Shear force transfer in base plate by friction and anchors

In some cases, shear force may be transferred via friction between a base plate and the concrete block with or without grout. This article discusses recommendations and developments in the design guidelines, with a focus on European standards. For a general introduction to anchoring design in IDEA StatiCa read the blog Safe and accurate anchoring design.

What EN 1993-1-8:2005 says?

Eurocode for the design of steel joints in Clause 6.2.2 states that the shear force may be distributed between anchors and friction. In other words, the resistance in shear is the sum of friction resistance and shear resistance of all anchors (Equation 6.3):

\[F_{v,Rd} = F_{f,Rd}+nF_{vb,Rd}\]

where:

• \(F_{f,Rd} = C_{f,d} N_{c,Ed}\) – friction resistance as defined in Equation (6.1)
  • \( C_{f,d} = 0.2 \) – coefficient of friction between base plate and grout layer for sand-cement mortar
  • \(N_{c,Ed}\) – design value of normal compressive force in the column; the minimal value should be used and the load combination where maximum shear and minimum compressive force act together. Infimum partial safety factors (e.g. \(\gamma_{G,inf}=1.0\) should be used for load effect increasing compressive force, typically self-weight.
• \(n\) – number of anchor bolts in base plate
• \(F_{vb,Rd} = \min \{F_{1,vb,Rd}, F_{2,vb,Rd} \}\) – design shear resistance of and anchor bolts
  • \(F_{1,vb,Rd} = \frac{\alpha_v f_{ub} A}{\gamma_{M2}}\) – design shear resistance of anchor bolt determined by Cl. 3.6.1
    • \(\alpha_v = 0.6 \) for classes 4.6, 5.6, and 8.8
    • \(\alpha_v = 0.5 \) for classes 4.8, 5.8, 6.8, and 10.9
    • \(f_{ub}\) – ultimate strength of anchor bolt
    • \(A = A_s\) – where shear plane passes through the threaded portion of anchor bolt
      • \(A_s\) – tensile stress area of anchor bolt
    • \(A = A_g\) –  where shear plane does not pass through the threaded portion of anchor bolt
      • \(A_g\) – gross area of unthreaded anchor bolt shank
    • \(\gamma_{M2} = 1.25 \) – partial safety factor for bolts (Table 2.1)
  • \(F_{2,vb,Rd} = \frac{\alpha_b f_{ub} A_s}{\gamma_{M2}} \) – Equation (6.2)
    • \(\alpha_b = 0.44-0.0003 f_{yb}\)
    • \(f_{yb}\) – yield strength of anchor bolt, where 235 MPa \(\le f_{yb} \le\) 640 MPa

These provisions originate from the research from Stevin laboratory at Delft Technical University in the Netherlands that are summarized in a paper in Heron Journal.  

Note that the shear resistance of anchoring with grouted base plate is much higher than that of EN 1992-4:2018 because larger deformations are allowed and second-order tensile forces develop in anchors loaded in shear.

Also note that no regard is paid to the resistance of the foundation concrete block. It is assumed that its resistance will be checked elsewhere according to EN 1992. For stand-off anchors a different approach is described in this article.

Fib Bulletin 58: Design of anchorages in concrete (2011) provision

An international fib Bulletin 58 deals with the effect of friction in Chapter 4.2. It states that not only when compressive force but also a bending moment acts in a base plate, friction will develop. However, it states that:

As a rule, frictional resistance should be neglected if:

• the thickness of the grout layer exceeds one-half of the anchor diameter
• the anchorage capacity is governed by a near-edge condition
• the anchorage is intended to resist earthquake loads

The concrete edge failure should be checked for the full shear force and not only for the shear force acting on anchors that is reduced by friction.

The coefficient of friction comparable to \(C_{f,d}\) in EN 1993-1-8 is \(\mu / \gamma_{Mf} = 0.4/1.5 = 0.267\).

Statement in EN 1992-4: 2018

Eurocode for anchoring design is very controversial because many designs that had been passing all the checks using the traditional designs suddenly failed. The Eurocode is primarily suitable for anchorages with short anchors where the results vary significantly, which is reflected by large partial safety factors. This is shown e.g. in this paper with 1 722 tests.

In Cl. 6.1 (2), it states that:

When a bending moment and/or a compression force act on a fixture, which is in contact with concrete or mortar, a friction force will develop. If a shear force is also acting on a fixture, this friction will reduce the shear force on the fastener. However, in this EN friction forces are neglected in the design of the fastenings.

So it does not specifically forbid using the contribution of friction but just does not use it at all.

FprEN 1993-1-8:2023

The final draft of Eurocode for design of steel joints strictly divides anchors into:

• Fasteners between steel and concrete – short anchors
• Anchor bolts – traditional long anchors

It is assumed that for anchor bolts, steel failure will govern. Shear resistance is in Cl. D.3.1.4 and again, it allows to sum the design resistance for friction and for anchor bolts. Note that it says nothing about fasteners between steel and concrete (short anchors where concrete or pull-out failures may govern).

\[ F_{v,Rd} = F_{f,Rd} + n F_{vb,Rd} \]

where:

• \(F_{f,Rd} = C_{f,d} N_{c,Ed}\) – friction resistance as defined in Equation (6.1)
  • \( C_{f,d} = 0.3 \) – coefficient of friction between base plate and grout layer for sand-cement mortar
  • \(N_{c,Ed}\) – design value of normal compressive force in the column
• \(n\) – number of anchor bolts in base plate
• \(F_{vb,Rd} = \frac{\alpha_{bc} f_{ub} A_s}{\gamma_{M2}} \) – design shear resistance of and anchor bolts
  • \(\alpha_{bc} = 0.44-0.0003 f_{yb}\)
    • \(f_{yb}\) – yield strength of anchor bolt, where 235 MPa \(\le f_{yb} \le\) 640 MPa
  • \(f_{ub}\) – ultimate strength of anchor bolt
  • \(A_s\) – tensile stress area of anchor bolt
  • \(\gamma_{M2} = 1.25 \) – partial safety factor for bolts (Table 2.1)

Note that coefficient of friction \(C_{f,d}\) is now increased to 0.3. This may be caused by generally better quality of grout. Also, \(F_{1,vb,Rd}\) that probably never governs was removed.

The design resistances of failure modes in concrete should be checked according to EN 1992-4 and should not govern. Different failure modes and code-checks of anchors in a concrete block are listed in this article.

Practice

In practice, the sum of friction resistance and anchors in shear resistance to the shear resistance is very rarely used. 

For cast-in anchors, the hole tolerances in base plates are typically large, e.g. +/-30 mm, that means in extreme the base plate may move 60 mm before hitting the opposite side of the hole. There may be washer plates with standard holes welded to the base plate, but in that case, the anchor is bent rather than sheared and its resistance is small. In result, the shear is typically transferred via friction only or via a shear lug.

Post-installed anchors may have standard holes in the base plate (and if they are intended to resist shear, they definitely should – EN 1992-4 – 6.2.2.1). But for these, the contribution of friction is typically neglected.

Summary

There is a development in the codes and the design guides converge to a solution where traditional long anchor bolts ended by a hook or a washer plate are allowed to benefit from friction between steel base plate and concrete foundation or grout under the conditions that:

• Steel failure mode governs
• There will be no earthquake loading
• Grout layer is thin

Do not use the contribution of friction, especially for cases:

• where concrete edge breakout governs
• with earthquake loading

For short anchors, typically post-installed and now defined by term fasteners between steel and concrete, the contribution of friction should be neglected.

In practice, the combination of friction and shear resistance of anchors is rarely used.

In IDEA StatiCa, there is no option to select a shear force transfer via a combination of friction and shear in anchors. If the user wishes to use this option in subsequent hand calcs, all the above-mentioned conditions should be fulfilled.

More about shear force transfer modes by friction, anchors, and shear lug in this article. IDEA StatiCa Connection allows the shear force transfer either completely by anchors or by friction.