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CROSS Safety Report

Use of Table B3 of Approved Document B for loadbearing external walls

Report ID: 1264 Published: 21 May 2024 Region: CROSS-UK


Overview

A reporter is concerned about the apparent selective reading of Table B3 of Approved Document B (ADB) by some designers.

Key Learning Outcomes

For designers and engineers:

  • Loadbearing walls, whatever their location or use should have the most onerous fire resistance guidance applied from Approved Document B Table 3
  • When considering the guidance of Table B3, the user should always consider their particular building situation, including the type of construction and associated sensitivity to heat exposure
  • The potential type of fire exposure a construction may face must also be considered

For building control bodies:

  • Ensure the guidance in Table B3 is only applied within its scope and in situations where it can be demonstrated that the overall functional requirements of the building regulations will be met, and stability will be maintained for the required period in ADB Appendix B

Full Report

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In the 2022 amendments to Approved Document B, a change was made to the wording in Table B3 for loadbearing walls. Previously the bracketed wording to item 2 in Table A1 said: 'Loadbearing wall (which is not also a wall described in any of the following items)'.

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Figure 1: Table A1 from ADB, 2006

When amended in 2022, this wording was updated to: 'Loadbearing wall (for a wall which is also described in any of the following items, the more onerous guidance should be applied)'.

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Figure 2: Table B3 ADB, current (part 1)
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Figure 3: Table B3 ADB, current (part 2)

Designers should be aware that ADB is for common building types, and this includes the type of construction technology being used. When considering the guidance of Table B3, the user should always consider their particular building situation, including the type of construction with associated sensitivity to heat exposure. This must be considered against the potential type of fire exposure that the construction may face.

Overall, a user of ADB must ensure the guidance in Table B3 is only applied within its scope and demonstrates that the overall functional requirements of the building regulations will be met so as to ensure that stability will be maintained for a reasonable period.

This update provided further clarity on the use of Table B3 in situations where building elements could be described by more than one item, for example, a loadbearing wall (item 2) that was also an external wall (item 5).

The concern is that some designers have been misapplying (and some still are) the guidance and selectively applying the items in Table B3. For example, in the case of an external loadbearing wall, by solely applying the external wall recommendations in ADB (item 5) and disregarding the loadbearing wall recommendations (item 2), then the recommended fire resistance exposure would be from the inside only, and not from the outside.

In the absence of any engineering justification, there is a risk that a lack of exterior fire resistance could compromise the stability of an external loadbearing wall at an early stage should there be an outside exposure (for example, from a fire plume projecting from a window – see illustration, Figure 4 below). This risk is especially apparent for loadbearing wall systems that rely on outer sheathing to protect internal elements.

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Figure 4: Potential fire condition exposing the exterior side of an external wall to heating

Another separate but related matter is that ADB guidance for loadbearing walls (in item 2) is for exposure from each side separately. A potential realistic exposure condition is for both sides to be simultaneously exposed e.g. where a projecting fire plume exits from an opening. Some newer construction technologies are particularly sensitive to the type of heating, and simultaneous exposure could significantly decrease the overall structural performance.

Expert Panel Comments

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The Expert Panel agreed with the general comment that designers should consider fire safety matters on a case-by-case basis rather than just assuming the applicability of relevant guidance to every scenario.

Construction and testing

There are a couple of points to highlight from both the construction and the testing perspectives.

From a construction standpoint, in midrise light frame construction, the walls are typically affixed to a fire rated floor system, and the load path transfers the load through the floors. 

This means that the walls are not exposed on both sides because the load path of the system will be separated at each floor level. In low rise construction where the loading from the roof is taken by the perimeter walls, detailing becomes crucial and is material dependent. For example, in a lightweight steel system under fire conditions, fire from either side might not be as onerous as fire from one side due to loading and bowing considerations. In timber construction, the situation differs because bowing is limited, but charring would be more significant if the exposure is on both sides. Separate considerations should be applied to concrete and masonry systems. 

the Expert Panel are not aware of any test houses in the UK that conduct fire tests on partition systems with fire from both sides

Therefore, the Expert Panel advise that, depending on the method of construction of the external wall and the designed load path, the loadbearing resistance of the wall and overall detailing of the construction method should first be considered.

From a testing perspective, it should be noted that the Expert Panel are not aware of any test houses in the UK that conduct fire tests on partition systems with fire from both sides. If fire resistance from two sides is required, this evaluation would primarily rely on performance-based design evaluation rather than direct fire test evidence. In addition, the designer needs to assess which scenario is more onerous from a structural perspective to determine whether the provided data is acceptable or if further calculations are required.

Loadbearing walls - most onerous guidance

The report highlights an important point on the application of Table B3 - elements must comply with all relevant requirements, not just one requirement.

In Table B3, item 2- loadbearing walls, makes clear that for any subsequent type of wall from then on, that is also loadbearing, then the most onerous guidance should be applied. The Expert Panel agree that this includes the type of exposure, and not just the R value required. Thus, it should be clear that for any loadbearing wall performing any other functions (for example, item 5- external wall, item 6 and 7- compartment wall) the exposure to each side separately is the exposure type required.

External walls (Table B3, item 5)

For an external wall, if it is loadbearing and more than one metre from a boundary, it must comply with item 2 and item 5b, that is exposure from either side separately, not just from the inside. This is the most onerous condition in the guidance.

Fire exposure to external walls can also occur for many reasons, for example from adjacent buildings within one metre of a relevant boundary (item 5a), or from below, for example from an adjoining podium, car park, roof, and other areas where a fire might develop and run up the side of an external wall. The risk is that an exterior fire attack on an exterior, loadbearing wall, may cause a premature collapse.

The Expert Panel further agrees that for an external wall over an opening (for example, a window), a both-side exposure is feasible. However, normally for this example, the external exposure would be nowhere near as severe as the internal exposure.

There is perhaps an opportunity for evidence-based research, utilising existing studies, to inform this issue

Typical construction typologies, that is common building situations, are likely to be such that the two sided exposure would be less onerous than a single sided exposure, as loadbearing capacity would be governed by asymmetric heating and buckling as opposed to material strength. Thus, in practice, this aspect might not be a problem for the structural design. Flames might emerge from windows, but the areas directly above a window are not usually loadbearing, even on the level above the window (as that would require a transfer structure above to take the load on either side). In addition, there tends to be insulation on the outside of the external wall which might protect the structure, if it meets BS EN 13501‑1 Class A2.

Although the risk might be lower in this case, the Expert Panel agree it should be considered. There is perhaps an opportunity for evidence-based research, utilising existing studies, to inform this issue, considering the specific design issues highlighted above and the development of improved guidance within ADB.

A related point is with item 1 of Table B3, which states that the method of exposure for elements such as beams and columns should be the exposed faces. It is feasible that the external faces could be exposed. As such the external faces of columns and beams should be adequately protected and often this is not considered.

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Figure 5: Extract from ADB table 4 showing minimum of 60 minutes for Compartment walls separating buildings

Internal walls

For most internal walls, the most onerous should be taken and where the requirement of B3 is less than that of B4, the fire should be considered from both sides. 

For compartment walls, the lesser REI is taken unless it is loadbearing. This is because in some occupancies in Table 4, up to eleven metres can have 30 minutes fire resistance, for example, sprinklered offices unless the compartmentation separates buildings.

In the case of structural concrete walls or columns in enclosure walls, unless the enclosure has the same fire requirement as the structural requirement, a designer should assume that the fire escapes the enclosure at some point and "attacks" the structure from both sides. 

For example, a structural lift wall may only need thirty minutes under ADB Table B3 but could require up to 120 minutes under Table B4. This assumes that the lift doors are not 120 minutes rated. If this is the case, then the fire will at some point enter the shaft. Consequently, the wall (at least for R) should be checked for 120 minutes for both cases, fire on one side and fire on two sides. 

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