For incremental launching method, the span depth ratio of bridges is normally low. Why?

Bridges constructed by incremental launching method are usually low in span depth ratio and typical values are 14 to17. With low span depth ratio, the bridge segments are stiff in bending and torsion which is essential to cater for the launching process. Such low span depth ratio could tolerate the discrepancy in vertical alignment on supports over which they slide. Such differential settlements may occur owing to the shortening of piers when the superstructure slides over them and the differential deformation of different piers.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

What are the main potential benefits in using the bridge form of precast prestressed beams supporting in-situ concrete top slab?

The potential benefits of using the bridge form of precast prestressed beams supporting in-situ concrete top slab are:

(i) For bridges built on top of rivers and carriageway, this bridge form provides the working platform by the precast beams so that erection of falsework is not required.

(ii) This bridge form generally does not require any transverse beams or diaphragms (except at the location of bridge supports), leading to reduction of construction time and cost.

(iii) It creates the potential for simultaneous construction with several spans.

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 parasitic forces for prestressing?

In statically determinate structures, prestressing forces would cause the concrete structures to bend upwards. Hence, precambering is normally carried out to counteract such effect and make it more pleasant in appearance. However, for statically indeterminate structures the deformation of concrete members are restrained by the supports and consequently parasitic forces are developed by the prestressing force in addition to the bending moment generated by eccentricity of prestressing tendons. The developed forces at the support modify the reactions of concrete members subjected to external loads and produces secondary moments (or parasitic moments) in the structure.

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.

Under what situation shall engineers use jacking at one end only and from both ends in prestressing work?

During prestressing operation at one end, frictional losses will occur and the prestressing force decreases along the length of tendon until reaching the other end. These frictional losses include the friction induced due to a change of curvature of tendon duct and also the wobble effect due to deviation of duct alignment from the centerline. Therefore, the prestress force in the mid-span or at the other end will be greatly reduced in case the frictional loss is high. Consequently, prestressing, from both ends for a single span i.e. prestressing one-half of total tendons at one end and the remaining half at the other end is carried out to enable a even distribution and to provide symmetry of prestress force along the structure.

In fact, stressing at one end only has the potential advantage of lower cost when compared with stressing from both ends. For multiple spans (e.g. two spans) with unequal span length, jacking is usually carried out at the end of the longer span so as to provide a higher prestress force at the location of maximum positive moment. On the contrary, jacking from the end of the shorter span would be conducted if the negative moment at the intermediate support controls the prestress force. However, if the total span length is sufficiently long, jacking from both ends should be considered.

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 three major types of reinforcement used in prestressing?

(i) Spalling reinforcement 
Spalling stresses are established behind the loaded area of anchor blocks and this causes breaking away of surface concrete. These stresses are induced by strain incompatibility with Poisson’s effects or by the shape of stress trajectories.

(ii) Equilibrium reinforcement 
Equilibrium reinforcement is required where there are several anchorages in which prestressing loads are applied sequentially.

(iii) Bursting Reinforcement 
Tensile stresses are induced during prestressing operation and the maximum bursting stress occurs where the stress trajectories are concave towards the line of action of the load. Reinforcement is needed to resist these lateral tensile forces.

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 is spalling reinforcement needed for prestressing works in anchor blocks?

Reinforcement of anchor blocks in prestressing works generally consists of bursting reinforcement, equilibrium reinforcement and spalling reinforcement. Bursting reinforcement is used where tensile stresses are induced during prestressing operation and the maximum bursting stress occurs where the stress trajectories are concave towards the line of action of the load. Reinforcement is needed to resist these lateral tensile forces. For equilibrium reinforcement, it is required where there are several anchorages in which prestressing loads are applied sequentially.

During prestressing, spalling stresses are generated in the region behind the loaded faces of anchor blocks. At the zone between two anchorages, there is a volume of concrete surrounded by compressive stress trajectories. Forces are induced in the opposite direction to the applied forces and it forces the concrete out of the anchor block. On the other hand, the spalling stresses are set up owing to the strain compatibility relating to the effect of Poisson’s ratio.

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 functions of grout inside tendon ducts?

Grout in prestressing works serves the following purposes:

(i) Protect the tendon against corrosion.
(ii) Improve the ultimate capacity of tendon.
(iii) Provide a bond between the structural member and the tendon.
(iv) In case of failure, the anchorage is not subject to all strain energy.