Seismic Design Category

Once you’ve determined the building’s risk category, the next step is to determine the seismic design category (SDC). SDC is determined based on several factors:

• Soil properties at the site, or site class, classified as A, B, C, D, E, or F: Site class A is associated with the presence of hard rock. Site class F is associated with peats and/or highly organic clays, very high plasticity clays and very thick, soft/medium stiff clays
• Mapped values of the seismic hazard
• Risk category of the structure

SDCs range from A to F, with A representing the lowest risk and F the highest. The SDC is used to determine permitted seismic force-resisting systems and structural height limitations. It also determines the applicability of special requirements associated with structural redundancy and structural irregularities in the building system.

SDC A represents a very low seismic hazard for which there are no seismic-specific limits (NL) on structural height, system type, structural redundancy, or structural irregularities. Structures located in this category are not subject to design forces determined in accordance with the ELF. Beginning with SDC B, seismic forces in accordance with ELF are applicable and consideration must be given to special requirements for structural irregularities. Special requirements and limitations become increasingly stringent in SDC C and higher. As can be seen in Table 2, as seismic design category increases, structural height limitations apply as well as limitations on the use of some lateral force-resisting systems.

Calculating SDC for Short-Period Structures

For short-period structures, such as most wood-frame structures, ASCE 7 allows determination of the seismic design category based on value of SDS (design spectral response acceleration) and risk category alone (see Table 3), provided alternative criteria are met for structure period and diaphragm flexibility and the site’s mapped spectral response acceleration period value (S1) is less than 0.75.

Structural Redundancy

Separate from inherent redundancies present in typical wood construction, the code encourages a redundant layout of seismic force-resisting systems throughout the building. The redundancy factor, ρ, which is either 1.0 or 1.3, is applied to the seismic design forces based on lateral force-resisting system configuration. In SDC B and C, the redundancy factor equals 1.0. For wood-frame shear walls in higher seismic design categories, use of a redundancy factor equal to 1.0 can often be accomplished for plans that have a regular layout and where lateral resistance is provided on all sides of the building perimeter by shear walls with aspect ratios (height-to-length) of 1.0 or less, or, for cases where the aspect ratio of individual shear walls is greater than 1.0, the total length of light-frame shear walls equals or exceeds the story height.

Special Design Conditions: Anchorage of Concrete or Masonry Structural Walls to Wood Diaphragms

Requirements for anchorage of concrete or masonry structural walls to wood diaphragms, found in both ASCE 7 and SDPWS, were developed to address instances where these structural walls became detached from supporting roofs, resulting in collapse of walls and supported bays of framing. The intent is to prevent the diaphragm from tearing apart during strong shaking by requiring transfer of wall anchorage forces across the complete depth of the diaphragm.

Minimum out-of-plane wall anchorage design forces are provided in ASCE 7 Section 12.11.2. Additional requirements, applicable in SDCs C, D, E, and F, are provided in Section 12.11.2.2. Provisions include the use of continuous ties between diaphragm chords to distribute wall anchorage forces into the diaphragm and permitting the use of subdiaphragms to transfer anchorage forces to main continuous cross-ties. Restrictions exist on the use of toe-nailed connections and nails subject to withdrawal, and on framing loaded in cross-grain bending or cross-grain tension. Special detailing provisions for wood diaphragms consistent with those in ASCE 7 were first added to 2015 SDPWS in Section 4.1.5.

Wind Design and Wood

Many regions of the United States experience high winds, with the accompanying threat of wind-related damage to buildings. According to the Insurance Institute for Business and Home Safety (IBHS), high winds cause billions of dollars of property loss each year.⁸ However, damage from wind can be minimized through engineered design which strengthens buildings against wind forces. As with the seismic provisions, codes and standards governing the design and construction of wood-frame buildings have evolved based on field data from prior wind events and related research.

Each building, with its own unique characteristics and site conditions, reacts differently to wind loads. Many complex and interrelated issues must be addressed before a building can be truly considered wind-resistant.

Structural Wind Loading

The wind loading requirements in the IBC are determined primarily through reference to ASCE 7-16. The minimum design requirements for wind loads are intended to ensure that every building and structure has sufficient strength to resist these loads without stressing any of its structural elements beyond the material strengths prescribed in the code. The code emphasizes that the loads prescribed in Chapter 16 are minimum loads; in most conditions, the use of these loads will result in a safe building. For this reason, it is important to ensure that wind loads are properly determined. The commentary that accompanies ASCE 7-16 is a good source for additional information, as it outlines conditions which may result in higher loading.