ASCE 7-10, Minimum Design Loads for Buildings and Other Structures, is a crucial standard for structural engineers. This guide breaks down key aspects of ASCE 7-10 for beginners, specifically focusing on uniformly distributed and concentrated loads as well as partition loads. We will cover how to interpret the standard and apply it to practical design scenarios.
Uniformly Distributed and Concentrated Loads
ASCE 7-10 Table 4-1 provides uniformly distributed and concentrated loads for different occupancies. This table is a primary source for determining live loads. However, it’s essential to carefully review the footnotes accompanying the table, as they contain important qualifications and exceptions.
Section 4.4 of ASCE 7-10 emphasizes a critical point: you must use either the uniform load or the concentrated load, “whichever produces the greater load effects.” These loads are not to be applied simultaneously. Unless otherwise specified, the concentrated load is assumed to be distributed over a 2.5 ft by 2.5 ft square area, positioned to maximize its effect on the structural member.
Figure 5.2.1 Distribution of Concentrated Load
Alt text: Diagram illustrating a concentrated load spread across a 2.5 ft by 2.5 ft square, demonstrating load distribution principles according to ASCE 7-10.
Concentrated loads can be distributed over a small area. Consider a floor joist with a 4 ft tributary width in an office building. According to ASCE 7-10 Table 4-1, the uniform distributed live load is 50 psf, and the concentrated load is 2000 lbs.
Figure 5.2.2 Live Load Cases
Alt text: Depiction of live load scenarios on a floor joist, showing both a uniform distributed load and a concentrated load positioned at mid-span and near the support for maximal load effect calculations.
In this scenario, the entire concentrated load is applied to the joist since the joist spacing (4 ft) is greater than the specified 2.5 ft square area. Designers often simplify this by applying the force as a point load at the center of the area without significantly affecting the calculations. The concentrated load is typically analyzed in two locations: at mid-span to generate maximum moment and near the support to generate maximum shear. The joist design must account for the envelope of moments and shears resulting from these live load cases, combined with dead loads.
Partition Loads
ASCE 7-10 Section 4.3.2 addresses loads from partitions that may be rearranged over time. This is particularly relevant for office occupancies where interior layouts are likely to change during the building’s lifespan. These partitions are typically non-bearing walls attached to the floor and/or roof system.
To account for these movable partitions, engineers commonly add a partition loading to the floor dead load, instead of relying solely on the interior walls shown on architectural plans. Typical values for partition loads range from 15 psf to 25 psf, depending on the engineer’s assessment of potential rearrangements and the expected weight of the partitions. The standard mandates a minimum of 15 psf, based on light-framed, 10 ft high walls spaced 10 ft on center. Consultation with the architect is recommended to get a better understanding of the expected construction types and partition layouts.
While partition loads have been considered in building codes for years, ASCE 7-05 was the first edition to formally introduce them into the standard. The commentary acknowledges that partition loads were sometimes treated as dead loads but doesn’t explain why they are now considered live loads.
However, ASCE 7-10 Section 12.7.2 explicitly requires that a portion of the partition load be included in the effective seismic weight of the building for seismic design considerations.
Other Distributed Live Loads
The commentary section, ASCE 7-10 C4.3.1, provides an excellent discussion on how the live load values in Table 4-1 were determined and suggests values for other common situations not covered in the main table. This information is valuable when negotiating live loads for unspecified conditions with local building authorities.
Additionally, ASCE 7-10 Table C4-2 offers insights into the nature of some live loads and the statistical basis behind the table values. This table presents the results of load surveys, including the interval and recurrence of peak transient loads, and is a worthwhile resource for understanding the underlying principles of live load determination. Spending time reviewing this table enhances the designer’s understanding of load probabilities and code development.
Conclusion
Understanding ASCE 7-10 is essential for any structural engineer. By carefully considering uniformly distributed and concentrated loads, partition loads, and supplemental information from the commentary, you can ensure your designs comply with the standard and provide safe and reliable structures. Always refer to the latest edition of ASCE 7 and consult with experienced engineers when needed.
