Precast Seating Requirements  

The loss of the back of the supported unit (slab, rib, hollowcore) is 30 mm and not the ZERO cited above by Cl 18.7.4.3 (c)(ii).

The concrete unit will have the back (the end) of the unit spall when it is seated on steel armouring. 

So, I may have this wrong – let’s do a “count-up”:  Cl. 18.7.4.3 (c) :-  at one end of the precast floor unit  (8m long, say?)

1. Bearing length, say = 10 mm
2. Spalling of the back of the unit (the supported member) = 30 mm
3. Spalling of the front of the supporting member – steel armoured = 0 mm
4. Allowance for creep, shrinkage and thermal movement = 2 + 2 + 1 = 5 mm

AND tolerances  Cl. 18.7.4.3   from the Precast Guidelines “The Grey Book” 1991. =  17 mm 

Total allowance for seating width in this example =   62 mm

All things considered 75 mm looks OK.

Also thinking about the placement of the 50mm wide bearing strip (still needed even when one edge is armoured) = 50 mm strip, ZERO allowance for chamfer (armoured supporting member) and 17 mm tolerance = 50 + 17 = 67 mm ?

The

It is also worth noting that although the requirements are very prescriptive there is still significant uncertainty when estimating both deformation demands (drift, elongation) and potential reductions in seating length (spalling, tolerances, etc).  It is common in design standard requirements to have a minimum (or maximum) that is based on engineering judgment.  In the case of precast seating 75 mm is considered an appropriate minimum given the potential uncertainties in seating calculation and construction.  Precast supports are also a binary failure mode and so additional conservatism is warranted to ensure a low probability of collapse.

Thanks Rick and Des,

Are there any comments on my second question in relation to seismic assessments?

Having worked in construction I have seen some Hollowcore units placed with too much seating at one end but too little at the other end. Workers are often oblivious to this as they want to (time and money) drop units as quickly as possible and lift the next one off the truck. I have had to get units shifted. I wonder how many construction firms have engineers or QA staff checking seating? As engineers, particularly, designers are often regarded as a cost and not paid to supervise very much, how much bad construction practice goes on? Also often units are not cut at 90 degrees so can have less than 50mm seating in part.

Now I am doing ISAs I have found it almost impossible to check actual seating as you cannot see into the floor unit or through seating angles. The drawings may say a minimum 50mm but this does not mean it actually is that. I wonder how many buildings there are with potentially insufficient seating? Possibly some buildings should have seating angles retrofitted as a matter of urgency?

We note that the previous SESOC guidance allowed for seating precast on steel without bearing pads, however, A3 specifically notes that low friction bearing pads shall be used, with a co-efficient of friction of 0.7 or less (= co-efficient of friction of steel/concrete).  The requirement for bearing pads was also emphasised at the course we attended in relation to the A3 changes late last year.  How do we get on with intumescent paint protection, where there is now a small gap at the underside of the units due to these bearing strips and the top surface is no longer fully protected? The intumescent paint would be damaged if pre-applied to the top surface during the erection of the units.

Posted by: Nick Calvert

Thanks Rick and Des,

Are there any comments on my second question in relation to seismic assessments?

Updated guidance on assessment of precast concrete floors (including seating) is in preparation currently as part of the revision of Section C5 of the MBIE Technical Guidelines ofr Engineering Assessments, and is anticipated to be released for public comment later this year.

Until that guidance is released – the existing Part C5 refers to Fenwick Bull & Gardiner (2010) – “the purple book” – for assessment of seating lengths – available at https://ir.canterbury.ac.nz/handle/10092/4211

In short – the check is not a binary pass/fail based on NZS3101, as various parts of the calculation depend on the drift demand. If the critical drift for the seating of units is less than the drift corresponding to other parts of the building then the capacity (%NBS) of the building would be limited by the seating.

Posted by: Steve Thomas

Having worked in construction I have seen some Hollowcore units placed with too much seating at one end but too little at the other end. Workers are often oblivious to this as they want to (time and money) drop units as quickly as possible and lift the next one off the truck. I have had to get units shifted. I wonder how many construction firms have engineers or QA staff checking seating? As engineers, particularly, designers are often regarded as a cost and not paid to supervise very much, how much bad construction practice goes on? Also often units are not cut at 90 degrees so can have less than 50mm seating in part.

Now I am doing ISAs I have found it almost impossible to check actual seating as you cannot see into the floor unit or through seating angles. The drawings may say a minimum 50mm but this does not mean it actually is that. I wonder how many buildings there are with potentially insufficient seating? Possibly some buildings should have seating angles retrofitted as a matter of urgency?

The location of precast units in plan and the squareness of their ends should be considered when assessing the reasonable tolerances for precast floor units – i.e. the tolerance is not only on the overall length of the precast unit. This is explained in detail in the CAE Guidelines on Precast Concrete (‘the grey book’) on pages 107 and 109 – available from https://ir.canterbury.ac.nz/handle/10092/11540

The same details above can be used as the basis for assessments where (as is common) it is not possible to check actual seating.

Hi Karen,

I am also interested about your query.  I do note that SESOC interim guidelines cite friction coefficient of concrete on steel as 0.35 – 0.7 based on NZS3404 steel-steel and NZS3101 concrete-cast-against-steel respectively.  Maybe there is confidence enough that friction less than 0.7 can be achieved without a bearing strip?  I’d be interested to hear a more informed opinion.

Just bringing this topic back to life.

How are others dealing with the seating of precast (say flat slab) onto 20 series blockwork walls, which are typically reinforced centrally, such that the cover to longitudinal bars is very large (horizontal bar tied to vert). Cl 18.7.4.3 (c) will give a very large seating requirement, completely covering the blockwork vertical bar which would often be folded inwards as a starter bar. 

say 10mm (bearing) + 30mm (unit spall) + 80mm (cover to longitudinal bar) + 5mm (creep/shrinkage/etc) + tolerance (say 15mm)

Total seating circa 140mm, which will make it very difficult for reinforcing continuity from block to topping. Does this preclude seating of precast onto blockwork altogether, requiring a shelf angle as the only option? or an insitu concrete bond beam with two longitudinal bars + links?

Shelf angles can have their own issues with fixing capacity when installed at the top of the wall due to edge distances, also with durability if the situation happens to be a suspended slab over a subfloor space. 

Cheers, Guy

@0249019

Hello Guy,

I typically spec an angle where seating flat slab on masonry. However, I have seen various ways of doing this and wondered if you found any guidance in your searching as to what best practice is here?

Regards,
Daniel