Design and Detailing of Steel Column Splices

Generally, in braced steel frames, column design actions are meant to reduce as we go up the column stack. This is because the typical load path in a structure is from top to bottom. Therefore, whilst it is safe and conservative to size a steel column using the maximum design values of the loads at the base level and adopts the same throughout the entire structure, it would however, be practically uneconomical to do so for a multi-storey arrangement. Hence the usual practice is to reduce column sections as we go up the structure particularly, where there is a significant reduction in the design loads. The connection between this column stacks of different section is what is referred to as a Column Splice.

In other words, column splices are essentially steel-plated bolted connection provided in multi-storey steel construction to serve as a connection between two columns of different sections. However, a column Splice might be introduced even for column stack of same sections, sometimes just to increase the ease of construction. For example, by breaking a column into convenient lengths, the fabrication process, transportation and erection is made easy.

Provision of column Splice on each floor is seldom practised because the savings in column materials is far-far outweighed by the cost of making a Splice. Thus, they are typically provided with every two or three storeys and are located just above the finished floor level.

Types of Column Splice

There are generally two types of steel column Splice in braced steel frames, vis a vis.

Bearing Splice

In a bearing type Splice connection, the actions are transferred by direct bearing, i.e. the upper column loads are transferred on the lower column directly, generally via the use of a plate known as a devision plate or sometimes the ends of the members are prepared to bear on each other (See figure 1).

Image showing a bearing splice  connection
Figure 1: Bearing Splice Connections

Non Bearing Splice

In the case of a non bearing splice connection, loads are transferred via the bolts and splice plates. There is no bearing between the interconnected columns. They are usually detailed with a gap in between the column stacks (See figure 2).

Image showing a non bearing splice  connection
Figure 2: Non Bearing Splice Connections

The bearing type of splices are usually more simpler, having lesser bolts than non bearing splices, hence they are typically economcal and most commonly encounterred in practice.

Detailing Requirement

In braced steel structures all joint must be detailed to remain a simple connection. In order words no joint should transmit significant moments. The detailing practice of column splices include the rules and practical considerations on fasteners, holes, fittings and geometry.

Fasteners

Generally the design of the fastener is in accordance with BS EN 1993-1-8. The fastener and spacing edge distance must comply with the rules giving below.

For majority of cases, M20 – M24 class 8.8 bolt should be sufficient for cover plates. Non preloaded bolts are normally used however, where preloaded bolts are used it is recommende that they’re designed as “non slip” resistant in accordance with the code requirement.

Fittings

Fittings are usually fabricated from S275 steel plates. If the Splice is a bearing Splice requiring a division plate it is necessary for it to be lightly welded to the column.

Holes

Holes are to be done in accordance with the laid down rules in BS EN 1090-2. They may be punched or drilled.

Geometry

As with all simple connections, there is a recommended geometry in order to ensure that the connection does not transmit a bending moment and remains a simple connection. The main objectives of detailing column splices are:

  • Ensure that the connection is capable of transmitting the design forces
  • Ensure all members are held accurately in position relative to each other
  • Sufficiently rigid to ensure that the upper column stack remains in position during erection
  • Provide nominal stiffness about both axes.

Design Requirement

The design of a bearing Splice involves checking the design forces and moments that induce net tension in every part of the steel connection. Clause 6.2.6 requires that a Splice connection must transmit 25% of the maximum compressive action in the column.

The design of a non-bearing Splice on the other hand is more involved and complicated. It requires all forces and moment to be transmitted through the bolts and Splice plates alone. Hence this would not be considered further in this post, a later post will attempt to explain the intricacies of non-bearing Splice connection.

The detailed design of a bearing Splice connection can be carried out in six steps. These are explained below.

Recommended Detailing Practice

The recommended detailing of column splices in bearing are rules based on past experience on column splices design. These guidelines are to be followed judicously, in order to ensure the connection remain simple and do not transmit moment. This can be summarized in the following figures.

Recommended plate dimensions for splice connections
Figure 3: Recommended detailing practices for external cover plates

Note that the bolt spacings and edge distances are to comply with the recommendation of Table 3.3 of BS EN 1993-1-8.

Recommended plate dimensions for splice connections
Figure 4: Recommended detailing practice for division plate

Verify for Presence of Net Tension

The next procedure is to check the detailed splice connection for the presence of a net tension. Where a net tension does not occur, the splice is only detailed to transmit only axial compression through direct bearing. However, if a net tension does occur, then further checks on the endplates must be carried out.

For net tension to occur, the following expression must be satisfied

{ M }_{ Ed }>\frac { { N }_{ Ed,G }h }{ 2 } 
  • MEd Is the column design moment at the floor level, below the splice
  • NEd,G is the axial compressive force due to the factored permanent actions only
  • h is conservatively the overall depth of the smaller column.

Verify Plates Resistance

Having checked for net tension in the plates, if a net tension does exist in the connection, the next step is to verify the plate resistance to the induced tensile stress. The following equation must be satisfied:

{ N }_{ Ed }\le { N }_{ t,Rd }=min\left( { N }_{ pl,Rd }\quad ;\quad { N }_{ u,Rd }\quad ;\quad { N }_{ bt,Rd } \right) 
{ N }_{ pl,Rd }=\frac { { A }_{ fp }{ f }_{ y,tp } }{ { \gamma  }_{ M0 } } ;\quad { N }_{ u,Rd }=\frac { { 0.9A }_{ fp,net }{ f }_{ u,tp } }{ { \gamma  }_{ M2 } } ;{ \quad N }_{ bt,Rd }=\frac { { f }_{ u,tp }{ A }_{ fp,nt } }{ { \gamma  }_{ M2 } } +\frac { { f }_{ y,fp }{ A }_{ fp,nv } }{ \sqrt { 3 } { \gamma  }_{ M0 } } 

Where:

  • Npl,Rd is the tension resistance of the gross sectional area
  • Afp is the gross area of the flange plates attached to one flange
  • Nu,Rd is the tension resistance of the net area
  • Afp,net is the net area of the flange cover plates attached to one flange
  • Nbt,Rd is the block tearing resistance for a concentrically loaded bolt group
  • Afp,nv is the net section area subjected to shear
  • Afp,nt is the net section area subjected to tension
  • fy,tp is the plate yield tensile strength
  • fu,tp is the plate ultimate tensile design strength
{ A }_{ fp,nv }={ 2t }_{ p }+\left( { e }_{ 1 }+\left( { n }_{ 1 }-1 \right)  \right) { p }_{ 1 }-\left( { n }_{ 1 }-0.5 \right) { d }_{ o }
{ A }_{ fp,nt }\quad =\quad { t }_{ p }\left( { p }_{ 2 }-{ d }_{ o } \right) \quad if\quad { p }_{ 2 }\le 2{ e }_{ 2 }\\ \quad \quad \quad \quad \quad =\quad { t }_{ p }\left( { 2e }_{ 2 }-{ d }_{ o } \right) \quad if\quad { p }_{ 2 }>{ 2e }_{ 2}
  • tf,uc is the flange thickness of the upper column
  • bf,uc is the flange width of the upper column
  • e1 & e2 are the edge and end distances of the plates
  • do is the diameter of the holes
  • n1 is a factor
  • p1 & p2 are the bolt spacing of the plates

Where there is a significant net tension in the plates, preloaded bolts are to be used. The basic requirement for a significant net tension to exists in a Splice connection is defined here as :

{ N }_{ Ed }>0.1{ f }_{ y,uc }{ t }_{ f,uc }{ b }_{ f,uc }

Verify Bolt Group Resistance

Once the check on the plate resistance to net tension is verified, the next step is to check the resistance of the bolt group. Where there is no net tension occurring in the plate this step can be skipped. The verification procedure depend on whether non-preloaded bolts or preloaded bolts are used.

Basic Requirement – Non Preloaded Bolts
{ N }_{ Ed }\le { F }_{ Rd,fp }
{ F }_{ Rd,fp }=n{ F }_{ b,Rd }\quad if\quad { F }_{ b,Rd }\le { F }_{ v,Rd }\\ \quad \quad \quad \quad =n{ F }_{ v,Rd }\quad if\quad { F }_{ v,Rd }>{ F }_{ b,Rd }
{ F }_{ b,Rd }\quad =\frac { { k }_{ 1 }{ \alpha  }_{ b }{ f }_{ u }d{ t }_{ p } }{ { \gamma  }_{ M2 } } \quad ;\quad { F }_{ v,Rd }=\frac { { \beta  }_{ p }{ \alpha  }_{ v }{ f }_{ ub }A }{ { \gamma  }_{ M2 } }
  • n is the number of bolts connecting one plate to a flange
  • α= 0.6 for property class 8.8 bolts
  • A is the tensile stress area of the bolt
  • d is the diameter of the bolt
  • do is the diameter of bolts holes including allowance
  • tpa is the thickness of the bolt packing
{ \beta  }_{ p\quad  }=\quad \frac { 9d }{ 8d+3{ t }_{ pa } } \quad if{ \quad t }_{ pa }\ge \frac { d }{ 3 } \\ \quad \quad =1.0\quad if\quad { t }_{ pa }<\frac { d }{ 3 } 
{ \alpha  }_{ b }\quad =min\left\{ \frac { { e }_{ 1 } }{ 3{ d }_{ o } } \quad ;\quad \frac { { p }_{ 1 } }{ { 3d }_{ o } } -\frac { 1 }{ 4 } \quad ;\quad \frac { { f }_{ ub } }{ { f }_{ u,fp } } ;\quad 1 \right\}
 { k }_{ 1 }\quad =min\left\{ 2.8\frac { { e }_{ 2 } }{ { d }_{ o } } -1.7\quad ;\quad 1.4\frac { { p }_{ 2 } }{ { d }_{ o } } -1.7;\quad 2.5\quad  \right\}

Refer to Clause 3.9.1 of BS EN 1993-1-8 for guidance on the procedure for verifying preloaded bolts. This refers to connections designed as non-slip under serviceability loads.

Verify Minimum Resistance of Cover Plate & Bolt Group

This check is required is required irrespective of whether a net tension occurs, it is the minimum resistance the cover plates and bolt groups are required to posess to ensure satisfactory performance.

For Cover Plate:

0.25{ N }_{ Ed }\le { N }_{ Rd }\quad =\frac { { A }_{ fp }{ f }_{ y,fp } }{ { \gamma  }_{ M0 } } 

For Bolt Group:

0.25{ N }_{ Ed }\le { F }_{ Rd,fp }
  • Afp is the area of flange cover plates
  • FRd,fp is the resistance of the bolt group as defined in the bolt group verification above

Verify Tying Resistance

Where it is required to comply with structural integrity requirement as its mostly the case, then the verification for plate and bolt resistances should be carried out using the value of NEd defined below

{ N }_{ Ed }=\frac { { N }_{ Ed,u } }{ 2 } 

NEd,u is the tensile force from BS EN 1991-1-7. However, it is usually conservative to adopt the values given from the plate and bolt verification.

Worked Example

A column splice is required on the fourth floor of a multi-storey steel building to connect a 203x203x113UKC to a 305x305x198 UKC. The design actions and permanent factored actions just above the floor level are 1685kN and 1275kN respectively while the design bending moment is 55kN.m. Using the recommended detailing practice, design a splice connection completely.

Recommended Detailing Practice

1) Cover Plates:

height\quad ={ h }_{ fp }\ge 2{ b }_{ uc }+{ t }_{ db }
width\quad ={ b }_{ fp }\ge { b }_{ uc }\quad =212.1\quad say\quad 250mm
Thickness,\quad { t }_{ fp }\ge max\left\{ \frac { { t }_{ f,uc } }{ 2 } ;\quad 10mm,\quad \frac { { p }_{ 1 } }{ 14 }  \right\} =\frac { 16.3 }{ 2 } ;10;\frac { 160 }{ 14 } =11.3\quad say\quad 12mm
 Packs,\quad { t }_{ pa }=\frac { { h }_{ lc }-{ h }_{ uc } }{ 2 } =\frac { 339.9-235 }{ 2 } =52.5mm\quad say\quad 30mm

2) Division Plate

Thickness\quad { t }_{ wp }=\frac { \left[ \left( { h }_{ lc }-{ 2 }t_{ lc } \right) +\left( { h }_{ uc }-{ 2t }_{ uc } \right)  \right]  }{ 2 } =\\ \left[ \frac { \left( 339.9-2\times 36.4 \right) -\left( 235-2\times 26.9 \right)  }{ 2 }  \right] =25.1mm\quad say\quad 25mm\\ \\ 

3) Web Cleats

Use\quad 90\times 90\times 8\quad angle\quad cleats\quad to\quad accomodate\quad M20\quad bolts
lenght\ge { 0.5h }_{ uc }=0.5\times 235=117.5mm\quad say\quad 150mm
packs\quad =\frac { { t }_{ w,lc }-{ t }_{ w,uc } }{ 2 } =\frac { 19.1-16.3 }{ 2 } =1.4mm\quad say\quad 2mm

4) Summary of Splice Details

  • Flange cover Plates: 2/250 x 12 x 525
  • Flange packs: 2/250 x30 x 240
  • Cleats: 4/90 x 90 x 8 Angles, 150mm long
  • Web packs: 2/85 x 2 x 150
  • Division plate: 265 x 25 x 310
  • Bolts: M20 8.8
  • Fitiing material: S275 steel
 connection details
Figure 5: Splice Connection Details

Check for Net Tension

{ M }_{ Ed }=55kN.m\quad ;\quad { N }_{ Ed.G }=2275kN
{ M }_{ Ed }=(55kN.m)\le \frac { { N }_{ Ed,G }h }{ 2 } =\frac { 1275\times 0.206 }{ 2 } =131.3kN.m

Since there is no net tension occurring in the splice connection, thus we can ignore verifying the plate and bolt group resistance and just proceed to Check 5: Minimum cover plate & bolt group resistance.

Verify Minimum Cover Plate & Bolt Group Resistance

1) Cover Plates

0.25{ N }_{ Ed }\quad \le { \quad N }_{ Rd }\quad \\ 
0.25{ N }_{ Ed }=421.25kN
{ N }_{ Rd }=\frac { { A }_{ fp }{ f }_{ y,fp } }{ { \gamma  }_{ M0 } } =\frac { 2\times 250\times 12\times 275 }{ 1.0 } =948.8kN
671.25kN<948.8kN\quad o.k

2) Bolt Group

verify\quad that\quad { 0.25N }_{ Ed }\le { F }_{ pl,tp }
{ F }_{ Rd,fp }=n{ F }_{ b,Rd }\quad if\quad { F }_{ b,Rd }\le { F }_{ v,Rd }\\ \quad \quad \quad \quad =n{ F }_{ v,Rd }\quad if\quad { F }_{ v,Rd }>{ F }_{ b,Rd }

A) Shear resistance of a single bolt

{ F }_{ v,Rd }=\frac { { \beta  }_{ p }{ \alpha  }_{ v }{ f }_{ ub }A }{ { \gamma  }_{ M2 } }
 { \beta  }_{ p }=\frac { 9d }{ 8d+{ 3t }_{ pa } } =\frac { 9\times 20 }{ 8\times 20+3(30) } =0.72
{ F }_{ v,Rd }=\frac { 0.72\times 0.6\times 800\times 245 }{ 1.25 } =68kN

B) Bearing resistance of a single bolt

{ F }_{ b,Rd }=\frac { { k }_{ 1 }{ \alpha  }_{ b }{ f }_{ u }d{ t }_{ fp } }{ { \gamma  }_{ M2 } }
{ \alpha  }_{ b }\quad =min\left\{ \frac { { e }_{ 1 } }{ 3{ d }_{ o } } \quad ;\quad \frac { { p }_{ 1 } }{ { 3d }_{ o } } -\frac { 1 }{ 4 } \quad ;\quad \frac { { f }_{ ub } }{ { f }_{ u,fp } } ;\quad 1 \right\} \\ \quad \quad \quad \quad min\quad \left\{ \frac { 40 }{ 3\times 22 } ;\frac { 160 }{ 3\times 22 } -\frac { 1 }{ 4 } ;1\quad  \right\} \quad =0.61
{ k }_{ 1 }\quad =min\left\{ 2.8\frac { { e }_{ 2 } }{ { d }_{ o } } -1.7\quad ;\quad 1.4\frac { { p }_{ 2 } }{ { d }_{ o } } -1.7;\quad 2.5\quad  \right\} \\ \qquad =min\left\{ 2.8\frac { 50 }{ 22 } -1.7\quad ;\quad 1.4\frac { 160 }{ 22 } -1.7\quad ;\quad 2.5 \right\} =\quad 2.5
{ F }_{ b,Rd }=\frac { 2.5\times 0.61\times 410\times 20\times 12 }{ 1.25 } =120kN

Since the shear resistance of a single bolt is greater than the bearing resistance value, the minimum resistance of the bolt group would be governed by the shear resistance value and hence used for the calculation.

{ 2F }_{ Rd,tp }\quad =2\times n{ F }_{ v,Rd }=\quad 2\times 4\times 68\quad =544kN
421.25kN<544kN\quad o.k

Having carried out all the required checks and the connection found to be satisfactory, the detailed connection provided in figure x above can therefore be adopted.

See: Simple Connections in Multi-Storey Steel Frames

References

Steel Construction Institute (2014) Joints in Steel Construction: Simple Joints to Eurocode 3[Online]
Available at: www.tatasteelconstruction.com/ file_source/StaticFiles/Construction/p358.pdf, Ascot, Berkshire: SCI

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