Development of a design procedure for timber concrete composite ...

Assessment of the axial stress is derived from the
flexural action. However, Equations 13a and 13b can be
manipulated to identify the (corresponding) design axial
force:
N
=σ
where the allowable axial capacities are defined as:
N
Note: The characteristic tension strength must be
reduced for timber member exceeding depth of 150 mm
or modified in accordance with the manufacturer’s
specification. 4
5.2.2 Strength Requirements for Flexural Shear
Strength
In the absence of structural reinforcement in the
concrete member, the flexural shear strength is
provided by the timber member, therefore;
Vd
where for rectangular sections:
' 2At
Vd
= φk1k4k6fs
3
Note: Some conditions, (for example use of a deep
notch), may require reducing the shear plane area by
using the net area of the (beam) cross-section.
5.2.3 Strength Requirements for Bearing Strength
The bearing strength is provided by the timber member,
therefore;
N
in which:
N
A
*
c c,
c c
= φ f A
'
dc , c c
N
N
= σ A
(14a); (14b)
*
t t,
t t
= φkk k f A (15a); (15b)
'
dt , 1 4 6 t t
*
≥ V
(16)
d,
p
≥ N
*
p
= φkkkk f A
'
d, p 1 4 6 7 p p
(17)
(18)
(19)
5.3 STRENGTH OF THE COMPOSITE ACTION
(CONNECTION CAPACITY)
The connection (or notch) transfers the shear force
occurring between the members under flexure. The
actual mechanics of this force transfer are relatively
complex. However a prescriptive approach that defines
connection capacities (based on empirical test data –
refer to Figure 6) that ensures the design procedure
remains user-friendly, has been adopted for this
document.
5.3.1 Shear Strength of the Connection
A global assessment of the connection strength is
performed. It includes the assessment of the strength of
the connection closest to the support, V max (shear force
)
N
Characteristic stiffness Qk (kN)
(k
Q k
th
g
n
tre
s
tic
ris
te
c
ra
a
h
C
130
120
110
100
90
80
70
60
50
40
30
20
at the support), and the connection located at the
quarter-span area, V L/4 (shear force at the quarter span).
Note: Refer to Figure 6, for empirical strengths of the
specified connections.
where:
and the effective shear force in the connection located
near the support equals:
and the effective shear force in the connection located at
the ‘quarter’ span:
5.3.2 Shear Strength of the Timber
The shear strength of the timber – tangential shear
action in the area located between the support and the
first connection is assessed and checked as follows:
where:
25 50 75 100 125 150 175 200 225 250
Beam width (mm)
Trapezoidal
Triangular
Figure 6: Characteristic strength (Q k) of trapezoidal and triangular
notches.
Nd,
j
N
≥ Q
*
= φk k k Q
d, j 1 4 6
γ
cEAas
c c c
Q
*
=−
( Vmax
) ( EI)
* min *
max
ef
γ
cEAas
c c c
Q
*
=−
( VL
/4 ) ( EI)
k
V
V
* max *
L /4
ef
Ndl
,
≥V
*
( )
'
dl , 1 4 6 s t s
(20)
(21)
(22)
(23)
(24)
N = φkk k f bl
(25)
5.4 SERVICEABILITY VERIFICATION
The load combinations and factors for the serviceability
limit states (SLS) are defined in the AS/NZS 1170 series
[4, 5]. Serviceability of the TCC structure is undertaken
by checking the deflections against the limits defined to
suit the functional requirements of the building being
4
Modification of the design approach enforces in AS 1720.1–2010 [3].
12 NEW ZEALAND TIMBER DESIGN JOURNAL VOL 19· ISSUE 2