To designate a joist, the project engineer should specify the Joist Depth, Slab Thickness, Mesh Size and the Design Loads (Dead, Live, and Total Load together with any concentrated loads where applicable). For maximum efficiency, joists will be designed to specifically meet these loading requirements. Live Load deflection will be limited to L/360.
The Total Load includes the uniform Live Load, the weight of the joist, mesh, concrete, partitions, all mechanical, ceiling, and miscellaneous loads. A slash separates the Total Load and uniform Live Load which include furnishings and people.
Example of Joist Identification Based on a 2-¾ inch Slab:
- Joist Depth = 16 in.
- Live Load = 40 lbs./sq. ft.
- Dead Load = 65 lbs./sq. ft.
- Total Load = 105 lbs./sq. ft.
Simply show H16 with the Live, Dead, and Total loads clearly listed on the framing plans. Example: H16 105/40.
Selecting a Joist:
The following tables are to assist you in selecting the most efficient joist depths for a given concrete thickness and typical spans. It is often best to select typical joist depths based on the typical spans as the cost of some heavier joists will be less than the cost of raising the building height. The tables indicate the most efficient depth based on various loads, spans, and slab thicknesses. The minimum joist depth is also provided.
It is important to separate the total uniform loads from other special loads such as wall loads, concentrated loads, line loads, point loads, and partial loads, which should be shown on the drawings and will be added to the design requirements accordingly.
Hambro can accommodate special loads in several different ways:
- Increasing slab thickness with the use of Hanger Plate.
- Adding another layer of mesh.
- Adding a joist under the load.
- Using headers to transfer the load to adjacent joists.
The method of choice will depend on the type and extent of special loading conditions. We stand ready to assist the professional engineer in his determination of the method to use to accommodate any special loading conditions.