Use of load triangles above lintels (BS 5977) 2

[MHobbs]

Regarding the Limiting conditions, does clause c imply that the minimum distance between the lintel and the floor above should be 600mm?
The other capture I have attached is from fig 4 of the document and clearly implies the distance could be less but it is not dimensioned.

 

[EireChch]

While not my field, I would not take the Figure as implying that less than 600mm of masonry is permitted. If it was to be implied there would be a dimension;.

 

[JLNJ]

I believe the 600mm minimum is so that you have enough mass to resist the thrust. Without the mass, the lintel can't act as an arch.

FWIW - I tend to design all my masonry lintels as flexural elements, that way I understand what's going on rather than hope I'm applying some prescriptive method properly. And I don't have to worry about the mason placing a joint in the field in a place which totally wrecks the arching assumption(s).

 

[KootK]

I believe the 600mm minimum is so that you have enough mass to resist the thrust. Without the mass, the lintel can't act as an arch.

I agree with it being about resistance to arching thrust but not, necessarily, that it's about mass. C & E taken together seem to speak to thrust resistance:

e) Provides enough wall adjacent to the opening such that the thrust can be resisted by the shear resistance of that wall secment.

c) Provides enough wall above the opening at the bearings such that you've physically got something for the thrust to push against in bearing before it is moved down into the wall in shear.

I agree that something ought to resist thrust wall overturning and that something may well be mass. I just don't know that these provisions speak to that given that:

1) The provisions say nothing about the overall height of the thrust resisting wall piers unless we're just assuming 6'-8" etc and;

2) For larger openings, the mass of 2' of brick alone certainly won't be enough to resist overturning.

Regarding the Limiting conditions, does clause c imply that the minimum distance between the lintel and the floor above should be 600mm?

I would say that the minimum distance between the lintel bearing and the floor above should be 600 unless you've provided a non-prescriptive way provide that same, thrust bearing resistance.

 

[HTURKAK]

- Clause c DOES NOT imply that the minimum distance between the lintel and the floor above should be 600 mm..but one of the limiting conditions described in clause 5 Limiting conditions
For the methods described in clauses 7 and 8

-BS 5977 Part 1 is for Lintels , Method for assessment of load and clauses 7 and 8 describe triangular load method which assumes that the masonry wall over an opening will arch over the opening and loads outside the interaction zone is not considered .

- I have written the the clause 4.2 Assumptions For the methods described in clauses 7 and 8,

a) all the weight of the masonry within the load triangle is carried as a load on the lintel;
b) any point or distributed loads applied to the masonry within the load triangle are dispersed at 45° and carried by the lintel [see Figure 2(a)];
c) any point or distributed loads applied to the masonry within the interaction zone are reduced by 50 %, dispersed at 45° and carried by the lintel
[see Figure 2(b)];
d) the weight of masonry in the interaction zone is not carried by the lintel.



- You have posted some part of the fig 4 .. When you look to the remaining , i am sure you will get the point since pretty straight forward...I have attached below..

 

[EireChch]

I stand very much corrected - I should keep in my lane Htu

 

[MHobbs]

Morning all,

Thanks for the replies.
HTU I know you said I'd get the point, but I am a bear of very little brain.

The concrete slab would often be hollowcore but this would be a fairly typical construction.
Is the same load dispersion mechanism not justifiable if it is concrete slab or planks as through masonry?
What is the difference if the floor load is instead coming from joists in the form of point loads?

To not worry anyone I'd also just like to say my design practice is the same as JLNJ and in the situation I have shown I would design the lintel as a beam carrying the full load of the floor and wall above. I am just trying to understand the mechanism being utilized and the intent of the wording of the standard.

 

[HTURKAK]

Pls look Appendix A Use of assessed loads for design or selection of lintels.

Figure 4 and 5 shows the dispersion of loads for applicable combinations. In case of floor load is coming from joists , pls look figure 4 point load .

EDIT :

Will you please post your load analysis so we can see lintel design loads on a worked example ??