When should engineers consider using truss with K-bracing?

In the arrangement of triangulated framework in truss structures, it is more economical to design longer members as ties while shorter ones as struts (e.g. Pratt truss). As such, the tension forces are taken up by longer steel members whose load carrying capacities are unrelated to their lengths. However, the compression forces are reacted by shorter members which possess higher buckling capabilities than longer steel members.

For heavy loads on a truss structure, the depth of the truss is intentionally made larger so as to increase the bending resistance and to reduce deflection. With the increase in length of the vertical struts, buckling may occur under vertical loads. Therefore, K-truss is designed in such as way that the vertical struts are supported by compression diagonals.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What are the characteristics of Vierendeel girder?

The Vierendeel girder design is sometimes adopted in the design of footbridges. In traditional truss design, triangular shape of truss is normally used because the shape cannot be changed without altering the length of its members. By applying loads only to the joints of trusses, the members of truss are only subjected to a uniform tensile or compressive stress across their cross sections because their lines of action pass through a common hinged joint.

The Vierendeel truss/girder is characterized by having only vertical members between the top and bottom chords and is a statically indeterminate structure. Hence, bending, shear and axial capacity of these members contribute to the resistance to external loads. The use of this girder enables the footbridge to span larger distances and present an attractive outlook. However, it suffers from the drawback that the distribution of stresses is more complicated than normal truss structures

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What are the differences among Warren Truss, Howe Truss and Pratt Truss?

A truss is a simple structure whose members are subject to axial compression and tension only and but not bending moment. The most common truss types are Warren truss, Pratt truss and Howe truss.

Warren truss contains a series of isosceles triangles or equilateral triangles. To increase the span length of the truss bridge, verticals are added for Warren Truss.

Pratt truss is characterized by having its diagonal members (except the end diagonals) slanted down towards the middle of the bridge span. Under such structural arrangement, when subject to external loads tension is induced in diagonal members while the vertical members tackle compressive forces. Hence, thinner and lighter steel or iron can be used as materials for diagonal members so that a more efficient structure can be enhanced.

The design of Howe truss is the opposite to that of Pratt truss in which the diagonal members are slanted in the direction opposite to that of Pratt truss (i.e. slanting away from the middle of bridge span) and as such compressive forces are generated in diagonal members. Hence, it is not economical to use steel members to handle compressive force.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Why are precast concrete piers seldom used in seismic region?

The use of precast concrete elements enhances faster construction when compared with cast-in-situ method. Moreover, it enhances high quality of piers because of stringent control at fabrication yards. The environmental impact is reduced especially for bridges constructed near waterways. In particular, for emergency repair of bridges owing to bridge collapse by earthquake and vehicular collision, fast construction of damaged bridge is of utmost importance to reduce the economic cost of bridge users.

The precast bridge piers are mostly used in non-seismic region but not in seismic region because of the potential difficulties in creating moment connections between precast members and this is essential for structures in seismic region.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Why are coatings sometimes provided at the back faces of abutments?

There are different views on the necessity of the application of protective coatings (may be in the form of two coats of paint) to the back faces of bridge abutment. The main purpose of this coating serves to provide waterproofing effect to the back faces of abutments. By reducing the seepage of water through the concrete, the amount of dirty materials accumulating on the surface of concrete would be significantly decreased.

Engineers tend to consider this as an inexpensive method to provide extra protection to concrete. However, others may consider that such provision is a waste of money and is not worthwhile to spend additional money on this.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What is shock transmission unit in bridges?

Shock transmission unit is basically a device connecting separate structural units. It is characterized by its ability to transmit short-term impact forces between connecting structures while permitting long-term movements between the structures.

If two separate structures are linked together to resist dynamic loads, it is very difficult to connect them structurally with due allowance for long-term movements due to temperature variation and shrinkage effect. Instead, large forces would be generated between the structures. However, with the use of shock transmission unit, it can cater for short-term transient loads while allowing long-term movements with negligible resistance. It benefits the bridge structures by acting as a temporary link between the structures to share and transfer the transient loads.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Should raking piles of a bridge abutment be placed under an embankment?

For a bridge abutment to be supported on raking piles with different orientations, the movement between the ground and the pile group is difficult to predict. For instance, if some of the raking piles of the bridge abutment are extended beneath an embankment, then the settlement of embankment behind the abutment may cause the raking piles to experience severe bending moment and damage the piles as recommended by Dr. Edmund C Hambly (1979).

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.