Test: Limit State & Working Stress Methods - Civil Engineering (CE) MCQ

The Indian and British code methods are limited to under reinforced sections in flexure based on the effective reinforcement ratio, the American code method provides empirical relations to compute the ultimate flexural strength of under and over reinforced sections as well as of sections with compression reinforcement.

The rigorous strain compatibility method of analysis, which involves the use of idealized stress strain curves of steel and concrete, can also be used for an accurate assessment of the flexural strength of sections with reinforcements in the tension and compression zones.

The current Indian, British and American codes have more or less similar provisions for the computation of the shear strength of prestressed members that fail in web – shear or flexure – shear cracking modes.

Serviceability limit state corresponds to excessive deflection and cracking and it is customary in most codes to safeguard against excessive deflection under serviceability limit state, either indirectly by prescribing minimum span/depth ratio for the member or directly specifying a maximum permissible deflection expressed as a fraction of the span.

According to the American code ACI: 318-1989, the permissible deflection vary from a maximum of span/180 to a minimum of span/480 depending upon the type of member and the seriousness of damage to the adjoining structural element.

Short term deflections of prestressed members of class-1 and class-2 types under service loads are influenced by: magnitude of the prestressing force and its profile, applied load and self weight of the member and flexural rigidity of the Un cracked concrete sections.

The limit state of cracking is particularly important in the case of partially prestressed members according to Abeles, who has contributed much information about members with limited prestress.

No tensile stresses are permitted under service loads and hence the structure is crack-free at the working load stage and the members are often referred to as fully prestressed, requiring higher quantity of prestressing steel associated with higher cost, generally class 1 type members are preferred for containment structures housing atomic reactors, pressure pipes and in important structures where cracking at service loads is not acceptable.

Class 2 structures limited tensile stresses of magnitude not exceeding the modulus of rupture of concrete are permitted under working loads members are often referred to as moderately prestressed visible cracks are not permitted in this type abeles has used such structures in British railways with very satisfactory results.

Tensile stresses of magnitude exceeding both the modulus of rupture of concrete and the visible cracking of limited width are permitted under service loads as compiled and members are referred to as partially prestressed class-3 type constitute the most economical of prestressed concrete structures due to limited prestressing forces and high tensile steel requirements.