A Beginner's Guide to Structural Engineering

Homework/Example Problem Structures

2007 T. Bartlett Quimby

Overview
Dormitory
Communications Tower
Truss Bridge
House
Miscellaneous Details
 
Report Errors or Make Suggestions

 

The Drawings

The PDF drawing set consists of seven, 11"x17" drawings.  They print well on 8.1/2"x11" paper as well.  Additionally, a 3D AutoCAD model is available.

The drawing set also includes a table of truss member forces.  The truss member axial forces due to dead load and vehicle loading are given.  The dead load is based on assumed values for member sizes plus a 6" concrete deck.  The vehicle loading includes a train of AASHTO H20 trucks that were placed in 100 different places (1/4 ft increments) on the bridge.  The shown member forces are the worst case loading for each member.

Note that even though the bridge is located in a heavy snow region, snow load is not considered.  This is because the bridge must be plowed to accommodate the vehicle loading and, hence, is not on the bridge at the same time as the greater vehicle loading.  Ice loading is included since it can be present with vehicle traffice.

Figure Bridge-1 is an isometric view of the bridge steel structure.  Click on the hotlinks by the labeled joints to see images of the similar joints in the actual bridge.

Figure Bridge-1
Isometric View

Discussion

This old truss bridge made from steel W, MC, and L sections riveted together.  For our purposes, the drawings do not include member sizes nor complete details on bolt patterns and layout.  These are subject to change as we redesign the bridge.

Some artistic license has been taken in the creation of the drawings.  They are not exact as-built drawings but closely follow the dimensions and theme of the existing bridge.

The basic structure is relatively simple, but as they say, the devil is in the details.

The following series of isometric drawings are intended to help you understand the erection sequence and the parts of the bridge.  On the final isometric, you can click on hot links within the image to bring up images of the actual connections.

Stage 1:  Arrangement of the most of the main members.

Figure Bridge-2
Stage 1

In this stage the main members are laid out.  For this bridge, all the sections shown are W sections of the same nominal depth.  All the flanges are in the plane of the truss.

Notably absent are the top chords of the trusses.  These will be added later.

Stage 2:  Addition of the connection gusset plates.

Figure Bridge-3
Stage 2

The gusset plates are placed on both sides of W sections.  The gusset plates connect the various elements of the truss together.

Stage 3:  Addition of the top chord members.

Figure Bridge-4
Stage 3

The top chord consists of two MC sections bolted to the outside of the gusset plates. 

Stage 4:  Competition of the top chords.

Figure Bridge-5
Stage 4

The top chord is a compression member, hence it requires are large moment of inertia to prevent buckling.  The MC sections alone would be too weak in compression, so a connecting plate is added to the top connection the two MCs.  Lattice is added to the bottom to supply lateral support to the bottom flanges.  The resulting built up section is shown in section 2/S3 of the drawing set.  This completes the trusses.

Stage 5:  Add in the girders.

Figure Bridge-6
Stage 5

The girders are added in this stage.  The girders are attached to the verticals via a pair of connection angles that are bolted to the verticals and to the web of the girders as shown in 4/S4.  All the deck load is transferred to the trusses through these connections.  Note that all the girders, with the exception of the end girders, are equally loaded (i.e. they each support the same amount of deck).  The end girders can be smaller since they support half as much load.

Stage 6:  Add the deck beams.

Figure Bridge-7
Stage 6

Now the deck beams are attached to the girders using connection angles as shown in detail 2/S7.  The deck beams are arranged so that they support about equal load, however the inner two beams carry slightly more than the outer two deck beams.  In our design, we will use the same size for all floor beams so we will design for the load on the inner deck beams.

Stage 7:  Cast the concrete deck.

The final stage is to add the concrete deck over the deck beams.  For this bridge, the deck is a cast-in-place slab.

The stages applied to a connection:

  Joint "D" Joint "C"
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
Stage 6