What's this steel bearing plate going to do under this steel I beam?

lvwarren

You might want to model this in BeamCalc or some other tool. The spreader plate will tend to distribute the load across the entire 330 mm span, modulo a decrease in shear as you go across. If this is in a seismic zone I would visualize the entire structure placed on a shaking table and secure it with bolts and rebar accordingly.

hokie66

The spreader plate will do little, if anything to distribute the load or reduce the bearing stress. As the beam is many times stiffer than the plate, the load will essentially all be carried by the brick which is directly under the beam. Bearing plates, or spreader plates, are used to distribute concentric loads, not eccentric loading like this.

nvn

TimSmall: (1) The steel bearing plate ("spreader" plate) is necessary. The thickness of your current steel bearing plate is adequate.

(2) Your I-beam web crippling stress is probably OK, but we cannot be sure unless you post your I-beam size.

(3) Nominal bearing pressure, p, on brick with cement mortar must not exceed the brickwork allowable bearing stress, Sbra = 1.70 MPa. Your current nominal bearing pressure is, p = V/(b*L) = (47 300 N)/[(100 mm)(150 mm)] = 3.15 MPa, which exceeds Sbra, where V = applied load, b = bearing plate width, and L = bearing length. Therefore, the masonry currently appears overstressed.

It appears your 150 mm bearing length instead needs to be, L = V/(b*Sbra) = (47 300 N)/[(100 mm)(1.70 MPa)] = ~280 mm. Therefore, if your given end reaction force (V) is correct, it currently appears you need to increase your 150 mm bearing length to 280 mm.

(4) You might want to also see the last sentence in post 18, if you are in a seismic zone, or if you might experience high winds, tornadoes, or minor hurricanes.

(5) By the way, always leave a space between a numeric value and its following unit symbol. E.g., 12 mm, not 12mm. See the international standard for writing units (ISO 31-0).

Studiot

You need to post more detail than just the proposed beam size

Since this design is for the UK, you need to indicate how the load is arrived at.
Is it factored?

What is in the space between the brickwork and the end of the beam?

What are the deflection criteria for this beam. They are not a code requirement, they will be determined by what is in the opening eg a window frame will place more stringent limits than a clear opening in a room.

hokie66

I know that this is a physics forum, not engineering, but as an old structural engineer, I would never rely on a extended plate in that manner to redistribute the stress under the beam. The principle is that the load follows the stiffness, so the plate extension beyond the end of the beam will only see load after the brick starts to crush, or after that corner shears off.

Studiot

Agreed.

But we are only guessing at the intended purpose of the plate.

Aleph Zero may be correct in that it may be intended to bed the plate onto mortar to create a landing surface for the beam.
I have already explained why I don't consider this a good idea.

AlephZero

I agree it dioesn't look a very good design. I was just trying to rationalize what the designer was thinking.

On the other hand if the plate was slightly arched upwards, or the end of the beam was shaped so the contact was guaranteed to be at the end of the beam and the center of the plate and the load would spread towards both ends of the plate, that would seem a better idea - assuming the end of the beam would support the concentrated load without damage.

nvn

TimSmall: I would also be concerned about the stability of your masonry wall, in the lateral direction. I thought the required minimum thickness of a one-story, load-bearing masonry wall is 150 mm, not 100 mm.

Also, we do not know if your wall is properly anchored, in the lateral direction, to a stable structure, every 450 mm in the vertical and horizontal directions.

Also, ensure your house would be safe before you begin structural alteration or wall removal.

To ensure your plan sounds correct, you might want to have your entire plan independently checked by one or two additional, competent engineers who have expertise in residential building analysis, construction, and foundations, as well as expertise in load-bearing brick masonry walls, and stability.

You might also want to try some on-line searches for, e.g., brick masonry wall stability, etc.

If your existing masonry wall is load-bearing, I am not sure you can remove it, then concentrate all that load on the edges of an unreinforced, 100-mm-thick brick wall. You might consider rebuilding the end of your masonry wall, such that it would be thicker. And you could perhaps embed, within the wall end edge, a steel column. This wall needs adequate lateral support. I think this wall end also needs an adequate, independent foundation, underneath your building slab foundation.

Studiot

Well 100 is not brickwork size although it could be blockwork size.

We have all said we are just guessing without more information

xxChrisxx

If this is a question where a real answer is required, and not a bunch of assumptions.

Why not just ask for the design justification from the person that come up with it?
You are the customer after all.

TimSmall

The engineer has implied (but has not yet confirmed - even when explicitly asked by email) that the intention of the plate is to evenly distribute the load evenly over its 330x100 area.

They have explicitly said by email that the location and orientation of the plate should be as pictured in my initial graphic.

I have been provided with factored loading calculations, and deflection calculations, but there is no calculation for the bearing plate provided - just the statement "ALL PADSTONES (sic) TO BE 330x100x12mm THICK SPREADER PLATE. ALL BEAMS TO HAVE 150mm END BEARING" - and a rough sketch of the bearing arrangement on plan.

When I asked by phone whether the bearing details were definitely right, I was told that it was (but I didn't feel comfortable or that my question had prompted much thought, so I then started this thread). The following day I put the same question in an email and I got this reply back "The steel spreader plate is required to be 20mm [my emphasis] thick for a 150mm end bearing. So, the size of the spreader plate is 330x100x20 thick." - but no calculation (which I've now asked for, so that I can see what assumptions have been made) - however from what they've said before, I'm 90% sure that the assumptions include even load distribution by the plate. There is also no explanation of why the 12 mm thick plate is now 20 mm thick.

I very much appreciate the other points made regarding wall stability, lateral loading, compressive strength of masonry units, beam deflection, beam restraint, web buckling etc. - I feel that they are all very much valid points (I've checked some of these points already using beam design software, and I won't continue with construction until all of my remaining concerns have been addressed to my satisfaction), but what I was trying to concentrate on, is the question of the load distribution under the plate - given that the plate is (as hokie66 has said) eccentrically loaded, and moreover that the centre of the plate->masonry bearing lies beyond the end of the beam.

From past conversations, I'm 90% sure that the engineer has assumed that the load distribution under the plate is the same as it would be if the plate was concentrically loaded (as I've said, I've asked for explicit confirmation of this from them - but not yet received it), and I'm now pretty sure that it isn't (I started off about 50% sure - so this thread has been very valuable, thanks!).

BTW, external to this forum, the professionals (and competent amateurs) which I've consulted have been divided on the matter of load distribution (although a rough consensus seems to now have been arrived at here - many thanks to all who've contributed!).

The reason for my focus on this one issue, is that without this having been established with some certainty, it's not really possible to check the rest of the design (or indeed any proposed modification to the design which may be required).

I intend to meet with the firm of engineers at the earliest possible opportunity, and also to meet with my local building control department's checking engineer (incidentally, the building control dept. have told me that they don't normally check "simple" designs from this firm - and a handful of other well-established local major companies - because they trust them to always get it right - I'll leave you to draw your own conclusions on that policy). I will also take the design to another independent engineer, if I don't feel that I'm getting much traction from my meeting at either of the above!

I'm also feeling somewhat up against it, as my builder is going to ask me at 8am on Tuesday (Monday is a public holiday here) how much of the original design he can continue to work to (the beam is cut, primed and on-site) - and how much he shouldn't (and it currently looks like he'll probably run out of things to do before the matter is fully resolved).

Again, this thread has been very useful in that respect.

I hope this explains my apparent tunnel vision on this matter of beam->plate->masonry load distribution, as I'm trying to get the relevant principles as clear in my head as possible (or alternatively an explanation from first principles of why my concerns were baseless).

I'm emphatically not ignoring the other points which have been raised - I've addressed some of them to my satisfaction already, and I'll ensure the others will be addressed. I'm not a builder, and this has been the first time I've had any significant dealings with a firm of structural engineers (the firm was selected-by and received their initial instructions from my architect - since I'd decided to leave all of this business to the professionals!), so I wanted to be as sure as possible of the facts before I started raising concerns - apart from anything else, if I go in as (in their eyes) a layman and immediately make a newbie-grade mistake, I suspect that they're much less likely to take-seriously, any other concerns which I might have!


Thanks once again for everyone's input.



p.s. as the graphics aren't visible to people who aren't logged into the physics forums, I've also posted them here:

http://buttersideup.com/files/steel/I-Beam-Bearing.png
http://buttersideup.com/files/steel/Bearing_Case_A.png
http://buttersideup.com/files/steel/Bearing_Case_B.png
http://buttersideup.com/files/steel/Bearing_Case_C.png


Tim.

hokie66

All the other issues are important, but as you requested, I was just trying to deal with whether or not the bearing plate achieved uniform pressure on the wall. It will not do that in the designed configuration. One way of achieving more uniform bearing would be to add an end plate to the beam which projects slightly below the beam. This would apply a point load to the bearing plate, sort of like a rocker connection. The plate would need to be checked for bending. You could also just provide a smaller plate under the end of the beam between the beam and bearing plate. Same principle, but your beam may still not be long enough to reduce the stress to acceptable limits.

nvn

TimSmall: If the I-beam flange and steel bearing plate are parallel after load is applied, then the actual masonry bearing stress distribution will be triangular or trapezoidal, extending over a bearing length, we could say, of L1 = L + t, where L = beam bearing length (150 mm), and t = steel bearing plate thickness.

However, I am about 70 % sure that the brickwork allowable bearing stress, Sbra, is an equivalent uniform bearing stress that already takes into account this triangular stress distribution for you. Therefore, you generally simply compute a uniform (nominal) bearing stress as shown in post 20. But I will say, the Sbra I listed in post 20 is for an unfactored applied load.

The current conclusion is, your masonry appears overstressed, even if you use bearing length L1, stated above, and use an unfactored applied load.

Secondly, I think you will find, for a one-story, load-bearing masonry wall, the absolute minimum wall thickness allowed by the codes is 150 mm, not 100 mm. But a better minimum wall thickness would be 200 mm.

Also, if your masonry wall condition is degraded, and/or is not average quality, it might not even meet the Sbra value listed in post 20.

Studiot

See note 11

http://www.lbhf.gov.uk/Directory/Env...n_Guidance.asp

There are two routes to satisfying the building regs.

The deemed to satisfy route as above where the code provides a figure that should be safe in all cases because of generous margins of safety.

The full calculations route where more accurate calculations or even tests are performed to achieve a more economical solution by reducing the margins of safety to known but acceptable limits.

NateB

PDF

Text is from McCormac's Structural Steel Design, 4th ed. The example is from AISC Design Examples v 14.0. It is specific to the US, so it may or may not be useful...