Steel Beams vs Cavity Lintels

[MortarThePoint]

For long spans the loading can rise considerably. Not only does the load rise in proportion to length, the resulting peak moment on the resisting member rises with the square of length:

 

The section of the member needs to get increasingly large to resist the increased moment and still the designers typically allow a deflection in proportion to length. As I understand it, this is typically L/325. That means that a lintel for a 3300mm opening will allow a 10mm deflection at the limit. Cavity lintels for large spans (e.g. >2400mm) start to become very expensive if there are roof or floor loads near by. For example, a 3750mm lintel with a total UDL capacity of 50kN runs to around £200-£400.

 

Steel beams have huge moment resisting capability depending on their section. There is a range of Universal Beams (aka RSJ) which have a width of 102mm well suited to a 100mm internal leaf. A 3800mm 305x102x25kg beam costs around £200. I want to understand when to swap to such a steel.

 

Considering just universal beams for a 3400mm opening with 50kN UDL:

  Section         Iy(cm4)   Deflection         Saving

  178x102x19kg    1356      L/364  (9.3mm)     22%

  203x102x23kg    2105      L/564  (6.0mm)     6%

  305x102x25kg    4455      L/1200 (2.8mm)     0%

NOTE: 254x102 UB omitted

A cavity lintel would probably have a deflection of 10.5mm (L/325).

 

Steel beam advantages: reduced deflection, greater capacity, lower cost

Steel beam disadvantage: rarely galvanised and so needs careful painting etc when exposed to cavity, needs boxing in for fire protection

Cavity Lintel advantages: galvanised, makes provision for outer leaf, stated capacity so no need for Structural Engineer's input

 

Ease of installation can go both ways. At length, both are heavy.

 

What experience do people have and when do they typically change from cavity lintels to steel beams?

[Dave Jones]

The issue i have with catnic type lintels is the deflection they tend to have when you go wide, more than 3m. It's only barely over a mm but when its a bifold underneath its enough to make it catch in the middle and not run smooth. Got bitten by this before, cantic was absolutely bang on level upto wallplate. once the roof was on it flexed by a tiny fraction, not enough to crack mortar or anything but just enough to pinch the bifold to make it not run nice. Still opened perfectly but I knew it was pinched every time I opened it which set off my OCD no end.

 

Never used one again on a bifold.

[tonyshouse]

I hate cavity lintels as they introduce so much thermal bridging - and +1 on deflection points 

[MortarThePoint]

1 minute ago, tonyshouse said:

I hate cavity lintels as they introduce so much thermal bridging - and +1 on deflection points 

The thermal bridging can be reduced at shorter lengths, but as they get longer there's no choice with cavity lintels

[wozza]

Thermally broken Lintels are now available.

[Gus Potter]

Good points @MortarThePoint

 

One way of looking at Mortar's question particularly, to sensitive doors (say some bifolds, sliding doors) in wide openings is to start with the finished article and work back.

 

Steel beams - First split the loading on the lintel up into two types; you have the dead weight (dead load) bricks, blocks, floors and roofs etc. You can calculate how much the beam will bend (deflection) under this load. Now the bottom of the beam will not be flat but bowed down in the middle. But the dead load tends to be fairly constant, give or take moisture content changes in the materials and so on..

 

You can allow for this initial defection when taking the ball park first door sizes. Next you calculate what the extra deflection will be when you get say snow on the roof, have a party, fill the house with book cases etc, that is called the live (imposed load) deflection. Now you can add the two deflections together and this gives the overall deflection. Just remember that when say the snow melts the beam will move back up a bit.

 

Let's say you have an opening 4000 mm wide. You can start by saying let's keep the deflection limit under imposed loading to say span / 360 = 4000 / 360 ~= 11.1mm, maybe a bit too much for sensitive doors. Plug this 11.1mm (or a lower value) deflection back into the formula that Mortar gives and this will give you the beam second moment of area, now go to the tables and find a beam with a greater value than this. Next go back and calculate the overall beam deflection under dead + live load, and just as a check do the imposed load only again. Have a look at the figures you get and cross check with the door manufacture's tolerances in terms of deflection, they generally won't be too forgiving.

 

Now you have got a handle on the beam size that you need to control the deflection. The next thing to do it to check that the beam will be safely carry all the loads, these are the "strength checks". Now you are nearly there.

 

It's tempting to stop there as you think.. well I know what I need to allow for beam deflection, I'll just give it a few millimetres for the mastic seal. But mastic is not as compressible as you think when it is thin, you can accidentally transfer load to the tops of the doors, they bend and start to stick. The last part of this jigsaw is to detail the seals / insulation so you avoid transferring load to top of the doors.

 

Cold formed steel "Catnic" type lintels

These are a bit of a different animal. Generally they are designed with similar deflection criteria to the above. However, you need to look closely at what is called the "load ratio" A cavity lintel supports the inner and outer leaves. The two leaves have different loadings and this can cause the lintel to twist a bit as well as bowing downwards. if you follow the manufactures guidance (particularly the installation instructions) to the letter then the lintel should be safe.

 

I think the key here is to get the detailing right in terms where and how you allow the beam / cold rolled lintel movement to take place. This is not that easy when you are trying at the same time to prevent cold bridging and so on.

 

With big expensive doors that are not that tolerant to vertical deflection, loading on the top side I would look to start with a lower deflection limit, design the beam for that, detail the vertical movement joint over the doors and see if it looks pleasing to the eye and fits with the insulation detail. All easier said than done as a lot of the aluminium sections are pretty slender these days.

 

If you have steel Universal beams and an expensive glazing system you can put a bit of precamber on the beams. In other words you work out the deflection, pre bend the beam up so that when the load is applied the bottom of the beam is flatter rather than bowed down. It's not that expensive to do on modest sections funnily enough.

 

 

 

 

 

 

 

 

 

look at how flat the underside of the beam needs to be.

[Dave Jones]

Never heard of arched rsj but they would surely look crap with stone headers as they wouldn’t sit level ?

[Gus Potter]

Hi Dave

3 hours ago, Dave Jones said:

Never heard of arched rsj but they would surely look crap with stone headers as they wouldn’t sit level ?

 

I chucked that in at the end about the pre camber as food for thought. Yes a bit unsual. You would only consider it on long span (wide openings) where you have say a lot of masonry above, maybe a heavy long span floor to support too.  All beams bend once loaded up with masonry so once the stonework is build and you string a line between the points of support you may find say 7 - 10mm mm downwards deflection  i.e  the underside of the beam is bowed down and it will stay like that.

 

With modern glazing systems and a lot of the slender frames anything near or against them that is not too straight (like the underside of a beam) can show up more to the keen eye. Now suprisingly it's not that difficulat to put a little upwards precamber in a modest sized beam...you're not arching it, just tweeking. There is a fabricator in Scunthorpe who uses a hydraulic ram (off an old crane) in a rig. They lay the beam in on it's side. Run a string line from one end to the other and crank up the ram, easing the beam back and forward until they get say a gentle 10mm precamber.

 

The beam is installed with the camber upwards, masonry on top. Hopefully you can then stand back and have a perfectly flat beam soffit...

 

Yes not run of the mill stuff but if you are spendings thousands on the glazing then why two or three hundred more just to get it a little more perfect? In reality it's on most folks wish list and thus unusual to see on site for domestic applications. It was pointed out to me the other day that quite a lot of heat loss occurs due to the window frame, particularly aluninium, thus the less frame you have exposed the better the performance... how much I'm not sure?

 

 

[MortarThePoint]

Separately, I'd like to replace the 2850mm lintels above the garage doors (2543 opening) with steel beams. The outer leaf would have a precast concrete lintel. I thought this was all good until I spoke to the Structural Engineer and they said they'd prefer a cavity lintel here as it's so near the top of the wall and a cavity lintel would join the leaves together. They asked about perhaps two UB steels with bolted on PFC between which all seems excessive. The cavity lintels are on a long lead time due to the wide inner leaf.

 

I've concocted the scheme below which feels like it should be pretty solid. The Frame cramps would be on e.g. 450mm centres. Can anyone think of a more substantial way of joining the leaves which is compatible with quick delivery and reasonable cost.

 

For clarity I've shown the block underneath the steel beam, but not underneath the concrete lintel. Obviously the wouldn't be blocks under the steel in the opening and would be blocks under the R15A in the panels ?

Edited November 11, 2020 by MortarThePoint

[PeterW]

32 minutes ago, MortarThePoint said:

The cavity lintels are on a long lead time due to the wide inner leaf.

Who’s telling you this ..? 2 working days from Condell 

 

https://www.condell-ltd.com/ig-lintel-l1-s-100-wil-2850mm

 

I think your frame cramps would need to be at 300 centres and I’d check with the SEng as it’s a non-standard use case and by the time you have finished discussing it your lintel will be on site and installed. 

[Dave Jones]

we used  a single RSJ with a plate welded to bottom to carry the inner skin. Clad the underneath (same RAL cladding as windows) to hide the steel

[Mr Punter]

45 minutes ago, PeterW said:

Who’s telling you this ..? 2 working days from Condell 

 

He is using Hi Therm lintels.

[MortarThePoint]

43 minutes ago, Mr Punter said:

He is using Hi Therm lintels.

 

I'm using HiTherm in the house but the garage uses non-thermally broken ones.

 

1 hour ago, PeterW said:

Who’s telling you this ..? 2 working days from Condell 

 

https://www.condell-ltd.com/ig-lintel-l1-s-100-wil-2850mm

 

I think your frame cramps would need to be at 300 centres and I’d check with the SEng as it’s a non-standard use case and by the time you have finished discussing it your lintel will be on site and installed. 

 

That's a weedy lintel, I need L5/100 WIL for this as the roof space of the garage is being used. Those are on 12-14 days and Catnic are only slightly better

[MortarThePoint]

The other work around that came to mind was to use a standard inner leaf lintel and widen the cavity just at the top. Lintels for a 120mm cavity are available quickly. The 100mm block at the top would be centred on the 140mm blockwork below so the load transfer should be OK. It leaves a 20mm recess in the inner leaf that could easily be filled with something like plasterboard.

 

Could use a cavity lintel for a 140mm cavity with the 100mm block not centred on the blockwork below, face flush instead. This feels less defensible structurally (a.k.a. a bad idea).

 

Edited November 11, 2020 by MortarThePoint

[MortarThePoint]

Witch option do you think I should go for?

 

I might be loosing the plot now:

Edited November 11, 2020 by MortarThePoint

[MortarThePoint]

The Structural Engineer seems to prefer the wide cavity lintel option which was a slight surprise to me.