PCI eLearning Course T125: Flexural Design of Precast, Prestressed Concrete—Service Limit States-PCI Aspire

PCI Aspire - Live Load Factor

Pdf Summary

At their 2015 annual meeting, the American Association of State Highway and Transportation Officials (AASHTO) Subcommittee on Bridges and Structures (SCOBS) adopted seven agenda items related to concrete structures. Agenda Item 2 focuses on major revisions to wind-load provisions, including changes to limit-state load combinations and load factors. The previous provisions were based on fastest-mile wind speed measurements, but the revised provisions use a 3-sec. wind gust speed with a 7% probability of exceedance in 50 years. The load factor for wind is now 1.0 for all load combinations applied to the wind pressure. Agenda Item 4 addresses load factors for the Service III limit-state load combination, specifically for checking tensile stress in prestressed components. The calibration of service limit states for concrete components concluded that components designed using the refined estimate of time-dependent losses have a lower reliability against flexural cracking compared to components designed using the pre-2005 prestress loss calculation method. As a result, an increase in the load factor for live load from 0.8 to 1.0 is required to maintain the level of reliability against cracking in prestressed concrete components. Agenda Item 5 integrates lightweight concrete (LWC) into Section 5 in a more consistent and accurate manner. Changes include a revised definition of LWC to include lightweight aggregates up to a unit weight of 0.135 kip/ft3, removal of specific types of LWC mixtures from the definition, and the introduction of the density modification factor, lambda (λ), to modify traditional resistance equations, stress limits, and development lengths. The shear strength reduction factor, phi (ϕ), for LWC has also been set equal to the factor for normal-weight concrete. The remaining concrete agenda items from the 2015 SCOBS meeting will be discussed in a future column.

Keywords

American Association of State Highway and Transportation Officials

AASHTO

Subcommittee on Bridges and Structures

SCOBS

concrete structures

wind-load provisions

limit-state load combinations

load factors

wind gust speed

load factor for live load