CONCRETE

Advi

ceCONCRETE ADVICE No. 05

Holding down bolt design: suggested procedures

Deryk Simpson, BSc CEng MICE

This document gives guidance on the design of holding down bolts for attaching steel or concrete stanchions to reinforced concrete foundations. Design approaches are given for resisting the uplift on the bolts and for the allowable bearing pressure behind 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 Society Technical Report No. 34. Proprietary fi xings are not included in this document. The manufacturer's technical literature should be consulted for the load capacity of proprietary fi xings.

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 bonded over the full embedded length and have relatively small anchor plates. The second method may be applicable if there are more than two bolts, if the bolts are not effectively bonded over the embedded length but rely on an anchor plate for embedment, or if the bolts are fastened to a relatively large stiff anchor plate embedded in the concrete.

METHOD 1 - Effectively bonded bolts

Check Shear StressThe following procedure can be used to check the depth and number of bolts in tension. Assume 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 depth to the top of the bolt anchor plate not the full depth of the bolt.

The uplift load is to be the factored design load not the characteristic load. If the uplift load value results from non limit-state design calculations assume a partial load factor of 1.6 unless a lower partial load factor can be justifi ed by calculation:

Cone shear stress = Design uplift force Surface area of

cone or cones

Note: Appendix A includes a method for calculating the surface area of non-intersecting cones and tabulated values for the combined areas of pairs of intersecting cones for different depths and spacings of pairs of bolts.

The actual cone shear stress should be less than the design shear stress value obtained from Table 3.8 of BS 8110(2). The enhancement in design shear stress for tension reinforcement is only applicable if top structural reinforcement is present in two directions at right angles. The lower of the two percentages should be used for the

Lic

ensed copy:

hyder, Hyder Consu

ltin

g, 3

0/08

/201

0, U

ncon

trol

led

Copy,

The

Con

cret

e So

ciet

y

As value in Table 3.8. If only nominal top reinforcement is provided then the foundation should be regarded as unreinforced for checking the shear stress around bolts in uplift.

If the cone shear stress exceeds the design shear stress then the bolts will need to be deeper and/or more bolts provided.

Check Bond Stress: Cast-in boltsIf the shear stress is less than the relevant design shear stress the anchorage of the individual bolts should be checked. The method for calculating the anchorage bond stress around a reinforcement bar in BS 8110 can be used, i.e.

fb = Fs/ ( x x D)

where:Fs = Design force in the bar = Diameter of the boltD = Depth of the boltfb = Anchorage bond stress, which should not

exceed fbufbu = Design ultimate anchorage bond stress

The method of calculating fbu is given in 3.12.8.4 in BS 8110-1. An assessment of the bond characteristics of the bolt will need to be made. Bolts that are plain round bar with only a limited length of thread at the top should be considered as Plain Bars for Table 3.26 BS 8110-1. Bolts that are threaded full length can be considered as Type 2 Deformed Bars in Table 3.26. NB: The amount of reinforcement provided has no effect on the anchorage bond stress.

If fb exceeds fbu the bolts will need to be deeper and/or more bolts provided.

Check Bond Stress: Post-grouted boltsIn the cases where bolts are grouted into drilled holes it may be prudent to check two anchorage bonds, i.e.

z On the grout/bolt interface. The calculation will be as for cast-in bolts above, except that a value of fbu will need to be determined for the grout material, based upon the grout characteristic compressive strength or the manufacturers technical information.

z On the grout/drilled hole interface. The calculation will be similar to that for cast-in bolts above, except that = hole diameter and fbu 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 smooth hole (e.g. produced by diamond drilling) the Plain Bar values could be used for Table 3.26 BS 8110-1. For a rough hole (e.g. produced by percussive drilling) the Type 2 values could be used for Table 3.26.

In all cases if fb exceeds fbu 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 punching shear failure. The anchor plate effectively becomes the loaded area for a punching shear design, which is under-taken in accordance with Section 3.7.7 in BS 8110(2).

For the purposes of design for resistance to uplift the symbols in BS 8110 would have the following meaning:

u0 = perimeter of anchor plateu = effective punching shear perimeter around anchor

plated = depth to top of anchor plate below mid-plane of

top reinforcement V = maximum design uplift load (i.e. factored uplift

load)vc = design concrete shear stress from Table 3.8.

If lightly reinforced or reinforced in one direction only, take 100As/bvd less than 0.15. If reinforced in the top in both directions take the lesser value for 100As/bvd.

Design procedureCheck the shear stress at the face of the anchor plate:

vmax = V

u0d

vmax should not exceed 0.8 fcu or 5 N/mm2 if less.

If vmax exceeds the above values it will be necessary to increase the size of the anchor plate.

Check the shear stress on the critical perimeter:

v = V ud

where

u = length of critical perimeter, located at a distance of 1.5d from the anchor plate.

Lic

ensed copy:

hyder, Hyder Consu

ltin

g, 3

0/08

/201

0, U

ncon

trol

led

Copy,

The

Con

cret

e So

ciet

y

v should not exceed the appropriate value from Table 3.8. If v exceeds the Table 3.8 value there are a number of options available:

z Lengthen the bolts, thus setting the anchor plate deeper into the concrete.

z Increase the size of the anchor plate. NB: it may require stiffening if increased in size.

z Increase the amount of top reinforcement to increase vc.

z Provide shear reinforcement. This would be regarded as a last resort due to the practical diffi culties and cost of installing shear links in foundations. In this instance the shear would have to be checked on the next shear perimeter, and if necessary subsequent shear perimeters.

Overall designThe bolts themselves should also be checked for direct tension stress (see BS 7419(3)). Also the foundations should be designed to resist the uplift.

2 Base plate sizing compression

The following procedures will give the absolute minimum stanchion base plate size and applies to pin jointed bases only. 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. If the compression load results from a non limit-state design assume a partial load factor of 1.6 unless a lower value can be justifi ed by calculation.

Base plate area =Maximum design compressive load Design ultimate bearing stress

Two cases should be considered and the maximum area used.

2.1 Infi ll material/concrete foundation interface

Take design ultimate bearing stress = 0.6 fcu(1) where fcu(1) is the characteristic strength of the foundation concrete.

2.2 Base plate/infi ll material interface

Take design ultimate bearing stress = 0.4 fcu(2) where fcu(2) is the characteristic strength of the bedding infi ll material. The characteristic strength will depend on the age at which the bedding/infi ll material is subject to the full load. Table 1 lists typical values for fcu(2). For proprietary materials, refer to manufacturers literature for design stresses at the appropriate ages.

2.3 Notes

z The information in Sections 2.1 and 2.2 above is based on Clause 5.2.3.4 in BS 8110(2) and Clause 3.8.4 in reference 1.

z In Table 1, the strength at 3 days is assumed to be 40% of the value at 28 days, and the strength at 7 days is assumed to be 60% of the value at 28 days. This is applicable to Portland cement materials only. Higher percentages may be used if confi rmed by testing or past knowledge of the materials.

z The design bearing stresses in Sections 2.1 and 2.2 can be used as the maximum values for the design of base plates that are subject to an overturning move-ment or non-uniform stress distribution.

z The procedures in Sections 2.1 and 2.2 assume a uniform distribution of stess below the base plate, i.e. that the base plate is stiff. If the stress is not uniform, i.e. a fl exible base plate, different procedures will be needed to size the base plate.

Table 1: Typical values for fcu(2)Material Cube strength at 28

days (N/mm2)0.4 fcu values (N/mm2)

3 days 7 days 28 daysCement grout 12 - 15 1.9 - 2.4 2.9 - 3.6 4.8 - 6.0Sanded grout 15 - 20 2.4 - 3.2 3.6 - 4.8 6.0 - 8.0Mortar 20 - 25 3.2 - 4.0 4.8 - 6.0 8.0 - 10.0Fine concrete Use 28 day cube strength 0.16 fcu 0.24 fcu 0.4 fcu

Lic

ensed copy:

hyder, Hyder Consu

ltin

g, 3

0/08

/201

0, U

ncon

trol

led

Copy,

The

Con

cret

e So

ciet

y

REFERENCES AND ADVICE

Impartial advice can be sought from The Concrete Society. Members are entitled to substantial discounts on services and products including site visits and investigations, dependent on status. For publications and information, The Concrete Society Bookshop holds a wide range of books and pamphlets along with an extensive library stock. We provide many services such as literature searches and notifi cation of new references to our extensive catalogue.

Contacts

Enquiries and advice The Concrete Society 01276 607140Bookshop 07004 607777www.concrete.org.uk

Issued December 2006

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 Societys 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.

References

1. CONCRETE SOCIETY, CEMENT AND CONCRETE ASSOCIATION AND CONSTRADO, Holding down systems for steel stanchions, British Cement Association (formerly the Cement and Concrete Association), Camberley, 1980.

2. BRITISH STANDARDS INSTITUTION, BS 8110, Structural use of concrete, Part 1: Code of practice for design and construction, BSI, London, 1997.

3. BRITISH STANDARDS INSTITUTION, BS 7419, Specifi cation for holding down bolts, BSI, London, 1991.

z The materials defi ned in Table 1 are as follows:

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.

z For further information refer to Clause 3.8.1 in reference 1.

APPENDIX A: Surface area of cones around embedded bolts

A.1. Single BoltsThe surface area [AS] of a 90 cone around a single bolt of embedded depth D is AS = 4.443 x D2.

NB: This equation cannot be used if bolts are closer together than 2D or closer to the edge of a foundation than 1.5D.

A.2. Pairs of BoltsD = Embedded depth of the boltsX = Horizontal distance between the bolt centresAD = Combined surface area of the two 90 cones

around each bolt.

A.2.1 If X is greater than 2D, AD = 8.886 x D2

A.2.2 If X is less than 2D the cones overlap. The values for AD are listed in the following table

X = 100 150 200 300 450 600 750 1000D (mm)

Effective conical area of 2 cones = 2D (x 103 mm2)

100 71.5 82.5 88.9 88.9 88.9 88.9 88.9 88.9150 141.6 160.8 178.0 199.9 199.9 199.9 199.9 199.9200 233.7 260.5 285.9 329.8 355.4 355.4 355.4 355.4300 484.3 525.8 566.4 643.4 742.0 799.7 799.7 799.7450 1027 1090 1152 1274 1448 1602 1728 1799600 1769 1853 1937 2103 2345 2574 2784 3072750 2711 2817 2922 3131 3439 3737 4021 44511000 4726 4867 5008 5288 5705 6114 6513 7149

Lic

ensed copy:

hyder, Hyder Consu

ltin

g, 3

0/08

/201

0, U

ncon

trol

led

Copy,

The

Con

cret

e So

ciet

y

Document BookmarksDocument Bookmarks