Civil Engineering Reference
In-Depth Information
The stress limit assumes that a minimum distributed reinforcement ratio of
0.003 in each direction is provided.
13.1.2.2
Ties
The tension ties are stirrups, longitudinal tension chord reinforcements, and
other special-detail reinforcements. All tension reinforcements should be
adequately anchored. Inadequate development of tension reinforcement will
lead to brittle failure at a lower load than at anticipated ultimate capacity.
According to Appendix A of ACI-318-2012, the nominal strength of a
tie shall be taken as
(
∆
F
=
f A
+
A f
+
f
(13.8) (ACI [eq. A-6])
nt
y
st
ps
sc
p
where
(
f
sc
+
Δ
f
p
)
shall not exceed
f
py
and
A
ps
= 0 for nonprestressed member.
13.1.2.3
Nodes
Nodes are the locations where struts and ties converge. In other words, nodes
are the locations where forces are redirected within an STM. Nodal zones
are classified as CCC if all the compressive forces meet and CCT if one
of the forces is in tension (Figure 13.6). Similarly, CTT and TTT are also
possible (Figure 13.6). One way of laying out a nodal zone is to create
equal pressure on each face of the node. By doing so, on a CCC node, the
length of the sides of the nodes
a
1:
a
2:
a
3 becomes the same proportion as
C
1:
C
2:
C
3. If one of the forces is in tension, the length of that side of the
node is calculated from a hypothetical bearing plate on the end of the tie,
which exerts the same bearing pressure as the compression member. Because
the in-plane stresses in the nodes are equal in all directions, such a node is
referred to as
hydrostatic element
(Figure 13.7). For a CCC node, this can
be easily applied but for other nodes it can be tedious. This can be simplified
by considering a nodal zone formed by the extension of all the members
meeting at that node (Figure 13.8). However, this allows unequal stress at
C
T
T
C
C
T
C
T
T
C
CCT node
C
CCC node
CTT node
TTT node
(a)
(b)
(c)
(d)
Figure 13.6
Classification of nodal zones.
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