What is mass timber construction fire rating, and why is it important? Mass timber is popular because of its strength, sustainability, and visual appeal. This building method uses large, solid wood panels or beams to create strong and attractive structures. However, safety, especially in a fire, is a major concern.
Fire-resistance ratings are crucial for the safety of mass timber buildings. These ratings tell us how long a material can withstand fire before it weakens or allows the fire to spread. Its’ accuracy is important to give people time to evacuate and to help firefighters control the blaze.
Understanding Fire-Resistance Ratings for Mass Timber
Fire-resistance ratings are crucial for building safety, especially in mass timber construction. These ratings show how long a material can withstand fire before it weakens or allows the fire to spread. For instance, a 1-hour fire-resistance rating means the material should resist fire for an hour. This, of course, gives people time to evacuate and firefighters time to respond.
Although wood is flammable, mass timber has a unique ability to resist fire. When it burns, the outer layer chars, forming a protective barrier that slows the fire and helps keep the structure strong. The fire-resistance rating of mass timber depends on factors like wood thickness and type.
Methods for Demonstrating Fire-Resistance Ratings
When it comes to demonstrating fire-resistance ratings for mass timber, there are a couple of key methods used. Let’s look at testing standards and alternative approaches.
Testing Standards
One of the main ways to assess fire resistance is through standard fire tests like ASTM E119. This test measures how long a material or assembly can resist fire under controlled conditions. In the test, mass timber products are exposed to fire in a furnace to see how well they hold up. The test checks if the timber can maintain its structural integrity and prevent the fire from spreading for a set amount of time. It ultimately tells builders how long the material can safely resist fire in a real-world scenario.
Alternative Methods
While standard tests are reliable, they’re not the only way to determine fire resistance. Performance-based design offers another option by using engineering judgment and computer simulations. This approach considers a building’s specific conditions. These aspects are things like layout, materials, and fire protection systems. These tools predict how mass timber will behave in a fire, allowing for flexibility and customization in safety design.
Calculation-Based Method Using NDS Chapter 16
NDS Chapter 16 is a critical part of fire-resistance design for mass timber. The NDS, or National Design Specification for Wood Construction, provides guidelines for calculating how long mass timber can resist fire. Chapter 16 focuses on determining fire-resistance ratings with calculation rather than physical tests.
Guide to Calculating Fire-Resistance Ratings
- Determine the Thickness: The thickness of the timber plays a big role in its fire resistance. Thicker timber generally resists fire longer.
- Calculate Char Rate: When exposed to fire, mass timber forms a char layer on its surface. This char acts as a protective barrier. The NDS provides formulas to calculate how fast the wood chars, depending on the type of wood and fire exposure.
- Adjust for Structural Integrity: As the wood chars, its structural strength decreases. Chapter 16 offers guidelines on how to adjust the design calculations to account for this loss of strength. This ensures that the timber can still support the building for a specific time during a fire.
- Apply Stress Adjustment Factors: Fire conditions cause additional stress on the material. NDS Chapter 16 includes factors that help adjust the design to account for these stresses. It can ensure everyone involved that the structure remains stable.
Design Calculations Under Fire Conditions
When mass timber is exposed to fire, a process called char formation occurs. As the outer layers of the timber burn, they form a charred layer on the surface. This char layer acts like a shield, slowing down the spread of the fire and protecting the inner, unburned wood. While this helps the timber resist fire, it also affects its structural integrity.
Impact of Char on Structural Integrity
As the char layer forms, the timber loses some of its strength because the charred wood no longer contributes to supporting the structure. The thicker the char layer, the less wood is left to bear the load. This reduction in structural capacity is an important factor to consider with mass timber buildings.
Adjusting Design Calculations
To ensure safety, engineers adjust their designs to account for char formation during a fire. Using guidelines like NDS Chapter 16, they estimate how much timber will char and then calculate the strength of the remaining wood. This involves reducing the timber’s assumed strength based on the char layer’s thickness.
By accounting for char formation, engineers can design buildings that meet standards and stay sound during a fire. This careful planning keeps buildings safe and stable under fire conditions.
Performance of Different Mass Timber Products
Cross-laminated timber (CLT), Glulam, and tongue and groove wood decking each handle fire differently.
CLT has a solid structure that forms a slow-burning char layer, helping it stay strong longer during a fire. Glulam beams and columns also form a protective char, but its fire resistance depends on the adhesive holding the layers together. Tongue and groove wood decking chars quickly due to its thinner profile, so it often needs extra fire protection.
Penetrations in Fire-Resistance Rated Assemblies
In assemblies like walls, floors, and ceilings, penetrations for pipes, wires, and ducts can weaken fire protection. These openings must be sealed properly to prevent fire and smoke from spreading.
Fire-stopping materials expand when heated. They’re to be used to seal penetrations, matching the fire rating of the assembly. This helps maintain the structure’s integrity during a fire.
Best practices include using approved materials, following installation guidelines, and regularly inspecting seals. Properly protecting penetrations ensures fire safety. It helps to keep fire and smoke contained and gives occupants more time to evacuate.
Safe and Secure Mass Timber Construction
We can never be careful enough with mass timber construction fire ratings. Understanding fire-resistance ratings, testing standards, and calculation methods is vital. Products like CLT and Glulam each have distinct fire performance traits that affect building design. Effective fire-resistance design ensures the stability of buildings and safeguards occupants and property. For specific fire-resistance needs and expert guidance, consulting professionals with experience is necessary.
Need more information or advice? Contact us today. Our expertise will help ensure any project is safe and compliant with all fire safety standards.
