44 3 Multispan Beams and Trusses
If loadPD 1 is located on the suspended beamH 2 H 3 , then fraction of this force
is transmitted to the primary beamH 3 CD.IfloadPD 1 located at pointH 3 of
suspended beamH 2 H 3 , then this force is completely transmitted to the primary
beamH 3 CD. Thus, influence line has no discontinuity at pointH 3.
If loadPD 1 is located at pointH 2 on the suspended beamH 2 H 3 , then this force
is completely transmitted to the primary beamH 1 BH 2 and thus, has no influence on
the reactionRD. Therefore, influence line ofRDhas zero ordinate at pointH 2 .If
unit load is located withinH 2 H 3 , then the pressure transmitted from the secondary
beamH 2 H 3 to the primary beamH 3 CDat pointH 3 varies proportionally to the
distance of the unit load from pointH 2. Therefore, ordinate of influence line at point
H 3 should be connected with zero ordinate at pointH 2.
If loadPD 1 is located on the beamH 1 BH 2 , then force is not transmitted to
the suspended beamH 2 H 3 ; the load does not influence on reactionRD. Therefore,
ordinates of influence line alongAH 1 BH 2 are zeros.
If loadPD 1 (or any load) is located on the beamH 2 C, then reactionRDis
directed downward. Maximum positive reactionRDappears if loadPD 1 will be
located at the extremely right point of the beam; maximum negative reactionRD
appears if loadPD 1 will be located at the pointH 3.
Influence Lines for ShearQk, and Bending MomentMkat Sectionk
Suspended beamH 2 H 3 is subjected to loads, which act on this beam only, while
a load from other parts of the beam (AH 2 andH 3 D) cannot be transmitted on the
beamH 2 H 3. Therefore influence lines for internal forces at sectionkare same as
for simply supported beam without overhangs.
Influence Lines for ShearQn, and Bending MomentMnat Sectionn
Influence linesfor sectionnshould be constructed as for simply supported beam
with overhangH 3 C. At pointH 3 this beam supports the beamH 2 H 3 , which is sus-
pended one. Therefore, ordinates of influence lines at pointH 3 should be connected
with zeros ordinates at pointH 2.
Influence Lines for ShearQs, and Bending MomentMsat Sections
When the load travels along portionAH 1 , then construction of the influence lines
for sectionsis exactly the same as for a cantilevered beam.
If loadPD 1 is located on the suspended beamH 1 H 2 , then fraction of this force
is transmitted to the primary beamAH 1 .IfloadPD 1 is located at pointH 1 of
suspended beamH 1 BH 2 , then this force is completely transmitted to the primary
beamAH 1. Therefore, influence line has no discontinuity at pointH 1.