Deflection/Settlement/Cracks

I have had my steels sized for my extension and had something raised by the SE as to weather or not i want to proceed with a larger deflection or stiffen up with an additional column. The calcs work both ways and i imagine they will prefer another column as it means belt and braces for them but the scenario to our preferred (no additional column ruining our open plan vibe) method is as below;

Steel in question will be holding the upper level or pebbledashed solid wall masonry, 1st floor roof structure etc With the assumed loads the calcs have spat out an expected deflection of 13mm over a 9m span. The SE has warned that such deflection may well result in cracks to the wall above almost straight away as the load settles after which movement should be minimal from live loads. He has identified a window in the centre of the upper floor where he anticipates cracks from the lower corners down as it settles following the steel install. He has also stated that this is the risk and not a forgone conclusion. There may be no cracking at all.

Has anyone worked with similar deflections / scenario. I guess the risk is a lot down to the quality of the install and level of packing etc? Is the worst case scenario after the settlement that the pebbledash upstairs has to be removed and then re plastered with reinforcement mesh or fibrous render to repair and prevent the cracks resurfacing at a later date.

For the record i am not having anything to do with the install it will all be done by professionals.

 

TBH, a dead load deflection of 13mm on a 9m span is not bad - your SE has given you a fairly substantial beam. But no one can say 100% there will be no cracking, and the SE is right to point out the possibility.
The design code the SE works to will specify a maximum allowable deflection for the live (usually span/360, so in your case 25mm) but gives no maximum deflection for dead load - this is left to the SE's judgement. Just be prepared for the possiblilty of repair to the rendering, and some filler here and there internally; cracks caused by the slight deflection of a steel beam are not usually of structural significance.

 

Eh? 9m? That's nucking futs.

 

Why is it nuts?

 

OP, post some pictures from the garden looking towards the property.

Andy

 

9m is a huge beam to retro-fit. Just the fact that the SE is nervous about it and is already covering his ass, speaks volumes.

 

I agree that it's a longer-than-average span, but we can assume that the SE has designed it carefully, as he is specific about the maximum deflection the beam will be subject to.
What a 9m span will produce is a high reaction at each end; presumably the SE has detailed sufficient padstones and wall-stiffening; or alternatively has designed a goal-post frame.
Be interesting to know what size of beam has been specified? And how they are going to get it in position? Splice?

 

No that's separate and can cause cracking irrespective of the design deflection. Deflection occurs once the beam is loaded.

What I'm curious about is why the engineer is actually giving you an option. Thats nonsense, as he should be designing to minimum deflection and if the design is for no additional pier then that's what he designs to.

BTW, with render coatings, you don't really want any cracking or debonding of render from hidden cracking.

 

Yes, but what minimum deflection should he design for on a 9m span? 10mm? 5mm? 2mm?
It's up to the SE to decide, based on his assessment of the fragility of the structure above. If it's old brick in soft lime mortar and relatively small windows, he could allow a bit more deflection than if it was - say - a wall built in celcon with a large floor-ceiling window. Whether or not 13mm deflection would be acceptable, no one can say.
Remember, if you want to halve the deflection, you need to double the "I" of the beam, which is a big step up.

 

If he is designing to BS5950 then it's defined. Presumably there will be something similar set out if he is designing to Eurocode 3.

I just think it's bizarre than an enginer would actually give the client a choice of how much he wants a beam to deflect, and how much he wants finnishes to crack!

 

NINE metres FFS. Who needs an opening that big in their house?

 

Only maximum live load deflection is defined in BS5950 and EC3 (span/360 for beams carrying a plaster finish). Neither code specifies dead load deflection, which is partly left to the SEs judgement.

BS 5950 actually states that the dead-load deflection should be 'agreed with the client', but by this they mean for specific applications, eg where sensitive fixed machinery or equipment is being installed. It doesn't mean asking a householder what deflection limit he would be willing to accept for his new beam, which would be unreasonable for a lay person to understand.

In effect, as you suggest, the SE might be trying to offload the responsibility on to the client.

 

I will try and respond in one go to points raised above. I am not a SE so don't know what is / isn't possible -That is why i have employed a professional.

- Photo below with rough location of beam marked on
- The span is large and bigger than usual. It was always going to be a challenge
- The SE is not nervous he is just pointing out the risk of cracking due to deflection and offering an option to reduce the deflection by introducing a mid point column. He has calculated the beam to work - not just a finger in the air and see if it works. I personally think this shows he is doing his job correctly and keeping his client fully informed
- The SE has not given me a choice on how much the beam will deflect. He has told me how much deflection the design results in and given me the option of 2 compliant designs
- He has not issued the final design to me yet as he wanted to know my preference before doing the final detailing. I will update when they are issued for accurate details but from my conversation with him he is proposing 2nr 400mm+ deep beams bolted together sat on a column at either end which will in turn be sat on new pad foundations. Full goal post so no load passing through the existing structure
- Beams will be in 3 sections to aid with installation. They will be spliced
- I will likely have the design CAT 2/3 checked once the final design and calcs have been issued

Tony 1851 are you a SE?
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Are the beams he is proposing one of the 406 x 140 sections? (that section is available in two weights). Or perhaps 406 x 178? If so, these are all fairly deep beams and will end up even deeper if using conventional bolted splices.
Deflection is obviously the main criterion here. If so, a Universal Column section would give you a shallower beam and with less deflection, depending on the weight of beam chosen. (These are not "I" beams but "H" beams used on their side). A possible size would be one of the 305 x 305 sections. They are heavy, but you would be having them in three sections and spliced anyway.
Perhaps ask the SE if he's considered that as an alternative?

 

Thanks I will mention that to him on monday