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Roderick Chisholm [08/05/12]

I have read with interest the Cross Newsletter No 26, in particular Report 296.

1. This deals with the compatibility of the concrete at column/slab junctions where the column concrete is of a characteristic strength much higher than that of the slab and where the slab is cast over the column head.

2. This is a common occurrence in tall buildings design.

3. The ACI Code 318 gives empirical guidance, stating that if the column concrete characteristic strength is limited to not more than 1.4 times the slab characteristic strength then the full column strength can be used. This removes the problem of having to cast a small area of the "column strength" concrete along with the much larger area of the "slab strength" concrete.

4. For example, slab concrete C37 can be used with column concrete C37 x 1.4 = C52

5. The example quoted, C85 column concrete and C37 slab concrete would seem extreme. It would benefit from having larger columns (with lower strength concrete) and higher strength concrete slabs at lower levels which are presumably the critical levels. C55 column concrete, C40 slab concrete would be satisfactory. For a given load the column area would be only about 35% greater with C55 concrete.

6. Although it appears that this ACI 318 concession will apply well to interior column/slab junctions, where the concrete at the column head is confined all around, edge and corner columns have less confinement if the slab is flush with a column face. It would be logical to provide additional "containment" reinforcement, extending to the column face, within the slab thickness. Where the slab extends beyond the column face by a full anchorage length then the edge or face column can be treated as internal.

7. The condition about tensile stresses in the top of the slab will occur for any concrete strength. If there is concern about this, simply continue the column links through the depth of the slab. This should provide sufficient confinement to the weaker slab concrete where it passes over the column head. In geographical areas of even medium seismicity it is common practice to provide links at close centres (eg 100mm c/c) for a short distance below and above the column/slab intersection.

8. This situation is referred to in the Concrete Society Publication "Guide to the Design of Concrete Structures in the Arabian Peninsula", 2008, Section 7.8.4

Chris Bolton [07/05/12]

I'm just reading Newsletter No 26. The piece on cracking of RHS columns due to freezing (report 253) is interesting, but I wonder if freezing is the only cause? The second photo (of a non-galvanised section?) shows cracking in the middle of the side, not at the corners. I would expect freezing to cause bending in the spans before enough tension developed to split the corner. My suspicion is that the corners may have been cracked in the galvanising (Liquid Metal Assisted Cracking) and the freezing has just opened them up. On cold rolled RHS, the corners are work hardened so vulnerable to LMAC. If I remember right, LMAC was first observed on the corners of RHS - they were posts for motorway barriers and had split from the end like banana skins. The cracks were not initially obvious but the posts would not fit in their sockets.

Any other comments on this phenomenon will be welcome.

Dr Richard Barnes [06/05/12]

Jørgen Munch-Andersen [06/05/12]

Regarding the report on Compatibility at RC column to slab joint (strong columns/weaker slabs) in newsletter no 26 I would like to draw attention to the fact that the robustness of a column-slap structure increases very significantly if the columns are continuous and the slaps rests on plinths. This could ensure that failure of a slab does not drag the column with it.

Roger Davies [30/01/12]

Newsletter No 25 Further to  Report 287 on Concerns about PV installations, and the comments,I have done a straw poll amongst those here who have had PVs installed on their houses (admittedly a small sample) – in no case was it treated as a material alteration and consequently the structure has not been assessed for the additional load. One of the manufacturer / installers of the ‘hot water panel systems’ that we have contact with had the same view. I would also have concerns about the design of fixings particularly the uplift case. I think that as most of these systems are installed under Competent Persons schemes that in practice the Local Authorities take a relaxed attitude particularly as it is considered to be a good thing on sustainability grounds. My personal experience is that triggering a request for structural justification for a material alteration is far less likely than a material change of use. When I put an extra floor on an existing building it was the coincidental material change of use that was used by the Local Authority to request more information I suspect that is because the definitions and requirements are better defined and wider in scope. Please keep up the good work in producing these reports, they are useful triggers for raising items for our Engineers to think about.

Michael Crick [30/01/12]

CROSS Newsletter No 25

Report 276 Licensing of Temporary Structures I could not disagree with some of the comments however I would expect the duties under the Occupier’s Liability Act 1957 to apply with the authority being the occupier or controller of the premises and hence holder of any duty of care. The items are certainly structures, are of metal or similar and are non-domestic and are constructed in the “furtherance of a business” or in line with “the conduct of their undertaking”. The people who erect and maintain them are at work therefore I would expect the Health & Safety at Work Act 1974 to apply including regulation 3(1) would apply. Although not necessarily notifiable, the construction (Design & Management) Regulations 2007 wherein designer duties under Regulation 11 would apply together with contractor duties under Regulation 13. It would seem that where there is concern there should be sufficient legislation in place to allow the serving of a Dangerous Structure Notice under the building Act 1984 Armed with this information it would seem reasonable for a level of encouragement be given to have matters rectified.I do not advocate a legislative approach in the first instance and usually seek co-operation and discussion but it is nice to know the law should there be a need to encourage when, on occasion, those that can, and perhaps should, do something claim they cannot.

Reports 266/280 PV panels on roofs I am currently working with a large energy organisation on installation of PV panels and can confirm that not only is a part of the process the need to carry a structural survey on the roof for ability to take both positive and negative pressure loads but also on a number of sites, roofs have required strengthening or been excluded from the programme. (I commend this company on ensuring such checks are carried out). The issue of snow load adds a further issue in that panels can cause a dam effect under slight thaw and instead of the snow sliding off it can be retained against panels and re-freeze overnight to ice. This not only increases dead-loads but at a point of thaw when the ice melts to a sufficient thickness to pass beneath the panel, has the potential to slide off in a single sheet of significant size and weight therefore the accessibility to persons by doors or paths beneath has to be considered. Such systems etc. will remain an issue as so much of the controlling legislation refers to “non-domestic premises”. “furtherance of a business” and “at work” which so often do not apply to residential properties. (albeit civil law may)

Nick Clarke [27/01/12]

In CROSS Newsletter No 25, January 2012, reports 266, 269, 280 and 287 refer to PV installations

Several BRE publications give guidance relevant to these reports: Digest 495 Mechanical installation of roof-mounted photovoltaic systems 2005 Digest 489 Wind loads on roof based photovoltaic systems 2004 Information Paper 8/11 Photovoltaic systems on dwellings: key factors for successful installations 2011 These are available from www.brebookshop.com In addtion, there is the NHBC Foundation publication (written by Paul Blackmore, BRE) NF 30 Guide to installation of renewable energy systems on roofs of residential buildings 2011 This is available from www.nhbcfoundation.org

Geoff Fletcher [14/12/11]

Thank you for the item in the CROSS Newsletter 24 concerning Report 244 “Failure of Epoxy Fixings due to High Temperatures”. A comment I would offer re this mobile phone tower failure incident is that in addition to questions of what was the temperature of the epoxy grout material and the receiving concrete substrate at the time of installation, note that just about ALL installed chemical anchoring products start to lose strength once they get to about 80degC. Even if installed within the stated temperature limits, a black tarmac covered concrete roof with a steel threaded anchor element embedded into the epoxy might attract, hold and conduct high temperatures into the epoxy on a sunny day & possibly for some time after that when storm conditions gather. On that basis I would think a mechanical anchor is a better potential solution.

Kubilay Hicyilma [07/11/11]

I was reviewing some drawings and had reservations about epoxy anchors in tension in high thermal conditions, so it was timely to comes across the fact that CROSS had noted this in the Newsletter No 24, October 2011. I will use this to help make my point, so that the detail is properly thought through.

Mike Crick [23/07/11]

In Newsletter 23 (July 2011) you ran an item on water filled containers for ballast.

I have similarly had problems of vandalism and a solution for temporary ballast was to place 2m dia manhole rings on visqueen and then fill with gravel. Can all be done with JCB 3, small tractor or similar but provides a vandal resistant 10 tonne balast block. On completion the ring can be lifted by same as it is light and will slide over gravel. Gravel can be recovered clean and re-used.

Same basic system, all quickly achievable with small plant but vandal resistant. (My use has been mainly for temporary road closure barriers that cannot be easlily removed but principle is the same.

Nick Clarke [22/04/11]

Newsletter 22 includes Report 163 about freestanding walls. This refers to the ODPM leaflet which can be downloaded from the websites of many councils, though oddly not from any government website, as far as I can tell. This leaflet refers to two BRE Good Building Guides (13 and 14), which provide much more detailed advice on the topic than the ODPM leaflet. There are in fact several BRE Good Building Guides and Good Repair Guides about freestanding walls that will be of interest to a professional readership, as opposed to the householder, who the ODPM leaflet is aimed at: GBG 13 Surveying brick or blockwork freestanding walls (1992) GBG 14 Building simple plan brick or blockwork freestanding walls (1994) GBG 17 Freestanding brick walls - repairs to copings and cappings (1993) GRG 28 Repairing brick and block freestanding walls (2000) This can be purchased through www.brebookshop.com. Nick Clarke IHS BRE Press, Garston, Watford WD25 9XX 01923 664170 GBG 13

Ian Smith [03/02/11]

In the CROSS Newsletter no 21, January 2011, there is a commentary (Report 216) on the failure of a group of M16 grade 8.8 bolts that connected a stub bracket to the web of a steel beam. The bolts had been excessively tightened by the erector, using an air-powered torque wrench. The fact that the grip length (and hence the thread length within the grip) was short was cited as one reason for the over-tightening of the bolts. The recommendation follows that “critical connections should be checked by an external consultant and that critical connections must use HSFG bolts with load indicating washers or TCBs” (attributed to HSE). While the external checking of critical connections is seen as being a good recommendation, the effective banning of the use of ‘untorqued’ bolts in all critical connections is seen as misguided, particularly if (as appears to be the case here) the bracket had a single load application and thus fluctuating stress at the base of the thread was not a design consideration. The logic behind this recommendation appears to be that an HSFG bolt will not be broken by a air-powered torque wrench. In practice this is known to be wrong. If the wrench used is too big, then an HSFG bolt can be broken and it is down to the skill of the erector to ensure that this does not happen. Erectors have been using air wrenches to tighten (not to torque) for many years and, if they are experienced, this method has been seen to be safe and very cost-effective. SCI Advisory Desk Note AD302 is topical, which in turn refers to the NSSS 4th edition stating “Bolts may be assembled using power tools or shall be fully tightened by hand” (this referring to non-preloaded bolts). The AD302 document, however, is concerned more with under-tightening bolts than over-tightening, though it does contain the caveat “Ordinary bolts particularly those specified to BS 4190, should not be torqued to the values used for preloaded (HSFG) bolts because they have thinner nuts than preloaded bolts”. Other European countries, and the builders of mechanical installations in the UK use grade 8.8 bolts with (usually) grade 10 (but not HSFG) nuts under conditions of controlled torque to create bolted connections that work in the same way as those which the bridge industry uses HSFG bolts. The fact that the nuts used in such connections are significantly smaller than the HSFG variety means they use lower torque values and achieve similarly lower axial preload in the shank of the bolt. The lower torque values are demanded because of the danger of stripping the nut’s thread due to excess hoop strain in the nut if the same torque is used as would be the case for an HSFG bolt. It has also sometimes been the practice to apply a grade 10 nut to grade 8.8 bolts when loaded in tension. Experienced erectors will, when tightening ‘untorqued’ bolts, either reduce the torque limit on the wrench, or throttle back on the air supply. The use of an air-powered wrench is not something for untrained erectors. The comments in Report 216 that experience and site control are necessary are therefore welcome. However, the effective banning of non-preloaded bolts from critical connections due to a single incident of improper site action is less appropriate - good guidance on the safe operation of air-powered wrenches would be a more useful tool.

John Carpenter [02/02/11]

I agree with Peter Hyatt’s suggestion that full plans are desirable for a wider range of work than currently catered for by the Building Act. The sophistication and complexity of basement (and loft) constructions were perhaps not envisaged when this was drafted. This is a subject under review by the Department of Communities and Local Government. Recent research work for the Health and Safety Executive (HSE) explores how closer co-operation and effectiveness could be achieved between Building Control, which does visit site during the construction phase, and the HSE. There are now arrangements in place between the Building Control Alliance and HSE to allow this to happen (http://www.buildingcontrolalliance.org/2010/09/hse-bca-sign-new-agreement-to-improve-health-and-safety-on-construction-sites/). Notwithstanding, it is not correct to say that safety legislation, and CDM specifically, does not apply to domestic alterations. It applies in full, with the sole exception of the client, who, if a householder, has no duties. Whether duties are discharged is another issue!

Brian Duguid BEng (Hons) MICE [02/02/11]

Dear CROSS, I am somewhat surprised to read in the newsletter's first report (Newsletter No 21 report 216)  that the HSE recommends use of load-indicating washers or tension control bolts in place of calibrated torque wrenches. All three methods are prone to the risk that they can apply the incorrect bolt strain, and as a result the part-turn method has been traditionally preferred in the UK. SCI guidance SCI 7-05 gives more details of the problems with the other methods, but essentially they are all prone to creating an incorrect strain in the bolts, due to uncertainties in the thread friction, and the load indicating washers have the additional flaw of introducing a corrosion-prone crevice into the steelwork.

Peter Hyatt C Eng M I Struct E [30/01/11]

Ref: Cross 214 (Newsletter No 21 January 2011) Local Authority Building Control are entitled to request additional information, including structural calculations, in connection with Building Notice applications and, in my experience, often do so. Approved Inspectors are not allowed to issue a final notice if they are aware that the works do not comply with the Building Regulations (according to Direct.gov.co.uk) and are entitled to withdraw the Initial Notice under these circumstances. This action would re-assign the Building Control function to the Local Authority, which has enforcement powers. There would appear to be only limited instances where Building Control can require Full Plans applications and these relate to means of escape and building use. It would appear that 'ordinary' residential projects do not fall into this category, even for relatively complex underpinning and basement construction. There would seem to be a good case to make for an amendment to the Building Act to require Full Plans applications for all works involving, say, substantial demolition, underpinning and the construction of basements - maybe in other situations, too. This might particularly apply to residential alterations and refurbishments, carried out for the private owners of the property, where many of the rigors imposed by the CDM regulations are not applied. There might also be a separate case to be made to extend the categories under which the full CDM regulations do apply, to include substantial structural works to privately owned residential properties. It would seem to be a complete anomaly that some of the most challenging building works, at risk of being undertaken by small contractors without adequate experience or resources, are excluded from much of the relevant health & safety controls.

Mike Banfi [30/01/11]

I have just read the the report on the fall of bridge deck support in the latest newsletter (Newsletter No 21 January 2011 Report 216 ) and the comments about the appropriate bolts to use. Unfortunately the comments do not reflect the current bolt standards. HSFG bolts used to be specified to BS 4395. That is obsolete and has been superseded by BS EN 14399 (High Strength structural bolting assemblies for preloading). There is a grade 8.8 bolt to this standard. Therefore it is not so much the grade of bolt that was wrong but the fact that they were to the wrong standard.

Name withheld [20/10/10]

I have just read Newsletter 20 and some of the issues raised in the Building Control article resonate with what I and colleagues have been discussing recently. In my experience, engineers do not carry out sufficient checking. I would go further and say that some, especially sole practitioners and small firms, rely on Building Control as their checking engineer. I think it is often the case that Building Control do not have the resources to carry out proper checking themselves and I know of cases where the commercial structure of how checking is sub-contracted out to consulting engineers by Building Control is not viable (e.g. contracts for checking let on ridiculously low hourly rates). One might see this as an argument for self-certification but my experience of working in such an environment saps still furher my confidence that checking is properly carried out. In this case, firms check and certify their own work (actually they employ certifiers) when they are under certainly no less commercial pressure to issue their design than when they were being checked by Building Control. In the case of 'self-certification', the certifiers are audited but our experiences of this does nothing to reassure. Audits seem to have focussed on minor or secondary issues such as the certifiers failure to specifically cerftify nailed fixings of domestic stud partitions and not looking at the certifiers' competence and dilligence in a broader sense. My belief is that independent review is a better system and should perhaps be mandatory.

Henry Dalton [05/08/10]

I was interested to read of the cases (in Newsletter No 19) in which:

1) An engineer proposed to underpin a wall with mass concrete without allowing for the bending moments ( and therefore tensile stresses ) which would arise due to soil ( and water ? ) pressures, and

2) A local authority failed to take action regarding a defective retaining wall.

Two years ago we were asked to design underpinning works to form a basement under a building. On one side of the building no soil would be retained ( since there was an adjacent basement ) and we specified mass concrete. On the other side about 3 m of soil was to be retained and we specified reinforced concrete. Unfortunately the builder used mass concrete where reinforced concrete was required. Although the Building Control Officer is fully aware of this he has taken no action to force the owner to take remedial work.

George Mathieson [04/05/10]

Re: demolition of large panel structures, and lack of continuity I remember working on this after Ronan Point, when many large buildings were being strengthened, mainly to introduce continuity between panels. General practice at the time was very poor in many cases, and I remember thinking at the time that a lot of the continuity reinforcement or bolting that we were designing would only add minimal strength in practice. Nothing I have seen since has caused me to change my mind on this. Demolition of any large panel building needs to be approached with great care, and a basic assumption that continuity is likely to be severely lacking should guide the work.

J Gray [23/04/10]

Newsletter No 18 contained a report: GAIN IN STRENGTH OF MORTAR SLOWER THAN CONCRETE (Report 177) and and a this issue can be addressed with the use of  a rapid hardening cement.  Not to confuse hardening and setting, the two are independent and can be designed separately.

Final hardness/strength is often taken at 28 days, but continues indefinitely at a slowing rate.  It is quicker at higher temperatures. Many modern cements gain substantial strength in much less time. Match testing can be done when strength on loading is important. Strength is a function of water/cement ratio for mortar as for concrete.

There are numerous specialised cements/mortars, and Contractors accustomed to using them.  Any of the larger firms' Intranets should refer, and it is impressive what can be found on the public www. Useful information can be found on the Sustainable Concrete web site http://www.sustainableconcrete.org.uk/main.asp?page=140