Steel over bifold

[Sunil237]

Received my structural calcs today and I'm not so sure of the solution provided by the structural engineer.

 

I have an extension going up which is 10m wide x 5.5m deep with a flat roof, and a bifold of 6.4m on the rear wall. The engineer has specified a length of 6.84m 250 x 150 x 8 RHS but it shows a deflection of 25mm which I'm concerned about as I need to minimise this for the bifold.

 

Any suggestions on improvements? I have already suggested to make the steel longer as I'll be charged for 7.5m either way and the wall has plenty of space.

 

thanks

[JohnMo]

Go back to structural engineer to seek clarification of the deflection?

[Sunil237]

Thanks John, I have queried this with him but wanted external thoughts too

[markc]

As John said, plus RHS isn’t the best section to minimise deflection as the “flanges” are only 8mm top and bottom agains the added self weight of 16mm of web (2x8mm sides.

yes a RHS looks better but it would really need a pre camber before install.

[nod]

The last two that we have done have been done with a 12 mill hit and miss welded plate on the bottom of the steel Which  gives 0 deflection and has worked fine 

Hes probably probably added the deflection due to the lack of weight above it Which is no use to you Five mill of movement will cause the doors issues 

 

[Conor]

Our engineer's solution was a 203SHS with 14mm plate welded to the bottom for 5mm deflection of 5m. Beam sat behind the insulation and only the steel plate formed a bridge.

[Sunil237]

4 hours ago, markc said:

As John said, plus RHS isn’t the best section to minimise deflection as the “flanges” are only 8mm top and bottom agains the added self weight of 16mm of web (2x8mm sides.

yes a RHS looks better but it would really need a pre camber before install.

What would be the better alternate type?

[Sunil237]

That @nod and @Conor, in my calcs he did say a shelf plate is optional. I have asked him to include this. Am I summonsed to use the same thickness as the RHS or just go thicker?

[nod]

31 minutes ago, Sunil237 said:

That @nod and @Conor, in my calcs he did say a shelf plate is optional. I have asked him to include this. Am I summonsed to use the same thickness as the RHS or just go thicker?

No the span will have already dictated the the thickness of your steel 

We hit and miss welded plates on three sides at our previous build Six years on No cracks or movement 

 

 

[scottishjohn]

question

  why RHS  and not RSJ of larger thickness  material

 what is the attraction of a box beam?

are  the  flat roof joists hanging on this as well?

 8mm seems thin to me and not suprised at deflection in that length 

plus easy to to weld or bolt in extra  thickness to centre of RSJ if still worried about deflection 

when all said and done it is the vertical part that gives the strength and flat plate is cheap

 and easy to fill with insulation 

[markc]

2 hours ago, Sunil237 said:

What would be the better alternate type?

A UB (universal Beam) has a thin web (vertical) and thicker top and bottom flanges making it stiffer when vertically loaded.  As mentioned above, adding a bottom plate to Box section will greatly improve its resistance to deflection because you are increasing the bottom flange

[scottishjohn]

1 minute ago, markc said:

A UB (universal Beam) has a thin web (vertical) and thicker top and bottom flanges making it stiffer when vertically loaded.  As mentioned above, adding a bottom plate to Box section will greatly improve its resistance to deflection because you are increasing the bottom flange

I would respectfully disagree

it is the centre web that gives the rigidity  ,so height and thickness of that gives the strength ,top and bottom  are to give it sideways rigidiity  to keep centre web vertical

 even a plastic 12" ruler is stiff wehn on its edge 

try hitting your knuckles with it edge on  ,then do same with it flat 

[saveasteading]

23 minutes ago, scottishjohn said:

It is the centre web that gives the rigidity  ,

Sorry, no.

The web (the central vertical bit) is there to keep the flanges apart. Then it has to resist deflection  but that is secondary.

Increasing the distance the flanges are apart increases the lever for the flanges, and making them thicker gives strength....but it's a class for another time.

 

There is permanent  deflection from the load above the beam, and that doesn't concern you ( it bends and stays put, and the door gear can be fixed kevel). Dynamic deflection from the doors moving is your concern. (Loading and unloading).

 

There is no such thing as zero deflection.

 

Thus it  may well be ok, but probably needs clarification.

[Iceverge]

I was always of the impression that in a horizontal "I* beam that the top and bottom sections did all the work.

 

It's how in my head I explained this. 

 

 

The bid in the top was being squashed together, the bit at the bottom was being pulled apart and the bit in the middle was just making sure they stayed put. 

 

Without knowing the different yield strength of metals in compression and tension I do wonder why beams aren't built asymmetrically to gain maximum strength for minimal material.

 

[saveasteading]

2 hours ago, Iceverge said:

why beams aren't built asymmetrically to gain maximum strength for minimal material.

They can be and are, for big structures.

The standard  ones are "hot rolled" through formers from a yellow hot balk of steel. 

But beams can be welded from flat plates which can be whatever thickness and width is required, in which case they don’t even have to be parallel.

 

Your picture shows the possibility of increasing the web height from a hot rolled section. Castellated beams are cut to  zigzag halves and moved relatively sideways and welded together. The weld is visible in your picture.

The circle option  is a prettier effect, with some offcut waste resulting.

 

Very strong and allows services through.

 

Also note plate girders on victorian bridges, made from flat steel plates riveted together.

 

 

 

Edited yesterday at 07:10 by saveasteading

[scottishjohn]

I guess we are not going to agree

,but even the picture you show above shows the centre web is  biggest giving the strength 

 the other picture with holes in the beam is to decrease weight as at its hieght it is big enogh to take the load 

as for victroisn bridges etc 

 that was their only choice as no rolling mills or fancy steels  and no welding 

  Im sorry you are just wrong

 

  if load is vertical then a tall centre web will take the load in a thinner section tha huge flat plates at top and bottom of beams 

 you dont believe me then  do the steel rule test i suggested above 

  and see how weight it will support if truly vertical and not allowed to bend sideways 

 or look at a truck chassis 

 

 the main beams are always about twice the height of the top and bottom plates  and usally a C section  again to save weight for load required

 even better look at a 100ton low loader chassis  and see how that is made 

 

and they get serius loading with all the transient loads of roads 

 or look at aircraft wing main spars   

plenty examples of what i am saying  is true 

 

[saveasteading]

1 minute ago, scottishjohn said:

Im sorry you are just wrong

@scottishjohnA relevant degree and 50 years working with steel buidings wasted?

Maybe I'm explaining badly.

 

I was expanding on the query by @Iceverge so maybe it doesn't coincide with your post.

 

3 minutes ago, scottishjohn said:

biggest

May be the confusion here.

 

8 minutes ago, scottishjohn said:

if truly vertical

Is the other confusing factor.  It will buckle unless restrained. 

 

[Sunil237]

10 hours ago, markc said:

A UB (universal Beam) has a thin web (vertical) and thicker top and bottom flanges making it stiffer when vertically loaded.  As mentioned above, adding a bottom plate to Box section will greatly improve its resistance to deflection because you are increasing the bottom flange

Thanks, I can see their thoughts on adding a plate

 

10 hours ago, scottishjohn said:

question

  why RHS  and not RSJ of larger thickness  material

 what is the attraction of a box beam?

are  the  flat roof joists hanging on this as well?

 8mm seems thin to me and not suprised at deflection in that length 

plus easy to to weld or bolt in extra  thickness to centre of RSJ if still worried about deflection 

when all said and done it is the vertical part that gives the strength and flat plate is cheap

 and easy to fill with insulation 

Not sure why they have suggested an RHS over RSJ, with the multiple opinions here about the two I will ask them to check which one is more suited for for my purpose.

Yes flat roof joists will be mounted on this - using posijoists.

thanks for the other info too