

The vertical restraint, Fi,f,Rd

, which can be applied to the end

of wall via its connection to a stud supporting a lintel over an

opening, can be calculated as:

) (kN)Fi,f,Rd= 0.5bopeningqi,vert,d+ brestraint(gwall,d+ Ff,Rd

Where bopening = width of opening (m)

qi,vert,d = design value of uniformly distributed vertical

load per unit length onto the wall comprising

permanent loads minus the vertical

component of any wind uplift (kN/m)

brestraint =,

,

int

int

v

h

.minbb

restra

restra

* for a window opening(m)= 0.0 for a door opening

brestraint,v

= height of panel to bottom of window (m)

(see Figure CD3.2)

brestraint,h

= width of window (m) (see Figure CD3.2)

gwall,d = design value of dead weight of reduced height

timber frame wall per unit length (kN/m)

Ff,Rd = effective fastener resistance per unit length

(kN/m)

If the term involving Ff,Rd

is utilised for a window opening, the

fasteners attaching the bottom rail of the panel to the substrate

should be either ringed shank nails or screws, with a design

withdrawal resistance equal to at least Ff,Rd kN/m (see the

Manual, Table 10.8).The connections between the end of the return wall and

the end of the racking wall should have a design resistance of at

least Fi,f,Rd kN/m. Values for some standard fixings are given in

the Manual, Tables 10.5 and 10.6.

bopeningbopening

qi,vert, d

gwall, d

brestraint,h brestraint = 0

brestraint,v

Ff,Rd per unit length

Resultant

= F i,f,RdResultant

= Fi,f,Rd

qi,vert, d

Restrained stud

Wind Wind

Restrained stud

Fgure CD3.2 Dimensions for openings



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CD3.4 Connecton of bottom ral to the foundaton or

wall beneath

Some overturning resistance will inevitably be provided by

the attachment of the sheathing to the bottom rail of the

timber framing, and by the attachment of the bottom rail to the

foundation of the building or the top of the wall beneath. In thetest method on which the racking resistance expressions given in

EC5 Method B were based, the bottom rail was bolted down and

the nails attaching the sheathing to the bottom rail and leading

stud resisted all the overturning moment. It may therefore be

assumed that any overturning moment which does not exceed

the calculated shear capacity of the wall can be resisted by the

sheathing-to-frame connection, but the connection between the

bottom rail and the substructure must be designed to provide the

required resistance.

Assuming that the wall acts as a rigid assembly rotating

about one corner, fasteners distributed along the bottom rail can

provide a design resisting moment ofRdist,db/3 Nmm, where Rdist,dis the design resistance of the fasteners to vertical load in N/mm and

bis the length of the wall (between openings or returns) in mm.

Table 10.8 in the Manual gives the design axial loadcapacities of some fasteners commonly used to connect the

bottom rail of a wall panel to a timber wall plate. Figure CD3.3

shows some brackets used to attach wall plates to the foundation.

If such brackets are relied on to restrain the rotation of individual

walls they should be spaced no more than 600mm apart.

CD3.5 Restrant straps and brackets

Vertical restraint straps or bolted brackets with adequate strength

and stiffness may be specified on the end studs at openings and

corners to attach the wall to the foundation or the wall beneath

(see Figures CD3.4 and CD3.5). Consideration should be given

to the effects of shrinkage across intermediate floors and to

the true stiffness of foundation straps as commonly installed.

Tables 10.5 and 10.6 in the Manualgive load capacities for nailsin steel straps. Typical straps are made from steel to BS EN 10142;

Fe PO2 G 1.5mm to 3mm thick and from 20mm to 75mm wide.

Angle brackets may be up to about 6mm thick.

a) Fixing brackets attached b) Fixing brackets attachedto side of sole plate to side from beneath wall plate

Galvanised brackets or shoes fixed withballistic or masonry nails into concrete slaband nailed to both sides of the sole plate.Nailing and spacing as calculated

Fgure CD3.3 Some methods of attac hing a wall plate to the foundation



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Sole plate nailed tolocate it prior tofixing of panelsand straps

Bend in holding down strap set below lean mix concretecavity fill. The strap should be stiff enough not to straighten

Concrete

cavity fill

Stainless steel holding down strap nailed to studs and builtinto external brickwork cladding. Nailing and thickness as calculated.The weight of the external leaf restrains the straps

Breather membrane may be behindor over straps depending upon sequenceof construction

a) Holding down straps showing correct installation method

Chord

Bolts

6mm bent steel plateone or two sides

Anchor bolt

b) Holding down bolts and brackets

Fgure CD3.4 Some methods of connecting the end of a wall to the foundation



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Upper shearwall

Connector bracket

Anchor bolt

Threaded rod

Bolts

Double continuous plate

Lower shearwall

Connector bracket

Bolts

Fgure CD3.5 A suggested method for connec ting the end of a wall to the floor beneath


cd contents - part 3 provision of restraint against the rotation of individual timber frame walls

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