021 - Recessed Holding Down Bolts

[cross]

http://www.dbp.org.uk/cs/DBP00021-REVA.pdf

This case study has been amended on the website to take into account the points raised below.
Thanks to Andy for raising these points.
Please leave your comments on this case study here.

[andy allan]

I have doubts about this. The rebate will fill with water and make it more difficult to locate and adjust levelling packs and wedges. There has been one fataility as a result of recessing baseplates where the column was 16m tall; the erector made the mistake of using a central pack because edge wedges couldn't be adjusted with a hammer.

The reason for the 120 square 20 thick washer plates isn't given (and they don't feature in the cross section). If this is to provide additional lateral tolerance then it seems a little excessive - adequate tolerances are normally achieved by providing anchor bolts with sufficient length.

The trip hazard can be managed by using small high-vis traffic cones with the bases cut off.

[Chris Achilles]

This is a good point!

I think this type of base tends to be used on a raft foundation as an alternative to having the base plate directly to the slab as it looks better when finished.

The note about the washer plates refers to the bolt washer plates and presumably they are 20 think to resist uplift (Should not be shown on plan or should be dotted).

I'm not a massive fan of recessed bases myself as they do fill up with water and unless they are large enough they will hinder the use of wedges.

The comments about reducing the chances of people falling onto the bolts is valid but the ability to drive wedges or place packs should not be overlooked. I'll recommend that this submission is revised to highlight potential access and drainage problems.

[Chris Achilles]

With regard to columns falling over during erection - This is a problem which is often overlooked by designers and it applies to all types of bases recessed or flat. All column bases should be designed to be free standing during erection and if additional packs/wedges are required to make this possible then they should be specified by the designer, shown on the drawings and raised in the Designer Risk Assessment. The recommended value for wind loading is Vs - 18m/s to BS6399:Part 2 (Technical Guidance Note T20.002). The size of the packs is also critical to adequately spread the load and this information should be clearly advised to the erectors. Unfortunatley very few designers seem to consider stability during erection and lives have been lost in the past!