design in trusses
A general article about steel trusses which touched on choice of members and their
orientation but did not go into detail about designing connections was published in 20171.
In the present article Richard Henderson of the SCI illustrates the implications of such choices
on the connection design.
The selection of members and their orientation and the impact
on the design of connections in a truss is best illustrated with
an example. The arrangement of the truss, the magnitude of the
forces and the orientation of the members all have an impact on
the form of the connections. A fundamental part of achieving
an efficient joint design is establishing an understanding of
the flow of forces through the joint. This is only possible if the
forces provided for the design of the joint are in equilibrium. If
envelope forces are provided, this compromises the designer’s
ability to develop an efficient connection design. In what follows
connections between open section members will be considered.
As an example, consider a transfer truss spanning 30 m
supporting two columns at third points, each carrying 10 MN from
floors above. The truss is divided into three bays of 10 m width by
the columns. The building storey height is 4.0 m and each floor
in the truss will carry a uniform load of 45 kN/m. The chords are
restrained out of plane by floor beams perpendicular to the plane
of the truss.
An early decision is what the depth of the truss should be. The
maximum bending moment is 100 MNm from the columns and
about 5 MNm from each floor. If the truss is one storey deep (ie a
span to depth ratio of 7.5), the maximum chord force is 27.5 MN
which exceeds the axial resistance of the largest UC section. In this
example, a two-storey truss is chosen, giving a maximum chord
force of about 14.4 MN which can be carried by a UC. The truss
can be conveniently divided into 5 m panel widths. An N-frame
or Pratt truss has shorter vertical members in compression
and longer diagonal members in tension. The connections in
the tension members are likely to prove the most difficult to
detail and the tension forces in the bracing could be reduced
by orienting the bracing so that the diagonal members are in
compression and the verticals in tension (Figure 1).
It can be seen from Figure 1 that the length of the bottom
chord carrying a force above 14 MN is more than 20 m long
and will need a tension splice for transportation. Adopting a
conventional N-frame is therefore considered to be preferable
as the necessary splices can be located in elements with lower
A truss with a single storey depth could be shop-fabricated and
transported to site in three pieces with two bolted site splices.
Erection time would be reduced but crane lifting capacity and
transportation would need to be considered if this option is
contemplated. The truss arrangement chosen, member forces and
some member sizes are shown in Figure 2.
Example connection design – orientation of members
Consider the joint at point A at the base of the column carrying
10 MN. The bottom chord member could be detailed as one
fabricated assembly with a joint at each end to connect to the
column, diagonal brace and the continuing chord member. At this
joint, there is a tension in the central section of bottom chord of
about 14.2 MN and a tension diagonal carrying about 13 MN. The
chord member carries about 40% of the tension in each flange
and 20% in the web. A conventional orientation of the members
might be considered with the webs vertical as in Figure 3.
Figure 1: Truss
Figure 2: Truss arrangement diagonals in tension
Bottom chord joint -