Concentrated Loads

In addition to the specified uniform live loads, building codes may require that concentrated loads (sometimes called "safe loads") be considered in the design of most non-residential floor systems. A joist must be designed for the maximum effects produced by either (1) the total loads or (2) the dead load plus the code-specified concentrated load. Multiple load cases must be considered to find the maximum induced stress in the joist.

Typical applications requiring design for concentrated loads include offices, garages, schools and most other non-residential floors. Loads are specified in each code. Note: additional considerations must be taken to address vehicular loads in garages. The RedSpec™� concentrated load analysis is not appropriate for vehicular loading.

Concentrated load analysis in RedSpec™ is included whenever the concentrated load input on the Application loads tab is non-zero. The analysis uses a conservative point-load approximation. Rather than using a uniform load distributed over 30", the I-joist and beam analysis places an equivalent point load in several locations so as to maximize shear and moment within each span. These locations are 15" from each end of the span and at midspan. For I-joists with holes, the concentrated load is also placed at the center of round holes and at each edge of square or rectangular holes. For trusses, the point load is placed consecutively at pin and panel locations.

To account for load sharing on closely-spaced joists, it is assumed that only half the concentrated load applies directly to any single member (while the other half is shared with neighboring joists) whenever the spacing is 24" or less. Where the member spacing is more than 24", the full concentrated load is applied to the member. These assumptions do not apply to the open-web truss analysis.

Background

A concentrated load analysis may be conducted manually by adding a uniform load in critical locations. As an example, say a 1500# concentrated load is specified for a library stack room. If the engineer specifies how this load is to be applied, those criteria must be followed. Without direction from the engineer, the following criteria are used:

Case 2 - Uniform dead load plus the concentrated load distributed as follows.

Codes typically mandate the load be applied over an area sized 2.5' by 2.5'.

For example,

1500# / (2.5' x 2.5') = 240 psf

Apply a 240 psf uniform load, 2.5' long, on the member to maximize shear, bearing length and moment. The 240 psf load will automatically be multiplied by the Spacing/Trib specified in the Design Tab, which will be accurate as long as the joist spacing is 2.5' or less. For a simple-span joist the load is placed from 0' to 2.5' to maximize shear and bearing length, and at midspan to maximize moment. Additionally, for joists with holes, the concentrated load must be located to maximize shear at the hole.