CONCRETE ADVICE No. 42 (November 2010)
Holding down bolt design to Eurocode 2
y t e i c o S e t e r c n o C © , y p o C d e l l o r t n o c n U , 2 1 0 2 / 3 0 / 9 0 , t d r a h n i e M , t h m h c w : y p o c d e s n e c i L
Deryk Simpson BSc Simpson BSc CEng MICE FCS
This document provides guidance on the design of holding down bolts for attaching steel or precast concrete stanchions to reinforced or un-reinforced concrete (1) foundations, using Eurocode 2 . Concrete Advice sheet 5 covers the design of (3) holding down bolts to BS 8110-1 . Design approaches are given for resisting the uplift on the bolts and for the allowable bearing pressure underneath the stanchion base plate. This document only covers bolts in tension or compression and does not cover bolts in shear. A method for the design of dowels in shear is included in Concrete (4). Society Technical Report No 34 Proprietary fixings are not included in this document. The manufacture’s technical literature should be consulted for the load capacity of proprietary fixings.
1 Uplift bolts in tension There are two possible ways of checking bolts in uplift. The first is applicable to single bolts and pairs of bolts, which are effectively fully bonded over their full embedded length and have small or no anchor plates. The second method is applicable if the bolts are not effectively bonded over the embedded length but rely on an individual anchor plate for embedment, or when a group of bolts is fastened to a relatively large stiff anchor plate embedded in the concrete. Method 1 – Effectively fully bonded bolts Check shear stress The following procedure can be used to check the depth and number of bolts in tension, for fully bonded cast in bolts and post-drilled and fully grouted bolts. This method assumes the tension in the bolts is resisted by shear stress on the surface area of 90° cones of concrete within the
foundation around each bolt. The depth of the cone is to be taken as the full depth of the bolt for post-fixed bolts and to the top of the bolt anchor plate for cast-in bolts. The uplift loads used in these calculations is to be the relevant factored design actions not the not the characteristic actions. Cone shear stress = Design uplift load Surface area of cone or cones
÷
Note: Section 4 includes a method for calculating the surface area of nonintersecting cones and tabulated values for the combined areas of pairs of intersecting cones for different depths and spacings of pairs of bolts. The actual shear stress on the area of the cones should be compared to the allowable shear stress obtained from Equation 6.47 in Eurocode 2. In unreinforced or nominally reinforced sections the ρ1 term is zero so the allowable shear stress becomes (v min min + k 1σcp).
CONCRETE ADVICE NO. 42 In most foundations the horizontal stresses will be low, thus the allowable shear stress becomes equal to v min. If the cone shear stress exceeds the allowable shear stress then the bolts will need to be deeper and/or more bolts provided.
y t e i c o S e t e r c n o C © , y p o C d e l l o r t n o c n U , 2 1 0 2 / 3 0 / 9 0 , t d r a h n i e M , t h m h c w : y p o c d e s n e c i L
Check bond stress: Cast-in bolts If the shear stress is less than the relevant allowable shear stress the bond anchorage of the individual bolts should be checked. The method for calculating the anchorage bond stress around a reinforcement bar in Section 8.4 of Eurocode 2 should be used. However Eurocode 2 only covers the anchorage of ribbed bars; the anchorage of plain bars is not covered. If the embedded section of the bolt is threaded full length or consists of a length of ribbed reinforcement, the allowable bond strengths in Eurocode 2 are appropriate. If the embedded length of the bolt is a non-ribbed plain bar then the calculations in Eurocode 2 are theoretically not applicable. Engineering judgement will be needed if the embedded bolts are of plain round bars. There are two possible alternatives. The method in Eurocode 2 could be used, but with the bond strengths reduced to 55%. (This 55% relationship is based upon Table 3.26 in BS 8110-1). Alternatively the bond calculations for plain bars in BS 8110-1 could be used. If the actual bond stress exceeds the allowable bond stress the bolts will need to be deeper and/or more bolts provided. Check bond stress: Post-grouted bolts In the cases where bolts are grouted into drilled holes it may be prudent to check two anchorage bonds:
hole for bond calculations purposes. In all cases if bond stress exceeds the allowable bond stress it will be necessary to deepen and/or increase the number of bolts. Method 2 - Anchor plate pull out This method assumes that the anchor plate embedded in the concrete tries to pull out of the concrete by a punching shear failure. The anchor plate effectively becomes the loaded area for punching shear design, which is undertaken in accordance with Section 6.4 in Eurocode 2, with the section depth h shown in Figure 6.12 being taken as the depth of embedment to the top of the anchor plate. The anchor plate must be stiff enough so that the uplift forces in the bolt(s) produce an even distribution of compressive stress on the top face of the plate. The majority of stanchion holding down bolts will fall into this category, because in practice the grouting up of the bolt cones around the bolts cannot be assumed to be fully effective in transferring shear from the bolt into the surrounding concrete. Design procedure Check the shear stress around the perimeter of the anchor plate. If the shear stress exceeds the allowable value it will be necessary to increase the size of the anchor plate. The plate may require stiffening if increased in size. Then check the shear stress on the first critical perimeter. If the shear stress exceeds the allowable value there are a number of options available: •
On the grout/bolt interface : The calculation will be as for cast-in bolts, except that a value of allowable bond stress will need to be determined for the grout material, based upon the grout characteristic strength or the manufacturers’ technical information. On the grout/drilled hole interface : The calculation will be similar to that for cast-in bolts except that the effective diameter will be the hole diameter and the allowable bond stress will be the lesser of that for the foundation concrete or the grout. An assessment of the bond characteristics of the perimeter of the drilled hole will need to be made. This will depend on the roughness of the inside of the hole. For a ‘rough’ hole (e.g. produced by percussive drilling) the bond stress appropriate for deformed bars could be assumed. For a ‘smooth’ hole (e.g. produced by diamond drilling) the bond stress values appropriate to plain bars should be used. If there is any doubt about the potential roughness of the hole assume a ‘smooth’
• • •
Lengthen the bolts, thus setting the anchor plate deeper into the concrete. Increase the size of the anchor plate. The plate may require stiffening if increased in size. Increase the amount of top reinforcement to increase the allowable shear stress. Provide punching shear reinforcement. This would be regarded as a last resort due to the practical difficulties and cost of installing shear links in foundations. In this instance the shear would have to be checked on the next punching shear perimeter, and if necessary subsequent shear perimeters.
Overall design The bolts themselves should also be checked for direct tension stresses. Also the foundations should be designed to resist the uplift.
CONCRETE ADVICE NO. 42
2 Base plate sizing – Compression
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The following procedures will give the absolute minimum stanchion base plate size and applies to pin jointed bases only. This procedure assumes the base plate has adequate stiffness to give a uniform distribution of compressive stress. For stanchion bases required to resist overturning moments refer to the relevant codes of practice and design guides for the design of the base plate size. Use factored design loads not characteristic loads.
Proprietary grouts : In practice proprietary grouts are likely to be used. The compressive strength given in the manufacturer’s literature should be used for design. If no cylinder strength is quoted in the literature the appropriate cylinder strength can be obtained from the cube to cylinder strength relationships indicated in Table 3.1 of Eurocode 2.
Base plate area = Maximum design compressive load ÷ Allowable ultimate bearing stress
Fine concrete : Designed concrete with a small aggregate to suit the intended thickness of the infill.
Two cases should be considered and the maximum area used.
The other materials defined in Table 1 are as follows (the proportions and strengths are taken from Reference 5);
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Infill material to concrete foundation interface Use the design method in Section 6.7 of Eurocode 2. The dimension h in Figure 6.29 will normally be the depth of the foundation. The f cd value in Equation 6.63 will be that for the foundation concrete. For axially loaded symmetrical bases Ac1 in Figure 6.29 may be the plan area of the base. Base plate to grout/infill material interface Use the design method in Section 6.7 of Eurocode 2, taking the ratio Ac1 / Ac0 in Equation 6.63 as 1.0. The f cd value in Equation 6.63 will be that for the grout/infill material. Table 1 lists typical values for the characteristic strength of grout/infill materials. Table 1: Typical values for grout/infill material characteristic cube strengths Material
, t d r a h n i e M
Cement grout Sanded grout Mortar
, t h m h c w
Fine concrete Proprietary grouts
: y p o c d e s n e c i L
The procedures given above assume a uniform distribution of stress below the base plate, i.e. that the base plate is stiff. If the stress is not uniform, i.e. a flexible base plate, different procedures will be needed to size the base plate.
Characteristic cube strength (N/mm²) 12 – 15
Characteristic cylinder strength 2 (N/mm ) 10 -12
15 – 20
12 - 16
20 – 25
16 - 20
Use 28 day Use 28 day cube strength cylinder strength Refer to manufactures literature
Notes The design bearing stresses can be used as the maximum values for the design of base plates that are subject to an overturning movement or non-uniform stress distribution.
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•
•
Grout: Mixture of cement (usually Portland cement) to water in proportion of about 2:1 by weight. Sanded grout: Mixture of cement, sand and water in approximately equal proportions by weight. Mortar: Mixture of cement, sand and water in proportions of about 1:3:0.4 by weight.
3 Surface area of cones around embedded bolts Single bolts The surface area (AS ) of a 90° cone around a single bolt of embedded depth D is: 2
AS = 4.443 × D
Note: This equation cannot be used if bolts are closer together than 2D or closer to the edge of a foundation than 1.5D . Pairs of bolts D = embedded depth of the bolts X = horizontal distance between the bolt centres AD = combined surface area of the two 90° cones around the bolts. 2
If X is greater than 2D , AD = 8.886 D If X is less than 2D the cones overlap. The values for AD are listed in Table 2, which is based on information in Reference 5).
CONCRETE ADVICE NO. 39
Table 2: Effective conical areas for overlapping cones.
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X (mm) 150 200 300 450 600 750 EFFECTIVE CONICAL AREA OF TWO CONES 3 2 ( ×10 mm )
D (mm)
100
100
71.5
82.5
88.9
88.9
88.9
88.9
88.9
88.9
150
141.6
160.8
178.0
199.9
199.9
199.9
199.9
199.9
200
233.7
260.5
28.59
329.8
355.4
355.4
355.4
355.4
300
484.3
525.8
566.4
643.4
742.0
799.7
799.7
799.7
450
1027
1090
1152
1274
1448
1602
1728
1799
600
1769
1853
1937
2103
2345
2574
2784
3072
750
2711
2817
2922
3131
3439
3737
4021
4451
1000
4726
4867
5008
5288
5705
6114
6513
7149
1000
, y p o C d e l l o r t n o c n U , 2 1 0 2 / 3 0 / 9 0 , t d r a h n i e M , t h m h c w : y p o c d e s n e c i L
REFERENCES 1. 2. 3. 4. 5.
BS EN 1992-1-1: 2004 Eurocode 2: Design of concrete structures – Part 1-1: General rules and rules for buildings Holding down bolts, suggested design procedures using BS 8110-1, Concrete Advice Sheet 5, Concrete Society 2009 BS 8110-1: 1997 Structural use of concrete , Part 1: Code of practice for design and construction , Concrete industrial ground floors – A guide to design and construction , Concrete Society Technical Report No. 34, Third Edition, 2003 Holding down systems for steel stanchions , Concrete Society, Cement and Concrete Association and CONSTRADO, 1980
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Issued November 2010 CONCRETE Advice Sheets are produced and published by The Concrete Society. The information and advice contained in the Advice Sheets is based on the experience and knowledge of the Concrete Society’s Technical Staff. Although The Society does its best to ensure that any advice, recommendation or information it gives is accurate, no liability or responsibility of any kind (including liability for negligence), howsoever and from whatsoever cause arising, is accepted in this respect by The Concrete Society, its servants or agents. Readers should also note that all Concrete Society publications are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.
