Laterally Loaded Screw connections

Introduction

In dealing with timber structures, the use of screws in connecting different members has become a very best practice. The applied loads on these screws can be decomposed to a tensile force along the axis of the screw or a shear force perpendicular to the screw axis. This blog post will be focusing on the resistance of screws for the component of the load that causes a shearing force on the screw. The different code approaches, EN 1995: 2004, screw suppliers ETA documents and AS 1720.1:2010 code approaches will also be discussed in this blog post.

Want to know more about the axial resistance of screws? Check our blog post here

Types of Screws

Reference: Designers guide to Eurocode 5 Design of timber buildings EN 1995-1-1 CL 8.7 and EN 14592 

EN 1995:2004 classifies screws based on their formation as self-tapping screws and smooth shank screws. Self-tapping screws are formed by hardening after rolling down the thread. In these types of screws, the outer diameter is referred to as the nominal diameter. There are also a second class of screws that are formed by threading down the original shank rod, and these produce a screw with the smooth shank diameter being the nominal diameter. EN 1995:2004 calculations for axially loaded screws are the same for both types of screws. 

Figure 1 Smooth shank screws and self-tapping screws according to EN 1995:2004

 

AS 1720.1:2010 has screws and coach screws in two different sections of the code. Screws have usually a lesser diameter and bigger length range than coach screws. 

Design Approaches of Laterally Loaded Screw Connections

EN 1995:2004 approach: The European yield model

In Eurocode, the characteristic load carrying capacity of laterally loaded metal dowel type fasteners is calculated based on Johanson’s formulas. These dowel type fasteners include screws, bolts, nails, dowels and staples. The failure modes involve embedment failure of the timber member or yielding failure of the fastener. The withdrawal strength also affects the lateral load carrying capacity of the fasteners. In Eurocode, the final lateral load carrying capacity is the sum of the Johanson’s part of the equation and the rope effect which is the withdrawal resistance. 

For fasteners in single shear, there are six failure modes. The first three involve the embedment failure of the timber due to the applied lateral load. Failure modes A and B involve the embedment failure of the head side member and the tip side member respectively. Failure mode C involves the embedment failure of both timber members. In failure modes D and E, yielding failure of the screw occurs in the head side and tip side timber member together with embedment failure. In the last failure mode, F, yielding failure of the screw and embedment failure in both members happen. In the European yield model calculations, all the above six failure modes will be computed and the failure mode giving a minimum characteristic capacity value will be the governing one.  

Reference: EN 1995:2004 Clause 8.2.2 (1)

Figure 2 EYM diagrams for timber-to-timber connections

With

Where, 

FV, RK is the characteristic load-carrying capacity per shear plane per fastener

ti is the timber or board thickness or penetration depth, with i either 1 or 2

fh,i,k is the characteristic embedment strength in timber member i

d is the fastener diameter

My,RK is the characteristic fastener yield moment

  is the ratio between the embedment strength of the members

Fax,RK is the characteristic axial withdrawal capacity of the fastener

On failure modes other than A and B, the fastener will engage in a small deformation. On these failure modes involving displacement of the fastener, a force component parallel to the fastener axis will develop. This force has a clamping effect on the two members, and this will increase the lateral load carrying capacity of the connection. This is considered in the European yield model equations as a rope effect. 

For screws the rope effect is given as one fourth of the axial withdrawal resistance of the screws. This rope effect however is limited to being at most equal to Johanson’s part of the equation. For different dowel type fasteners, different percentages of the Johanson’s part are considered as the rope effect contribution. 

 

Rope effect for screws= min

EN 1995:2004 Clause 8.2.2 (2)

 

In situations involving steel-to-timber connections subjected to lateral loads, five potential failure modes are identified. This includes two failure modes for thin steel plates and three for thicker plates. Echoing the timber-to-timber connection scenarios, the failure modes range from embedment failure in the timber, yielding failure of the fastener, or a combination of both.”

In the EN 1995:2004, the design approach for screws under lateral loads varies based on their diameter. Screws with a diameter less than 6mm are treated as nails, where the load angle does not influence their capacity, implying a load capacity independent of the angle. Conversely, screws with a diameter greater than 6mm are designed as bolts. In this case, the load angle impacts the capacity formula.

Figure 3 EYM diagrams for steel-to-timber connections

For a thin steel plate in single shear:

For a thick steel plate in a single shear:

In the above equations, the characteristic lateral load carrying capacity equations consider the fastener diameter, member thicknesses, characteristic embedment strength in all the failure modes. In failure modes that involve yielding of the fastener, characteristic yield moment is also involved. As mentioned above the withdrawal capacity of the fastener is also considered in failure modes that involve deformation of the fastener.
The above equations are general equations and can be used in any of the dowel type fasteners. 

The characteristic embedment strength depends on the fastener type. For screws with an effective diameter less than 6mm, the embedment strength calculations will be done by considering the screw as nails whereas for screws with diameter greater than 6mm, the calculations will be done by considering the screws as bolts. The effective diameter of screws will depend on the type of screw and the ratio of inner thread diameter to outer diameter of the screw. (Reference EN 1995:2004 clause 8.7.1)

 

As can be seen from the above formulas for screws with an effective diameter greater than 6mm, the angle of the load to the grain must be considered and an angle adjustment is included. 

For screws with an effective diameter less than 6mm, the embedment strengths are given as nails and the value depends on the timber type, timber density, diameter and pre-drilling.

For smooth shank screws, the effective diameter of the screw will be equal to the shank diameter of the screw provided that the smooth shank penetrates into the point side member by not less than 4d. For self-tapping screws and smooth shank screws not satisfying the above requirement, the effective diameter of the screw will be equal to 1.1 times the inner thread diameter of the screw.

Supplier ETA documents

Screw suppliers have a modification of the formulas on EN 1995:2004 to fit their specific screws. For laterally loaded screws, the supplier ETA documents provide a formula for the characteristic embedment strength values. The European Yield Model formulas are applicable here with a modification of the rope effect and the embedment strength. The rope effect, a function of the withdrawal capacity, may vary from supplier to supplier. Similar to the embedment strength, the withdrawal capacities are also provided in the ETAs. The ETAs also provide an effective diameter of their screws. On EN 1995:2004, effective diameter of self-tapping screws is given as 1.1 times the root diameter while in most of the ETAs the effective diameter is taken to be equal to the outer thread diameter of their screws.

 

AS 1720.1:2010 Approach

On the Australian standard, the characteristic lateral load carrying capacities of screws are read from tabulated data. The design capacities are then found from the characteristic capacities by multiplying the characteristic capacities by design factors. For screws embedded in the end grain direction, there is a grain orientation factor of 0.6. The number of shear planes is also considered by the shear plane factor, K14. To consider the type of material on the head side, there is a head fixity factor introduced, K16. The number of screws is also accounted using the factor K17.

Lateral loads at an angle to the grain

For loads at an angle to the grain direction, following the usual approach, the embedment strength of the timber member as well as the fastener yielding, and withdrawal capacity must be checked against the applied load. 

Additionally, due to the load angle, there will be a component of the applied load perpendicular to the grain direction and tends to split the timber member. This component of the applied load must be checked against the splitting capacity of the timber. This is well discussed on EN 1995:2004.

Figure 5 Splitting caused by the perpendicular to grain component of the applied load.

 

On AS 1720.1:2010, for coach screws, there are distinct characteristic capacity tables for loads parallel and perpendicular to the grain. The capacity for loads at an angle is determined by interpolating between these two tables.

Spacings, End and Edge Distances

In arranging dowel type fasteners, different codes recommend different requirements. EN 1995:2004 has a separate distance requirement for axially loaded and laterally loaded fasteners. For laterally loaded fasteners, EN 1995:2004 puts the distance requirements depending on the load angle, the diameter of fasteners and the timber density.

The end and edge distances depend on whether the end/edge being considered is loaded end or unloaded end. The spacing requirements are given for a general case. 

Figure 5 Distance requirements according to EN 1995:2004

Similar to the approach followed for the calculations, geometry requirement checks for laterally loaded screws are taken by considering the screws as nails if their effective diameter is less than 6mm and the requirement of bolts will be considered if the screws effective diameter is greater than 8mm.

The screw supplier ETA documents refer to the spacing and distance requirement provided on EN 1995:2004’s nail section (EN 1995:2004 clause 8.3.1.2) as a general requirement. For some specific screws and specific materials like CLT, however they provide their own requirements. 

In both cases the nominal diameter of screw is taken for calculations.

AS 1720.1:2010 has a similar end/edge distance and spacing requirements for axially and laterally loaded screws. The requirements also do not consider the loading angle. These requirements depend on the shank diameter of the screws.

For coach screws, AS 1720.1:2010 recommends using spacing requirements of bolts which somehow considers the angle between the applied load and the grain. There are two general considerations, when load is applied parallel to grain and when load is applied perpendicular to grain direction. For loads acting at an angle less than 300 to the grain, the requirements are taken similar as for loads parallel to grain. For loads acting at an angle between 300 and 900 to the grain, the distance requirements are taken to be similar with those loads acting perpendicular to grain.

Figure 7 Geometry requirements on AS1720:2010 for Coach screws

The Laterally Loaded Screw Module in CLT Toolbox

CLT Toolbox has delivered two separate modules for timber-to-timber connections and steel-to-timber connections in single shear. The user also has an option to choose different timber types. We have also incorporated different screw suppliers, and the user can either choose a screw from a supplier or insert screw geometric and material properties manually. For screws manually input, the user has an option to design based on EN1995:2004 or AS1720:2010. For screws selected from screw suppliers, users will have an option to design based on EN1995:2004, the supplier ETA document or AS1720:2010.
Our timber-to-timber module has an option for different embedment of screw options. Screw heads can be embedded inside the timber member with a custom embedment length, or the screws can be embedded such that their length in both connected members is equal (centered about horizontal tension perpendicular plane option).

Figure 7 Different approaches of laterally loaded screws

The design guidelines on EN 1995:2004, supplier ETA documents and AS1720:2010 are well followed in our modules with an educational feature of the input section. EN 1995:2004 uses European Yield Model (EYM) for the design of laterally loaded screws. The embedment strengths on these EYM formulas are obtained from the section referring to nails for screws with an effective diameter less than or equal to 6mm. For larger diameter screws, EN 1995:2004 recommends using the formulas developed for bolts. The Rope effect, which is a term on the EYM equations is given by one fourth of the withdrawal resistance of the screw. If user chooses the supplier ETA document for the design of screws, the embedment strengths and the rope effect calculations are taken from the supplier ETA documents and the EYM equations can be used here.

Regarding AS1720:2010, the characteristic capacities are read from tabulated values based on the screw shank diameter and the joint group of the timber used. 

Geometry checks are also done according to code recommendations in our modules.