Attachment - 10 Structural Construction Standards for Alaska.pdf

Structural Construction Standards for Alaska's Climate Zone 8

This report outlines the key structural construction standards and reference codes applicable to building in Alaska's Climate Zone 8. This zone, which includes much of the state's Interior, Southwest, and Northwest regions, presents unique engineering challenges due to extreme cold, significant snow loads, high seismic activity, and the widespread presence of permafrost. Construction in this environment demands strict adherence to specialized building codes and practices to ensure the safety, durability, and longevity of structures.

Governing Codes and Standards

Construction in Alaska is governed by a combination of international codes with state and local amendments. It is crucial for any project to consult with the local building authority for specific requirements, as these can be more stringent than state-level codes.

Primary Building Codes: While Alaska has adopted the 2021 International Building Code (IBC) and the 2021 International Residential Code (IRC) at the state level, many local jurisdictions operate under di erent versions. For many projects in Climate Zone 8, the prescriptive measures in the IRC may be insu icient, necessitating a fully engineered design.

Load Standard: The American Society of Civil Engineers (ASCE) 7, "Minimum Design Loads and Associated Criteria for Buildings and Other Structures," is the primary referenced standard for determining structural loads, including snow, wind, and seismic forces. Either ASCE 7-16 or the more recent ASCE 7-22 may be referenced, depending on the specific edition adopted by the local jurisdiction.

Material-Specific Standards:

Wood: The National Design Specification for Wood Construction (NDS) provides the basis for calculating the capacity of wood members and their connections. The American Plywood Association (APA) also provides critical design guidance.

Steel: The American Institute of Steel Construction (AISC), particularly the Seismic Provisions for Structural Steel Buildings (AISC 341), governs the design of steel structures, especially in high seismic zones.

Concrete: The American Concrete Institute (ACI) provides the governing standards, including ACI 318 (Building Code Requirements for Structural Concrete), ACI 301 (Specifications for Structural Concrete), and ACI 306 (Guide to Cold Weather Concreting).

Energy E iciency: While not strictly structural, the Alaska Building Energy E iciency Standard (BEES), which is based on the 2018 International Energy Conservation Code (IECC) with Alaska-specific amendments, dictates thermal and moisture protection requirements that are integral to the building envelope's structural integrity and performance.

Local Jurisdictions: Municipalities such as Anchorage, Valdez, Sitka, and the Fairbanks North Star Borough (FNSB) have their own building departments and enforce local amendments.

Fairbanks North Star Borough (FNSB): As a key example within Climate Zone 8, the FNSB and its cities (Fairbanks and North Pole) have adopted the 2018 IBC and 2018 IRC, along with the 2020 National Electrical Code. The borough's specific regulations are found in Title 15 ("Buildings and Construction") and Title 18 ("Zoning") of its Code of Ordinances.

Structural Design Loads

Structures in Climate Zone 8 must be designed to withstand significant environmental loads. These are determined based on the provisions of ASCE 7 and specific local amendments.

Snow Loads

Heavy snowfall is a major consideration. Ground snow loads (p_g_) are used to calculate roof snow loads and vary significantly by location and elevation.

Reference: The Structural Engineers Association of Alaska (SEAAK) provides a comprehensive table of ground snow loads for numerous Alaskan locations for both ASCE 7-16 and ASCE 7-22.

Example Ground Snow Loads:

Fairbanks: The City of Fairbanks specifies a minimum ground snow load of 60 psf and a minimum roof snow load of 50 psf. This aligns with the SEAAK value for ASCE 7-16. For the newer ASCE 7-22 standard, the reliability-targeted ground snow loads for Fairbanks are 77 psf (Risk Category I), 96 psf (Risk Category II), 110 psf (Risk Category III), and 120 psf (Risk Category IV). The City of North Pole also requires a 50 psf live load capacity for trusses.

Other Locations (ASCE 7-16, 50-year MRI): Bettles (80 psf), McGrath (65 psf), Nome (70 psf), and Valdez (160 psf).

Design Calculation: The flat roof snow load (p_f_) is calculated using the formula:

p_f = 0.7 * C_e * C_t * I_s * p_g

Where C_e is the exposure factor, C_t is the thermal factor (often 1.1 or 1.2 for cold roofs), and I_s is the importance factor.

Unbalanced and Drifting Snow: Designs must account for unbalanced loads and snow drifting as specified in ASCE 7. The City of Valdez, for instance, explicitly notes that hip and gable roofs must also be designed for an unbalanced snow load of 160 psf on the leeward side.

Seismic Design

Much of Alaska is in a high seismic risk area, requiring robust structural design to resist earthquake forces.

Seismic Design Category: Many locations in and around Climate Zone 8 fall into Seismic Design Category D for commercial structures and D2 for residential structures.

Ground Motion: Design is based on mapped spectral response accelerations (S_s_ and S_1_). For example, the City and Borough of Sitka specifies S_s_ = 0.97g and S_1_ = 0.60g.

Force-Resisting Systems: Structural systems must be designed and detailed in accordance with the seismic provisions of the IBC and ASCE 7. This includes requirements for load paths, connections, and material-specific detailing to ensure ductility and prevent collapse during a seismic event.

Seismic Design of Nonstructural Components (ASCE 7, Chapter 13)

In high seismic zones like SDC D, the performance of nonstructural components is critical for life safety and building functionality post-earthquake.

Seismic Forces (Fp): The seismic design force, Fp, applied to a nonstructural component is calculated using a formula that accounts for the component's weight (Wp), its location in the building, the site's seismic hazard (SDS), and factors for component amplification (ap) and ductility (Rp).

Displacement Demands: The design must accommodate the relative movement between the nonstructural component and the structure, which is crucial for elements that cross between floors or connect to di erent structural parts.

The Component Importance Factor (Ip): This factor is critical for ensuring post-earthquake safety and functionality.

Ip = 1.5: This higher factor is assigned to components required to function for life-safety after an earthquake, such as fire sprinkler systems and egress stairways. It also applies to components containing hazardous materials and those in essential facilities (Risk Category IV) needed for continued operation. This increases the design force, resulting in more robust anchorage and bracing.

Ip = 1.0: This factor is used for all other nonstructural components where failure is not a significant life-safety hazard.

Prescriptive Anchorage and Bracing: ASCE 7 Chapter 13 provides specific bracing and anchorage requirements.

Architectural: Suspended ceilings in SDC D, E, and F require seismic separation joints and specific perimeter supports. Partitions taller than 6 feet must be laterally braced to the structure.

Mechanical, Electrical, and Plumbing (MEP): Ductwork, piping, and conduit require bracing to prevent failure. Heavy equipment like generators, boilers, and chillers needs robust anchorage to prevent sliding or overturning. Exemptions are available for some lightweight systems if they have flexible connections.

Wind Loads

High winds are another critical load factor, especially in open or coastal areas.

Basic Wind Speed: Design wind speeds are specified by local jurisdictions. Examples include:

Fairbanks: 90 mph (3-second gust).

Valdez: 120 mph (3-second gust).

Sitka: 150 mph (3-second gust).

Kodiak: 160-175 mph (3-second gust, depending on the specific map).

Design Procedure: Design wind pressures are calculated based on the basic wind speed, exposure category, building height, and other factors as outlined in ASCE 7.

Material-Specific Structural Design

The extreme loads in Climate Zone 8 require specific detailing for wood, steel, and concrete construction.

Wood-Frame Construction

Load Path Continuity: A continuous load path is paramount to transfer lateral (wind/seismic) and vertical (snow/gravity) loads from the roof through the walls to the foundation.

Shear Wall Design: Wood structural panel (WSP) sheathing is essential for creating shear walls to resist lateral forces. Sheathing thickness, fastening schedules, and hold-down requirements are critical and depend on the calculated loads.

Sheathing and Fastening: The American Plywood Association (APA) provides design tables for combined shear and wind uplift. In high-wind areas, increased nailing at panel edges and, in some cases, ring shank (RSRS) nails are necessary to resist high uplift forces.

Hold-Downs and Connections: Continuous tiedown systems are often used to resist overturning forces. The NDS for Wood Construction provides the basis for calculating connection capacity, and specialized hardware is required in high-wind regions.

Structural Steel Systems

The design of steel structures in Alaska must prioritize preventing brittle fracture at low temperatures, especially in Seismic Design Category D.

Material Selection and Toughness: Steels must exhibit adequate notch toughness. ASTM A333 Grade 6 is common for low-temperature service due to its specified Charpy V-notch (CVN) impact toughness at -50°F (-45°C). Other steels like ASTM A992 and ASTM A913 can be used if toughness requirements are specified.

Charpy V-Notch (CVN) Testing: CVN testing is a critical quality control measure to verify a steel's toughness at a specified low temperature. The required absorbed energy depends on the steel grade, thickness, and design service temperature.

Seismic Provisions and Connections: The AISC Seismic Provisions for Structural Steel Buildings (AISC 341) provides mandatory requirements for designing steel structures in SDC D. Connection details must be designed to minimize stress concentrations.

Welding in Cold Regions: Welding in sub-zero temperatures requires a qualified Welding Procedure Specification (WPS) that includes preheating the base metal and using low-hydrogen consumables to prevent cracking and embrittlement.

Structural Concrete Design (ACI Standards)

Durable concrete in Climate Zone 8 requires strict adherence to ACI 318, ACI 301, and ACI 306 to combat severe freeze-thaw cycles and ensure proper strength gain.

Concrete Mix Design for Durability (ACI 301):

Air Entrainment: This is the most critical factor for freeze-thaw durability, with a required total air content of 5% to 8%.

Water-Cementitious Material Ratio (w/cm): A low w/cm ratio (maximum of 0.40 to 0.45) is essential for low permeability.

Compressive Strength: A minimum 28-day compressive strength of 4,000 psi is standard for exterior concrete.

Admixtures: Non-chloride accelerating admixtures are often used to speed hydration.

Admixtures containing calcium chloride are prohibited due to corrosion risk. Water reducers help achieve a low w/cm ratio while maintaining workability.

Reinforcement Specifications (ACI 318):

Seismic Detailing: Reinforcement must comply with ACI 318-19 seismic provisions. ASTM A706 steel is often specified for members resisting seismic forces.

Material and Placement: Standard reinforcement is Grade 60 (ASTM A615) and must be free of ice and placed with the correct concrete cover.

Cold-Weather Concreting Procedures (ACI 306):

Definition: Cold weather is when the average daily air temperature is below 40°F for more than three consecutive days.

Protection from Freezing: Freshly placed concrete must be protected from freezing until it reaches a minimum compressive strength of 500 psi. Early freezing can reduce its ultimate strength by up to 50%.

Protection Methods: The concrete temperature must be maintained above 50°F using insulating blankets or vented heated enclosures. The protection period is typically 5-7 days but can be reduced if accelerators are used.

Foundation Design in Permafrost and Cold Regions

The presence of discontinuous and sporadic permafrost is a primary challenge in Climate Zone 8. The goal is to keep the permafrost frozen to prevent thaw settlement. In areas without permafrost, preventing frost heave is the main concern.

Site Investigation: A mandatory geotechnical investigation is the critical first step for any project on permafrost, resulting in a formal report with specific foundation recommendations.

Foundation Types for Permafrost:

Pile Foundations: Piles transfer loads through the unstable active layer to frozen ground below, relying on the adfreeze bond. Design is governed by limiting long-term settlement from soil creep.

Post-and-Pad Foundations: This "floating" foundation system accommodates ground movement. Modern systems integrate adjustable jack systems (screw jacks) that allow the building to be periodically re-leveled to correct for settlement or heave.

Thermosyphon Foundations: These passive heat transfer devices preserve the frozen state of the ground by moving heat from the soil to the cold winter air. Their design is governed by CSA S500:21 and requires detailed thermal analysis to determine the number and placement needed to maintain a target ground temperature, accounting for future climate change.

Foundation Best Practices for Cold Regions (Non-Permafrost):

Frost-Protected Shallow Foundations (FPSF): Uses rigid foam insulation to trap geothermal heat and prevent the ground from freezing and heaving.

Insulated Concrete Forms (ICFs): Provide continuous insulation for foundation walls.

Frost Depth: While specific prescriptive frost depths vary, the significant ground frost is a major consideration. For example, utilities in Fairbanks mandate a minimum cover of four feet for water mains to prevent freezing.

Building Envelope and Insulation Requirements

A well-insulated and airtight building envelope is critical for energy e iciency and preventing heat loss that could thaw permafrost.

Alaska Building Energy E iciency Standard (BEES): This standard sets minimum thermal envelope requirements.

Prescriptive Requirements for Climate Zone 8:

Ceiling Insulation: R-59

Wood Frame Wall Insulation: R-30

Floor Insulation: R-38

Fenestration (Windows): U-Factor of 0.22 or lower

Best Practices: A continuous vapor barrier is required on the warm-in-winter side of the insulation. It is critical to insulate the entire area under a slab-on-grade floor to prevent heat loss.

Executive Summary

Structural construction in Alaska's Climate Zone 8 is governed by adopted international codes with significant state and local amendments, with structural loads determined according to ASCE 7. For example, the Fairbanks North Star Borough has adopted the 2018 IBC/IRC and specifies a 90 mph wind speed and a 60 psf ground snow load. The extreme environmental conditions—heavy snow loads (60-160 psf), high seismic activity (typically Category D), and high wind speeds (90-175 mph)—dictate the primary design considerations.

A critical and detailed aspect of seismic design is the treatment of nonstructural components per ASCE 7 Chapter 13. This involves calculating seismic forces (Fp) and designing robust anchorage and bracing for architectural, mechanical, and electrical systems. The Component Importance Factor (Ip) is a key parameter, with a value of 1.5 assigned to life-safety systems like fire sprinklers to ensure their functionality after an earthquake.

For wood-frame construction, an engineered design is often necessary, focusing on a continuous load path and robust shear walls. For structural steel, preventing brittle fracture is paramount, achieved through material selection (e.g., ASTM A333), mandatory Charpy V-Notch (CVN) testing, and adherence to AISC 341 seismic provisions. For structural concrete, success hinges on following ACI 301, 306, and 318, requiring a durable mix with air entrainment (5-8%), a low water-cementitious ratio (max 0.45), and strict cold-weather protection procedures.

The most unique challenge is foundation design on permafrost, which begins with a mandatory geotechnical investigation. The goal is to prevent thaw settlement. Solutions include pile foundations designed for creep settlement, elevated post-on-pad foundations with adjustable jacks, and thermosyphon systems designed per CSA S500:21 to actively cool the ground. Finally, the building envelope must meet the stringent requirements of the Alaska Building Energy E iciency Standard (BEES). Due to the variability of conditions, a site-specific geotechnical investigation and consultation with local building o icials are imperative for any project.