Steel beam selection

[russellsmiler]

The snow kept me inside all weekend!!

As i was at a loose end i have gone through some old textbooks etc when i was studying HNC civil engineering.

I feel confident now that i can calculate a structural design and selection of the right size steel beams for my (pretty full on) diy house extension Knock throughs, I can provide the calculations for BC.....heres the question though!!

Am i right in saying I dont need to be a proffessional strut engineer with PI indemnity, and that Building control will check my calcs anyway, and that if i was to make a mistake ( i would submit calcs well before work being undertaken) then it would be flagged up by LAs own engineers.

My ethos for this is that if the design is right..then its right....

Any comments

Thanks
Russell

 

[noseall]

What about third party indemnity like say if things go south whilst a builder is installing for instance?

If your standards are anything like current S.E. spec's then the beam will be twice as deep, three times as heavy and double the width it needs to be.

Should be interesting to see what your calc's throw up.

 

[russellsmiler]

well its me who will carry the can when it all goes south as I will be doing the installation.

Must agree that structural design things always seem overdone and oversized, gotta remember though that with steel beams its the Deflection parameters that mean you end up with what seems overkill, also when calculating loads you times by 1.4 as a factor of safety for the element.

I have the need to design in 12 no steel beams in my renovation so really need to DIY as the cost would be quite a bit.

Unless of course theres a charitable Strut eng who wants to help as well.

 

[tony1851]

The council has no legal right to insist that the calculations are done by a qualified engineer. Their job is simply to check the calcs as submitted.
However, this particular aspect of the Building Regulations has been under review and might change in the near future.
(by the way, you mentioned 1.4; don't forget it's 1.6 for live loads).

 

[RonnyRaygun]

If your standards are anything like current S.E. spec's then the beam will be twice as deep, three times as heavy and double the width it needs to be

Typical builder

 

[^woody^]

When I use structural engineers, I always have a contract clause specifying that the engineer is to help lift the beam. I get wafer thin beams.

Back OT, structural design is like chiropody ...... anyone can do it

 

[RonnyRaygun]

I have the need to design in 12 no steel beams in my renovation so really need to DIY as the cost would be quite a bit.

Are you OK designing pad stones and checking masonry, if necessary? Foundations might be taking extra loads too...

 

[RonnyRaygun]

With regard to beam sizes I feel I should defend all the SEs out there.

I'm not going to say I disagree completely, as we all know that beams are often oversized...

But there are many reasons for this:

Sometimes the beam is simply the most suitable size with regard to size of joists or coursing, and a smaller beam will hinder construction.

Lack of lateral stability means an unrestrained beam has a fraction of its full bending capacity. Some engineers are much more conservative with their assumptions regarding restraint than others.

Deflection is an important consideration, especially in long beams, especially in beams supporting existing masonry, and especially in UC sections used as beams. So where a beam needs to be shallow, and supports an existing wall, for instance knocking through to an extension, they often end up pretty heavy.

The BS5950 safety factors for bending and shear are 1.4 and 1.6, which are pretty large, but smaller than the safety factors for steel have ever been in previous design codes. Only Eurocodes have smaller safety factors. They are also smaller than for reinforced concrete, and far smaller than for timber or masonry design.

So the huge, heavy beams are often pretty tight on the limit of what is acceptable.

Sometimes not, I admit...

 

[^woody^]

LOl we got a bite nosey, lets reel him in

 

[RonnyRaygun]

LOl we got a bite nosey, lets reel him in

Was waiting for a smart comment...you know I'm right

Right, off to bed, I've got some beams to overdesign tomorrow

 

[noseall]

LOl we got a bite nosey, lets reel him in

I've got broad shoulders, i can take it.

Mind you i NEED broad shoulders what with all this heavy steel hefting.....

It's funny, we were doing a typical extension with a large knock-out, steel beams to take the cavity walls etc.
Got the steels cut and drilled ready, propped and needled, did the knock-out and discovered a beam landing on our wall internally and perpendicular to.
This beam was an angle steel orientated so that it was like an inverted 'v', more like this...^ .The floor joists landed on this and also had a v cut out of them so as to sit on the inverted angle steel.
This angle steel had to be cut so that it would be accommodated in the web of our new steels along with a slight rounding so that it fit snug into the slight curve where the web meets the flange.
The bearing of this angle steel was little more then 50mm and it would be sitting eccentrically on one steel beam flange only.
The S.E. (who had been summonsed upon discovery) took one look and said the extra load's fine i will alter my calc's to suit and went home!

So the huge, heavy beams are often pretty tight on the limit of what is acceptable.

On the limit "my ar5e"!

 

[russellsmiler]

Ronny

Regarding Padstones, I plan to use a precast 225 x 450 x 102 C35 concrete patstone, each beam will have bearing of 150mm at each end, masonry of existing house is very solid extremely hard brick, built well in the forties by our local council. Foundations?, Having dug down to get new water service in, and dug adjacent for foundations to extension i can see that they are 300mm thick very hard concrete, and ground conditions are very hard mudstone (not rock, just about diggable with excavator and no breaker)...So all well i would say......However!!! One thing as cropped up that i have come accross on a few occasions and never really know the definitve answer!!.

Beams of this nature are specified as always having 150mm End bearing on each end onto suitable strength concrete padstones, however I have seen previously, and in fact want to in one situation in my project, bear of at 90 degrees of an existing cavity wall, therefore unable to use outer leaf due to cold bridging are only able to have 102mm (brick size) bearing... now i know that this is generally acceptable on the odd occasion and espesially if the brick/ block panel is not over slender.

Any comments from our resident SEs

 

[RonnyRaygun]

Ronny

Regarding Padstones, I plan to use a precast 225 x 450 x 102 C35 concrete patstone, each beam will have bearing of 150mm at each end, masonry of existing house is very solid extremely hard brick, built well in the forties by our local council.

Probably OK, but depends on the loads at the ends of the beams. You need to make an assumption about the strength of the masonry, but if it is decent quality then you should be OK. I can send you an example pad stone calc if you like. I'd probably assume 20N brick in M4 mortar based on your description, but up to BC to accept that.

Foundations?, Having dug down to get new water service in, and dug adjacent for foundations to extension i can see that they are 300mm thick very hard concrete, and ground conditions are very hard mudstone (not rock, just about diggable with excavator and no breaker)...So all well i would say

Yeah, again, as with masonry, without a soils report you can only really make assumptions about the bearing strength of the soil - although your BCO may have knowledge of the local conditions. Foundations are only usually an issue where large areas of masonry are removed either side of a bearing, meaning that the load cannot spread far through the footing. It's more of an issue in shallow foundations as the load is assumed to spread outwards at 45 degrees to the base.

However!!! One thing as cropped up that i have come accross on a few occasions and never really know the definitve answer!!.

Beams of this nature are specified as always having 150mm End bearing on each end onto suitable strength concrete padstones, however I have seen previously, and in fact want to in one situation in my project, bear of at 90 degrees of an existing cavity wall, therefore unable to use outer leaf due to cold bridging are only able to have 102mm (brick size) bearing... now i know that this is generally acceptable on the odd occasion and especially if the brick/ block panel is not over slender.

If you are bearing at 90 degrees to the wall, 100mm bearing is fine. You've just got to ensure the pad stone is long enough to spread the load.

I've never yet got to the bottom of why 150mm bearing is recommended for lintels and beams bearing parallel to the wall. As long as the masonry bearing stress isn't exceeded 100mm should be fine. I've installed quite a few short lintels with 100mm bearing.

I wonder if it's because more heavily loaded lintels and beams that require large pad stones will load the pads eccentrically, meaning there will be a higher stress concentration at the beam end. The longer the beam's bearing, the more this is reduced. Ideally I guess the bearing should be the full length of the pad stone to get a true axial load to the masonry, but rarely done.

If anyone knows the answer please enlighten me!

 

[RonnyRaygun]

Mind you i NEED broad shoulders what with all this heavy steel hefting.....

I dunno, you builders these days...heaviest thing you lift is your beer down the pub in the evening I remember when you still had the man-size bags of sand and cement...probably need a man on each corner to lift one of those these days

 

[russellsmiler]

Those good old days i remember well.

No wonder i used to fall asleep in front of the fire before my ma had tea on the table,

Who remebers.

Artic loads of 50kg bags of cement turning up, and they were red hot on the shoulder as you 2-poss 3 of you spent a good morning one after another carrying into the cement shed.

Digging foundations to dwellings by hand, and having scaffolding planks to form barrow bridges to dump the arisings.

(my worst job) working with a full blooded irish utility service installer, Drunk every night, and then we used to spend a solid 9 hours digging and shovelling , laying, backfilling and then loading surplus onto wagons by hand, hardest work i ever done, shirts off sometimes in mid winter...However, breakfast time of steak and eggs with massive pots of tea to slurp.

Gawd, I havent even mentioned the clywedog dam!!

Hey ronny, thank you for sharing your knowledge!!