Surface Loads

With every type of live load that can be reduced, there is some stipulation on the magnitude of the load.

• For regular live loads, the surface load must be less than 100 psf.
• For heavy live loads, the surface load must be greater than 100 psf.
• For roof live loads, the reduced surface load should be between 12 and 20 psf.

SCIA Engineer calculates and uses the average magnitude of all surface live loads in a load group and warns if the load value is too high or too low.

Calculation of average magnitudes of surface loads

For each reduced live load group, the magnitudes of all the surface loads in the load group multiplied by the area over which they are applied and added together to get the total load. This includes surface loads applied to a 2D member as well as free surface loads (note: this does not include any point or line force or any other non-surface types of loads). For free surface loads which are non-uniform, the value of the average surface load value is used. This total load is then divided by the total area over which these surface loads are applied.

For example, if there 2 loaded concrete floors of a building, each with an area of 800 ft2, and the first loaded floor has an applied surface load of 25 psf and the second loaded floor has an applied surface load of 40 psf, the average surface load will be (35 psf x 800 ft2 + 25 psf x 800 ft2)/(800 ft2 + 800ft2) = 30 psf. This is the value that will be used for comparison to the upper or lower bound load limits.

Considerations and Potential issues

Because of the way average surface loads are calculated, if two surface loads exist over the same area, that overlapping area is counted twice and not just once. This means that, for example, if a single floor has a 60 psf surface load applied directly to the 2D member (e.g. composite deck) and a 50 psf free surface load, the average load acting on the floor is 55 psf and not the total load of 110 psf. If this load group is a regular live load there will not be a warning that the load is over 100 psf since the average load is not over 100 psf.

Only surface loads for which the value of the load is manually defined and pre-defined loads (i.e. library of pre-defined load values) are taken into account when computing the average load value. Other types, (such as snow, wind, water pressure, soil pressure, temperature) are ignored.