Structural Calculations for Stair Trimmer and Padstone


Postby wongcna » Tue Nov 04, 2008 10:43 pm

Hi Greeting, I am currently working with a friend of builder on my house loft conversion. We are nearly finish the project. Unfortuately, we still haven't solved out the FULL Plan building reguations approval. We have submitted all the drawings, calucations and building control inspector said the building works are excellent. Submitted timber floor joists and RSJ beam calculation seems to be ok. However, we just received a letter from building control department. They want to see the timber "Stair Trimmer" and RSJ Beam Support (pad stone) calculation. I am not quite sure how to calculate these 2 parts. Can anyone point me to the right direction for these calculations?s

Thanks in advance!
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Postby eljaybee » Wed Nov 05, 2008 8:50 am

Calculating a stair trimmer is done is the same manner as a beam, padstones are more complex, depends upon the loading, type of brick or block, etc.

Easiest way - get an engineer to do them.
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Postby wongcna » Wed Nov 05, 2008 12:02 pm

[quote="eljaybee"]Calculating a stair trimmer is done is the same manner as a beam, padstones are more complex, depends upon the loading, type of brick or block, etc.

Easiest way - get an engineer to do them.[/quote]

Hi, Many thanks for your reply. I am nearly finished the poject and I would like to do the calculation myself. I am not sure about the loading on the Stair Trimmer. Do I need to take account the staircase loading? or just consider the normal floor loading will be enought?

About the padstones, it is a precast concrete beam which is around 100 x 75 x 600 sit on top of the brick work. How can i consider the bottom brick work if I don't have the strenght of it?

any reference book or site will be very appreciated!

Thanks in advance!

Cheers
wongcna
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Postby eljaybee » Wed Nov 05, 2008 4:45 pm

It all depends upon how the trimmer is loaded, if it supports the staircase then you should allow some loading onto the trimmer.

Calculating a padstone is not the same as a concrete beam, it is all to do with the crush from the reaction load of the beam. You need to know what the construction of the wall is, although sometimes assumptions must be made (normally on the cautious side), as the type of brick and the type of mortar can influence the calculations. Guidance is given in the british standard for this. Thus a visual inspection needs to be done of the wall, loading, etc.
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Postby wongcna » Wed Nov 05, 2008 5:19 pm

[quote="eljaybee"]It all depends upon how the trimmer is loaded, if it supports the staircase then you should allow some loading onto the trimmer.

Calculating a padstone is not the same as a concrete beam, it is all to do with the crush from the reaction load of the beam. You need to know what the construction of the wall is, although sometimes assumptions must be made (normally on the cautious side), as the type of brick and the type of mortar can influence the calculations. Guidance is given in the british standard for this. Thus a visual inspection needs to be done of the wall, loading, etc.[/quote]

Again, thanks for your reply! Appreciated!

I graduated from civil enginnering. So, the basic structural design and calculation is ok for me.

About the Stairs Trimmer, please see the plan for more details. [url]http://www.wongcna.co.uk/engineering/Floor_Joist_Layout1.pdf[/url] It is a triple joist in the loft level. Staircase wlll be attached near the wall.
When I design the load, do i just put the stairs loading to the end of the beam? What loading can I assume for the stairs?

About design the padstone, The loading is 28kN sit on top of padstone. I think I will assume the concrete strength is 50N/mm2 and masonary strength(not sure) and then do the calculation. the padstone is 600x100(w)x75(d), would the depth is short?

I am happy to upload more drawing if help you guys for comment.

Thanks again![/url]
wongcna
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Postby eljaybee » Fri Nov 07, 2008 8:49 am

Having just looked at the drawing can you confirm, is it the timber trimmer off the drawing (span 2450mm takin the 3250mm span of floor) or is it that you wish to replace the 203x203 UC with a timber beam???

If it is the timber trimmer then you do not need to include the stair.

With regards to the padstone then a depth of 75mm is a little small as doesn't allow the load to be spread over the whole of the padstone bearing - remember the 45degree spread rule. So I would say go for atleast a 2 course deep padstone. Thus even if the brick is take as the poorest crushing then at 600mm wide it should be sound.

[i]Padstone calc example below for a 450mm wide padstone with 27.7kN load on lowest masonry crushing strength

Bearing details - Masonry: 5N/mm2 brick, class (iv) mortar, normal const/normal mfr Local design strength (factored) = 2.2/3.5 = 0.63N/mm2 (BS5628-1:2005 Table 2a)

Reaction load (27.70kN factored): 450 x 100mm padstone Factored stress under padstone = 27.70 x 1000/450 x 100 = 0.62 N/mm2[/i]
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Postby wongcna » Fri Nov 07, 2008 1:18 pm

Many thanks again your reply. I really appreciated.

I did run some calculation over it. You are right, the Stairs Trimmer 2450mm we are talkingabout. General speaking, double joist should be sufficient but I put triple joists in. I assumed PL 200KG or 2KN from the stairs at the middle of span + UDL from DL and IL.

Is that OK if I added to PL (2kN)? or I am just too stupid to do it?

About the pad stone, I think I will need to change to 600 x 100 x 150(depth) rather than 75mm. 27000/(100x600) = 0.45N/mm2
Local design strength (factored) = characterics strangth of Mansory/saefty factor = 0.63N/mm2.
As the design strength 0.63 is greater than stress below pad 0.45. I presume it is ok.
About the Local design strength. Do I need to time the Enhanced stress bearing (1, 1.25 or 1.5)?

Many thanks for your reply. I think you deserve at least a pint from me.
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