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Steel design Topics related to the design of structural steelwork to BS449 or BS5950

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Old 1st February 2018, 09:09 AM   #1
motorphotos
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Join Date: Apr 2011
Posts: 44
Default plates to bottom of beams

Currently there is a trend for large openings in extensions in the UK

This poses the problem of the lintel as the longest catnic/IG lintel made is 4800mm so less bearings becomes about 4400mm clear span

Catnic do a cavity lintel CXL which is effectively a 203 x 133 UB with a plate welded on the bottom however these are special order and costly

Fabricators want to recreate these lintels by using a UB or UKPFC and welding a plate to the bottom to take the outer leaf

Generally these would be in single storey extensions so the inner leaf load is probably a flat roof and maybe a course of bricks and then an outer leaf taken on the plate of say 300mm high

The question is how to design the plate

The load on the plate would be light something like 2.7 X 0.3 = 0.81 kN/m

Considering a cantilever off the beam flange to the centre line of the load the moment becomes 0.81 x 0.15 = 0.12 kN.m per metre length of beam

Z required would be 0.12 x 10^6 / 165 say 727 N/mm^3

To calculate the Z bd^2/6 the d would be the plate thickness but what would be the b used in the design

For a beam spanning between 2 supports the moment is at a specific point and the section properties are known however in this case the load is spread along a nominal 1.0m for plate so the same principle could not apply as the situation would be 1m or load on the plate meaning you could not really use the plate length of 200mm from the face of the beam to the outside of the outer leaf

Neither could you use the 1000mm long length the load is spread over

The same principle applies to calculating the deflection of the plate

given deflection of cantilever is WL^3 / 8EI where I is bd^3/12 the deflection is on the plate under the load so the b becomes say 200mm and the d is the plate thickness but the load is over 1.0m of plate not just at the point at which the check occurs so is not really correct

So the question is what b is used to calculate the section properties of the plate ??

motorphotos
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