This post is an overview of the British standard BS 5977-1:1981 which looks specifically at the method for assessment of load for lintels. This applies to both steel lintels and prestressed concrete lintels. It is an intrinsic part of understanding how our products are used within construction under current legislation.
This standard applies to lintels up to the length of 4.5 metres and assumes certain criteria are met regarding the weight of masonry, and applied point and distributed loads. These assumptions will be more specifically addressed within the post, but for a more in-depth understanding of this, please get in touch.
Load Triangle and Interaction Zone
Lintel load triangle and interaction zone
The load triangle is ‘a 45° triangle having 1.1 times the clear span of the lintel at its base’. The interaction zone is ‘a zone outside the load triangle but within a 60° triangle having 1.1 times the clear span of the lintel at its base.’ These two triangles form the basis of this standard and this diagram shows how they are applied above the lintel.
Assumptions
The considered load will depend upon openings that intersect with the interaction zone.
The standard also makes the following assumptions:
- The masonry load within the load triangle is carried by the lintel;
- Point loads within the load triangle are dispersed at 45° and carried by the lintel;
- Point loads within the interaction zone are reduced by 50%, dispersed at 45° and carried by the lintel;
- The weight of masonry in the interaction zone is not carried by the lintel.
Assessment of loads
There are three load scenarios which need to be assessed and considered when selecting and specifying lintels.
How loads are dispersed and calculated in the load triangle and the interaction zone.
The first is the assessment of load for lintel supporting a wall without openings within the interaction zone. This is the simplest scenario and basically, means that to calculate the load you take the load of the masonry above the lintel within the load triangle. If any additional loads are applied to the masonry within the load triangle or interaction zone you apply them at 100% and 50% respectively and disperse them at 45°.
All wall, floor and roof loads in the area bounded by vertical lines A and B are applied across the section XX to the lintel under consideration.
The second scenario is the assessment of load for lintel supporting wall with one or more openings within the interaction zone. This scenario is more complex to understand but is based on an opening intersecting with the interaction zone (not going across the span of the interaction zone.) If an opening intersects the interaction zone and additional load is applied to the lintel. The calculation of the additional load depends on the width of the interaction zone at the point of the opening minus where the opening overlaps – the diagram best illustrates this point.
The final scenario is the assessment of load for lintel directly supporting roof or point loads. Under these conditions, the full imposed load plus the self-weight of the lintel is applied.
Lintel Selection
When specifying or selecting a lintel it is obviously imperative that the correct lintel is used for the load to be carried. Our load span tables are a useful guide to selecting the right lintel, but there are times where you may need or want to convert the figures to a UDL (uniform distributed load). There are many things to consider, so if you require assistance with lintel specification or selection then give us a call on 01455 272457, email us or leave a comment below.