Summary
Highlights
The video introduces the continuation of the lecture on reinforced concrete, specifically focusing on load combinations used in both Strength Design Method (Ultimate Strength Design) and Working Stress Design, as per NSCP 2015 sections 203.3 and 203.4. It explains that structural members are subjected to various loads like dead, live, and environmental loads (earthquake, wind, fluids, lateral earth forces), and load combinations are necessary to account for simultaneous occurrences of these loads.
The speaker elaborates on the use of multipliers (load factors) in the Strength Design Method, which are generally greater than 1.0, to increase design loads. This is contrasted with reduction factors used in analysis to underestimate strength. The purpose of these multipliers in design is to provide an allowance for uncertainties, potential future increases in load, and the natural degradation of structural capacity over time, ensuring the structure's integrity is not compromised.
The video outlines seven load combination formulas for the Strength Design Method. Key parameters are defined: D for dead load, L for live load, W for wind load, E for earthquake load, Lr for roof live load, T for temperature load, F for fluid load, H for lateral earth pressure, and R for rain load. Special attention is given to F1, a live load factor modifier, whose value depends on the type of occupancy (e.g., public assembly, garage, or ordinary live loads).
In contrast to the Strength Design Method, the Working Stress Design (WSD) approach, mainly used in older NSCP codes, does not employ multipliers for loads but rather adds them directly. The video presents five load combination formulas for WSD. It also highlights that in the Philippines, common design considerations often focus on dead load, live load, wind load, and earthquake load due to available data and local conditions.
An example problem is presented for a reinforced concrete roof girder of a public assembly building, subjected to various service loads (dead, live, roof live, rain, wind). The task is to determine the governing load using both the NSCP 2015 Strength Design Method and Working Stress Design. The first part of the example details how to apply each of the seven Strength Design load combinations, identifying the highest resulting value as the governing load.
Following the Strength Design example, the video demonstrates the application of Working Stress Design load combinations to the same building scenario. It emphasizes that in WSD, no multipliers are used, and loads are added directly. The calculation process for each of the five WSD load combinations is shown, and the highest value is selected as the governing load.
A second example involves a reinforced concrete basement wall of a warehouse with dead, live, earthquake, temperature, and lateral earth loads. The process for calculating the governing load using the Strength Design Method is detailed, including identifying which loads are applicable for each combination and substituting the given values. The highest calculated load dictates the design.
The video concludes the second example by determining the governing load for the basement wall using the Working Stress Design approach. Similar to the previous WSD calculation, loads are directly summed without multipliers. The final governing load for WSD is then identified by comparing the results from all relevant combinations for this method. The lecture ends by assigning a third example for students as a seatwork activity.