WO2005019555A1

WO2005019555A1 - Placement of shear connectors

Info

Publication number: WO2005019555A1
Application number: PCT/AU2004/001142
Authority: WO
Inventors: Mark Patrick
Original assignee: University Of Western Sydney
Priority date: 2003-08-26
Filing date: 2004-08-26
Publication date: 2005-03-03
Also published as: GB2421965B; GB2421965A; NZ546195A; GB0606094D0

Abstract

A profiled decking sheet for use in the construction of a composite slab is disclosed. The sheet includes: a pan and side edge formations that allow adjacent steel decking sheets to be positioned in side by side overlapping relationship. The sheet is characterised in that the sheet is formed to identify one or more than one selected areas for connecting shear connectors to an underlying structural support.

Description

PLACEMENT OF SHEAR CONNECTORS

The present invention relates to composite slabs that form floors in buildings (which term includes car parks) .

A standard form of "composite slab" comprises: (a) a plurality of profiled steel decking sheets arranged in side by side, and often overlapping, relationship with each sheet comprising (i) side edge formations that facilitate overlapping adjacent sheets and (ii) a plurality of flat pans separated by ribs or other forms of strengthening members; and

(b) a layer of concrete on the profiled steel decking sheets.

The present invention is concerned with ensuring that shear connectors that are provided to connect composite slabs to underlying structural supports, such as steel beams, are fastened to underlying structural supports at optimal positions in relation to the sheets.

The term "shear connectors" includes welded studs, power-fastened brackets (e.g. Hilti HVB shear connector), or any other type of connector that can be fixed directly through the sheeting.

Optimal positions are where: (a) design codes and standards specify or intend that shear connectors be positioned; and (b) the performance of shear connectors is optimal . There are a number of examples of misplacement of shear connectors during construction of composite slabs, including the following examples:

(a) placing connectors too close to the ribs of decking sheets, which increases the adverse effects of rib punch-through failure and other undesirable failure modes such as longitudinal splitting and concrete pull- out;

(b) welding or fastening connectors through pan strengthening members which can significantly increase the chance of the fastening failing, e.g. the weld can become very porous and it is possible to break a shear connector off by bending it through a small angle;

(c) placing too many connectors on underlying structural supports at one location or cross-section; and

(d) placing connectors partially or wholly on decking sheets in situations in which the optimal positions of the connectors are directly on the underlying structural support and not on the decking sheets that are adjacent the optimal locations. Currently steel decking sheets are produced in a wide range of profiles. It is possible to weld shear connectors almost anywhere in the pans of most decking sheets, which makes stud misplacement a real possibility and something that commonly occurs in practice.

Also, as indicated above, there are optimal positions for shear connectors that can lead to significant improvements in the strength and ductility of the connectors. However, the very latest international design codes and standards do not allow designers to normally take advantage of certain optimal positions due to concern about how it can be ensured on site that the connectors will be placed correctly.

Placing shear connectors reliably can minimise the number of connectors required along a beam, which improves economy, but even more importantly gives specifiers and users of this form of construction confidence in its safety.

The present invention applies to any situation in which shear connectors could be connected to underlying structural supports .

Typically, the underlying supports include so- called "primary" beams (the ribs of the sheets are deemed to be parallel to the beams) , so-called "secondary" beams (the ribs of the sheets are deemed to be perpendicular to the beams) , and in fact any composite beams with the ribs of the sheets on either side at any angle to the steel beams.

In the case of composite slabs connected to primary beams, lateral rib punch-through failure occurs if the ribs are open (i.e. they create a void in the concrete) and the shear connectors are located too close to a side of the rib located over the beam. In the case of composite slabs connected to secondary beams, rib punch-through failure occurs if the ribs are open and the shear connectors are located too close to the side of each rib that is being thrust upon due to the composite action of the beam. If the shear connectors are off-centred such that the amount of concrete cover is reduced, then this is termed the "unfavourable" or "weak" side. The likelihood of rib punch-through failure is diminished by placing the shear connectors further away from the side of the ribs being thrust upon due to the composite action of the beam, viz. the "favourable" or "strong" side. For example, consider a uniformly loaded secondary beam. In that case, in practice, the shear connectors are positioned by locating a first shear connector mid-span of the secondary beam and then locating successive shear connectors from the mid- span connector towards the ends of the beam, with each shear connector being positioned on the side of the pans nearer that end of the beam.

Push-out and beam tests have shown that shear connectors located on the favourable side in secondary beam arrangements are stronger than if placed on the unfavourable side. (It should be noted that the terms "favourable" and "unfavourable", meaning off-centred in the favourable and unfavourable directions, respectively, is a loose description, because the size of the eccentricity is not defined, e.g. by 1 mm or 30 mm? It can be assumed in this discussion that the eccentricity is a significant amount when using these terms. More specifically, it will be assumed that in pans with only a single central longitudinal strengthening member or lap joint, that it is intended that the shear connector will be placed as close as possible to the lap joint or strengthening member. This is what is normally intended in practice when shear connectors are to be placed on the favourable or unfavourable side. Of course, it is normally possible for the eccentricity to be greater, which is naturally even more detrimental on the unfavourable side, while it may not be beneficial on the favourable side, e.g. the pull-out resistance of the shear connector may be impaired.

It has been the general opinion of those responsible for writing the rules in composite beam design codes and standards that it is not reliable to assume that shear connectors can be placed systematically on the favourable side.

Several issues are: (i) the difficulty of identifying from the top side of profiled decking sheets which are the strong and weak sides (i.e. one can not normally see the beams below, which are covered by the sheets and one needs to be able to understand the structural arrangement of the beams to be able to decide) ; (ii) this task being left to a stud applicator without any formal engineering training; and (iii) the potential lack of suitable on-site engineering supervision.

For example, in British Standard BS 5950:3.1 (1990) for composite beam construction it is stated "Where it is necessary for the studs to be located non-centrally in the (concrete) rib, the studs should preferably be placed in the favourable location such that the zone of concrete in compression in front of the stud is maximized. " However, no additional strength may be assumed. The much newer Eurocode 4:1.1 (1 June 2003 prEN 1994-1-1) for composite beam construction deals with the issue even more conservatively and states "Where the sheeting is such that studs cannot be placed centrally within a trough (i . e. pan) , they should be placed alternately on the two sides of the trough, throughout the length of the span . " Thus shear connectors must be treated, even if on average, as being centrally located, and no benefit can be gained placing all the shear connectors on the strong side. (This rule in EC4 is not of itself insurance that the studs will on average be effectively in the middle of the pans. Even if the rule is followed by the designer, there is nothing stopping shear connector applicators placing the studs alternately right next to the webs, which tests show can be less than having all the studs in the middle.) In the absence of engineering site supervision or suitably skilled labour (which is a common situation) , it is possible for unsafe buildings to be constructed that have been designed using the international standards BS 5950 or Eurocode 4. For example, all the shear connectors could end up being welded on the unfavourable side. One could argue that Eurocode 4 should have been even more conservative, insisting that the design strength of the shear connectors be based on all of the connectors being placed in the "most unfavourable position possible" . Although actually possible, the resulting penalty would have been unacceptable to industry.

In the very latest American design rules (AISC LRFD Specification for Structural Steel Buildings printed in 1999, Ballot TC5/03-09 June 2003 for amendment) it is stated that "Insuring that studs are placed in the strong position is not necessarily an easy task. Stud installation procedures are such that it is not always easy for the installer to determine where along the beam the particular rib is located, relative to the end, mid- span, or point of zero shear. Therefore, the installer may not be clear on which is the strong and which is the weak position . " They now normally recommend alternating shear connectors on the strong and weak sides .

An object of the present invention is to make it possible to minimise, if not eliminate altogether, construction errors due to misplacement of shear connectors, which can significantly reduce the shear strength and ductility of the connectors. In general terms, the present invention provides a profiled decking sheet for use in the construction of a composite slab that is formed to identify selected areas for connecting shear connectors to an underlying structural support.

According to the present invention there is provided a profiled decking sheet for use in the construction of a composite slab that includes: a pan and side edge formations that allow adjacent steel decking sheets to be positioned in side by side overlapping relationship, and is characterised in that the sheet is formed to identify one or more than one selected areas for connecting shear connectors to the sheet and to an underlying structural support.

According to the present invention there is also provided a profiled decking sheet for use in the construction of a composite slab that includes: at least two pans, a lengthwise extending strengthening member such as a rib that separates adjacent pans, and side edge formations that allow adjacent steel decking sheets to be positioned in side by side overlapping relationship, and is characterised in that the sheet is formed to identify one or more than one selected areas for connecting shear connectors to the sheet and to an underlying structural support .

The above-described decking sheets are suitable for use in situations in which the decking sheets are positioned over the underlying structural support. The above-described decking sheets are also suitable for use in situations in which the decking sheets are positioned to the sides of the optimal positions for the shear connectors on the underlying structural support and the shear connectors are connected directly to the support .

The decking sheets may have a coating on upper surfaces thereof that identifies the area or areas for connecting shear connectors to the underlying structural support .

The coating may be on the identified area or areas .

Alternatively, the coating may be on the area or areas other than the identified area or areas.

By way of further example, the decking sheets may comprise a plurality of indentations or projections or the like deformations of the pan that identify the area or areas for connecting shear connectors to the underlying structural support. The decking sheets may be formed to prevent shear connectors being located on the area or areas of the sheets other than in the identified areas.

By way of example, in a situation in which the shear connectors are welded through the decking sheets to the underlying structural support, the sheets may have a coating on the area or areas of the sheets other than the identified area or areas that prevents or inhibits shear connectors being welded to the sheets and the underlying structural support. Alternatively, the plurality of indentations or projections or the like deformations of the pan may prevent shear connectors being welded through the decking sheets and to the underlying structural support.

Embodiments of profiled steel decking sheets in accordance with the present invention include the following options.

(a) Situations in which the shear connectors are welded to underlying supports - directly or through decking sheets positioned on the underlying supports.

• Paint applied in localized areas to top or underside surfaces of decking sheets. (If on the top, the bond between the sheets and the concrete will be locally impaired, but being in the pans and localized, will normally be of no consequence when designing the composite slab.)

• Clear signage that either says or is understood to say "Do not weld here" .

• Any type of coating that prevents or significantly hinders the welding operation to cause the operator to stop the operation and place the connectors correctly, e.g. strip or patch of plastic adhesive. • Local deformations or indentations in pans of decking sheets designed to keep the operator away from these regions .

(b) Situations in which the shear connectors are power fastened to underlying supports - directly or through decking sheets positioned on the underlying supports. • Clear signage that either says or is understood to say "Do not connect here" .

• Obstacles that will prevent fasteners being power fastened, e.g. small cellular plate glued into position that will stop fastener from being fired into beam flange.

• Local deformations or indentations in the pan, designed to keep the operator away from these regions .

(c) Other situations. • Anything that prevents or significantly hinders the attachment process, or makes it obvious to the operator that connectors must not be placed in the designated no-go zones. There are a range of options for manufacturing embodiments of profiled steel decking sheets in accordance with the present invention by modifying standard profiled steel decking sheets, as summarised below.

1. In general, modifications to standard profiled steel decking sheets could be done in the factory before the decking sheets are delivered to site.

2. The modifications could be done as a general operation along the full length of the profiled decking sheets to allow for varying site conditions, e.g. local deformations or indentations in the pans forming no-go strips that otherwise could inadvertently be fastened through.

3. The modifications could be made very locally (e.g. small patch) if the position of the underlying structural supports, such as steel beams, is known when the sheets are being roll-formed.

4. The modifications could be done on site by a qualified agent of the decking manufacturer, depending on the modification involved, e.g. paint. This could be done on site independently of a shear connector applicator.

5. If a decking manufacturer proposes to use an asymmetric shear connector pattern, e.g. shear connectors on the strong side (for a uniformly-loaded secondary beam, this requires the modifications to be on one side of the pans for half of the beam, and on the other side of the pans for the remaining half of the beam) , then otherwise identical decking sheets would have to be marked or numbered so that they can be correctly placed on site. This may be require some site supervision to ensure that the desired outcome is achieved.

6. The present invention is particularly well suited to creating symmetrical no-go zones. For example, for composite slabs on secondary beams, placing no-go zones between the edges of the longitudinal strengthening members and lap joints of the sheets and where the shear connectors may be placed at least precludes the gross error of placing all the shear connectors near the webs and lap joints, in particular all on the weak side. By way of further example, for composite slabs on primary beams with adjacent sheets being positioned with adjacent lap joints in side by side relationship on the beam, then placing no-go zones on the pans between the ribs and lap joints prevents the placement of the shear connectors near the ribs. The present invention is described further by way of example with reference to the accompanying drawings, of which: Figure 1 illustrates incorrect and correct placement of shear connectors to a primary steel beam with a profiled decking sheet positioned on the beam; Figure 2 illustrates incorrect and correct placement of shear connectors to a secondary steel beam with a profiled decking sheet positioned on the beam; and

Figure 3 illustrates incorrect and correct placement of shear connectors to a steel beam with profiled decking sheets positioned on opposite sides of optimal positions of the shear connectors on the beam.

The arrangements shown in the Figures illustrate 3 examples of incorrect and correct positions for welding or otherwise fastening shear connectors 3 to supports in the form of beams 5 in the construction of composite slabs. Figures 1 and 2 illustrate composite slabs that include profiled decking sheets 7 that extend across the beams 5. In these arrangements, the shear connectors 3 are fastened to the beams 5 through the sheets 7. Figure 3 illustrates a composite slab that includes split decking sheets 7, ie sheets that do not completely cover the beam 5. In this arrangement the shear connectors 3 are fastened directly to the beam 5. The incorrect positions are shown on the left- hand side drawings of Figures 1 and 3 and on the crossed- out sections of Figure 2.

The Figures also indicate the use of coatings 9 to mark areas on the decking sheets 7 on which the shear connectors 3 should not be positioned. The coatings provide a clear indication of the correct positions for the shear connectors 3.

Many modifications may be made to the embodiments of the present invention described above without departing from the spirit and scope of the invention.

Claims

CLAIMS :

1. A profiled decking sheet for use in the construction of a composite slab that includes: a pan and side edge formations that allow adjacent steel decking sheets to be positioned in side by side overlapping relationship, and is characterised in that the sheet is formed to identify one or more than one selected areas for connecting shear connectors to an underlying structural support.

2. A profiled decking sheet for use in the construction of a composite slab that includes: at least two pans, a lengthwise extending strengthening member such as a rib that separates adjacent pans, and side edge formations that allow adjacent steel decking sheets to be positioned in side by side overlapping relationship, and is characterised in that the sheet is formed to identify one or more than one selected areas for connecting shear connectors to an underlying structural support.

3. The decking sheet defined in claim 1 or claim 2 includes a coating that identifies the area or areas for connecting shear connectors to the underlying structural support.

4. The decking sheet defined in claim 3 wherein the coating is on the identified area or areas.

5. The decking sheet defined in claim 3 wherein the coating is on the area or areas other than the identified area or areas .

6. The decking sheet defined in any one of the preceding claims includes a plurality of indentations or projections or the like deformations of the pan or pans that identify the area or areas for connecting shear connectors to the underlying structural support.

7. The decking sheet defined in any one of the preceding claims wherein the sheet is formed to prevent shear connectors being located on the area or areas of the sheet other than in the identified areas.

8. The decking sheet defined in claim 7 wherein, in a situation in which the shear connectors are to be welded through the sheet and to the underlying structural support, the sheet includes a coating on the area or areas of the sheet other than the identified area or areas that prevents shear connectors being welded to the sheet and the underlying structural support.

9. The decking sheet defined in claim 7 wherein, in a situation in which the shear connectors are welded through the sheet and to the underlying structural support, the sheet includes a plurality of indentations or projections or the like deformations that prevent shear connectors being welded through the sheet and to the underlying structural support.

10. A profiled decking sheet for use in the construction of a composite slab substantially as hereinbefore described with reference to the accompanying drawings .