Engineering Tips: Building Code Updates

The International Building Code (IBC) has a three-year release cycle, with the most recent edition being the IBC2024. This building code references the American Society of Civil Engineers (ASCE) “Minimum Design Loads and Associated Criteria for Buildings and Other Structures” standard designated as ASCE7-22. Many revisions have been made in this code/standard cycle regarding environmental loads. As jurisdictions are beginning to adopt this latest code cycle, it is worth highlighting some of the most notable changes as follows:

Basic Wind Speed Maps: The basic wind speed maps have been revised. The bulk of these revisions occur in hurricane-prone areas. From FEMA’s Fact Sheet, ‘Highlights of Significant Changes to the Wind Load Provisions of ASCE 7-22’, Figure 1, we see the new contours in BLACK compared to ASCE 7-16’s contours in RED. Figure 1 is for Risk Category II buildings and structures. Similar changes have been made to the wind speed maps for Risk Category I, III and IV.

Wind-borne Debris Region (WBDR): Areas in hurricane-prone regions where the basic wind speed is equal to or greater than 140 mph remain defined as being within a WBDR. However, the verbiage for an area in a hurricane-prone region where the basic wind speed is equal to or greater than 130 mph but less than 140 mph has been revised. This is now defined as being within a WBDR if it is within 1 mile (1.6 km) of the mean high water line where an Exposure D condition exists upwind of the water line.
Buildings with Gable and Hip Roofs with Slopes Greater than 7 degrees: The roof wind zones have been simplified and some adjustments have been made to the component and cladding pressure coefficients.

Additional information and description of other wind load changes can be found at the following link:

https://www.fema.gov/node/american-society-civil-engineers-minimum-design-loads-and-associated-criteria-buildings-and

Tornado Loads: Consideration of tornado loading has been included in this code cycle. A flowchart has been developed to aid in determining if a building or structure is required to be designed for tornado loading. Below is a version of the flowchart from FEMA/NIST’s ‘Design Guide for New Tornado Load Requirements in ASCE 7-22’. If the end user desires the structure to be designed for tornado loading then those requirements are to be considered. If the building or structure is classified as Risk Category III or IV, tornado loads are to be investigated further.

The flow chart indicates the location must be checked if it is within the tornado-prone region. The tornado prone region is defined as the shaded region of Figure 32.1-1 as shown below.

The flowchart has a check to determine if the Tornado Wind Speed, V_T is greater than 60mph. To determine V_T, proper site and building information is required to calculate the Effective Plan Area of a facility. The effective plan area for buildings and structures that are not considered an Essential Facility is equal to the smallest convex polygon or a ‘simplified’ smallest rectangle enclosing the plan of the building as shown in Figure 5 below from FEMA/NIST’s ‘Design Guide for New Tornado Load Requirements in ASCE 7-22’.

For Essential Facilities, the effective plan area includes the smallest convex polygon enclosing both the essential facility (or facilities) and all the buildings and structures required to maintain the functionality of the essential facility or facilities. Examples below are from FEMA/NIST’s ‘Design Guide for New Tornado Load Requirements in ASCE 7-22’.

The flowchart includes one more check as can be seen in Figure 3 previously displayed.

Lastly, consideration for the protection of glazed openings should be dealt with the end-user and their expectations for the building or structure’s overall performance as this may impact the enclosure classification affecting the pressure coefficients.

Additional information regarding tornado wind load considerations can be found at the following link:

https://www.fema.gov/sites/default/files/documents/fema_asce-7-22-tornado-loads-design-guide_012023.pdf

Snow Loads: The design approach for Snow Loads has been revised. There is no longer a Snow Importance Factor, rather, there are new ground snow load maps for each building risk category. Also, the minimum snow load values for low-slope roofs has been revised, the thermal factors have been revised, and the rain-on-snow value has been increased. Snow drift calculations have been revised and depend upon the ground snow load value (p_g) and the roof’s insulating R-value. A Winter Wind Parameter, W₂, has been incorporated into snow load calculations and must be provided by the Customer/Building EOR. It should be noted that even though it appears the ground snow load values have increased, the resulting design snow load value is typically comparable to the previous ASCE 7-16 edition after applying the various factors involved.
Rain Loads: There have been changes to the ponding calculations to include ponding head (dp). Secondary drainage system inlet is to be vertically separated from the primary drainage inlet by 2 inches. Existing roofs are to be evaluated if they receive discharge water from any new construction. The design storm return period by Risk Category have undergone a revision.
Seismic Loads: Seismic ground motion and long-period transition maps have been revised. Always check the site-specific values in the ASCE Hazard Tool. The Site Classifications have been revised and Site Classes BC, CD, and DE have been added as described in the following table.

When site class is unknown, the design is to be based upon the worst case of site class C, CD, or D. The code now requires that the seismic parameters S_MS and S_M1 are to be provided by the Customer/Building EOR. Always check the site-specific values in the ASCE Hazard Tool.