Description Cellular beams are beams with openings at short regular intervals along their length. The beams are either fabricated from 3 plates or made from rolled sections. Openings, or ‘cells’, are normally circular, which are ideally suited to circular ducts, but can be elongated, rectangular or hexagonal. Cells may have to be filled in to create a solid web at positions of high shear, such as at supports or either side of point loads along the beam. The size and spacing of the openings can be restricted by the fabrication method, as well as by the required strength of the beams.
The main two companies specialising in design and supplying these beams are:
Westok Ltd, supply cut and re-welded rolled sections, which can be of different weights or serial size, and can incorporate a camber when re-welded.
These cellular beams generally have circular openings at regular centres.
Fabsec Limited, supply beams fabricated from three plates with openings cut in the web. These beams can have a wide range of opening types and spacings, and can be supplied with a camber.
Cellular beams can be arranged as long-span secondary beams, supporting the floor slab directly, or as long-span primary beams which are aligned parallel to the span of the slab supporting other cellular beams or conventional rolled section secondary beams.
Typical beam
span range 10 – 18 m
Figure 5.6 Long-span secondary cellular beams with regular circular openings
6.0 m7.5 m
6.0 m
130
2 No. 19ỉ x 100lg studs @ 300ctrs
3.0 m 3.0 m
660
450
Decking
1.0 thk A193 mesh
20 No. holes @ 650ctrs.575
650
PART PLAN Side bracing
Wall bracing
203 UC 46 (3
&4) 203 UC46 (3&4)
203 UC 46 (1&
2)
S275 203x133 UB25
S275 203x133 UB25
S275
Stairs
Side bracing
A A
Slab
203 UC 46 (3
&4) 254 UC7
3 (1&
2)
457 x 152 UB52 S275
203 UC60 (1&2) 203 UC46 (3&4) 356 x 171 UB51
S275
Slab
254 x 146 UB31 S275 254 x 146 UB31 S275
457 x 152 UB52 S275 254 UC73 (1&2)
B
B
254 UC73 (1&2) 203 UC52 (3&4) 203 UC46 (1&2)
203 UC46 (3&4)
254 UC 73 (1&
2)
203 UC52 (3&4) S355
S355 S355
203 UC 60 (1
&2)
203 UC46 (3&4) S355
S355
S355
S355
SECTION A-A
457 x 191 UB67 S275 406 x 178 UB60 S275
254 x 146 UB31 S275
356 x 127 UB33 S275 356 x 171 UB45 S275
457 x 191 UB74 S275 + 457 x 191 UB82 S275 Cellular beam
406 x 178 UB60 S275
= 150 mm raised floor + 130 mm slab + 660 mm beam
+ 150 mm ceiling & lighting Lift
Overall floor zone
=1090 mm 1100 mm
Figure 5.7 Cellular beams (long-span secondary beams) and composite slabs – example of floor steelwork arrangement for a 4 storey rectangular plan building
Main design considerations for the floor layout
Secondary beams should be spaced close enough to avoid propping the decking during construction. (3 – 4 m).
Large (elongated or rectangular) openings should be located in areas of low shear, e.g. in the middle third of span for uniformly loaded beams.
Consider integration of the services within the beam depth to minimise the overall floor zone.
Advantages Long, column-free floor spans.
Relatively lightweight beams compared with other long-span systems.
Economic long-span solution.
Precamber can be accommodated during the fabrication of the members.
Disadvantages Increased fabrication costs compared with plain sections.
Services
integration Regular openings in the web allow ducts to pass through the beams. Larger items of equipment are located between the beams. Openings need to allow for any insulation around the services. Ensure web openings align through beams.
Governing design criteria for beams
Critical check may be elements within the beam – for example, the web posts between openings, particularly near high point loads or adjacent to elongated openings.
The dynamic response of the floor may be critical in some circumstances.
Opening size is ideally within 80% of the finished beam depth, and with a maximum opening length/depth ratio of 2.5. Stiffeners may be required for large openings.
Governing design criteria for decking/slab
Strength or deflection of the decking in the construction condition.
Fire resistance (affects the concrete cover to the decking and mesh reinforcement size).
Strength or deflection of the slab in the composite condition.
Design approach 1. Assume long-span secondary beams at 3 – 4 m spacing, supported by primary beams on a 6 m, 7.5 m or 9 m column grid
2. Choose the decking and slab, using decking manufacturer’s design tables or software. Use lightweight concrete unless there is a directly-bonded floor covering. Assume LC35/38 concrete, and unpropped decking during construction. Ensure the chosen slab and reinforcement meet the fire resistance required.
3. Design the beams using manufacturer’s software. If stud spacing is not automatic within the software, try shear studs at approximately 300 mm spacing on secondary beams (to suit trough spacing), and at 150 mm spacing on primary beams (The development of moment resistance will often be more severe on a primary beam, demanding closer spacing of studs.) Note that the orientation of the decking will differ between secondary and primary beams. As the services are likely to be integrated, ensure cell sizes and positions are agreed with the services engineer.
Typical section
sizes 700 mm deep steelwork for 15 m span at 3.75 m centres.
(Beam + slab depth) span/16-19.
Grade of steel S275 and S355.
Lighting Ceiling
Raised floor 150
140
100 50 Ducting + insulation
400 dia.
760
1200
Figure 5.8 Cellular beam - Typical cross section Type of
concrete Either normal concrete, 2400 kg/m3, or lightweight aggregate concrete (LWAC), typically density class D1,8 to BS EN 206-1 (1600-1800 kg/m3) can be used.
Normal concrete has better sound reduction, so is better for residential buildings, hospitals, etc.
LWAC is better for overall building weight/foundation design, better span capability of slab, and has better fire insulating properties, enabling slightly thinner slabs (10 mm less) to be used. It is not available in all parts of the UK. LWAC is not considered suitable for directly-bonded floor coverings.
Overall floor zone 1200 mm for 15 m span with 400 mm opening.
Fire protection Intumescent paint, often off-site. Consult fire protection manufacturer; References 23 and 24 give background information on fire protection of beams with web openings.
Connections End plate, in shear only.
Design guidance For choice of decking and composite slab design (including fire resistance);
manufacturer’s design tables.
For best practice advice in design and construction, refer to P300[17]
For design charts and worked example for decking and beams, refer to P055[18]
For the basic design of orthodox cellular beams, refer to P100[25]
Software Cellbeam software from www.westok.steel-sci.org Fabsec software from www.fabsec.co.uk
Figure 5.9 Fabricated beam with off-site fire protection