Essentially, beams provide vertical support. In building structures they generally transfer loads from slabs to columns and walls. They are designed to resist resulting ultimate bending moments and shear forces and then checked against serviceability requirements. In-situ beams offer strength, robustness and, above all, versatility, for instance in accommodating cladding support details.
In overall terms, wide flat-beams are less costly to construct than narrow deep beams: the deeper and narrower, the more costly they are to construct. The following comments also apply.
If beams and columns are of the same width, the common planes can lead to efficient
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working as formwork can proceed along a continuous line. However, used internally, these relatively deep beams result in additional perimeter cladding. They also tend to disrupt progress and service runs.
Downstand edge beams may limit the use of flying form systems on the slab. Upstand
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perimeter beams (designed as rectangular beams) can reduce overall cost. Parapet wall beams are less disruptive and less costly to form than deep downstand beams.
Upstand beams and shallow downstand band beams can be easier to construct and have
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less impact on horizontal services distribution and floor-to-floor heights than relatively deep downstand beams (see Figures 3.B and 3.C).
Inverted L-beam Span
T-beam (internal) Overall depth
Overall depth
Span Band beam (wide T-beam) Upstand (or
spandrel) beam Overall
depth Slab depth
Overall depth
3.2
3.2.1
Figure 3.B In-situ concrete beams:
T- and inverted L-beams
Figure 3.C In-situ upstand beams and band beams
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In-situ beams
The charts and data
The intersections of beams and columns require special consideration of reinforcement details.
Sufficient width is required to provide room for both beam and column reinforcement; end supports need to be long enough to allow bends in bottom reinforcement to start within the support yet maintain cover for links and/or lacers.
The charts for in-situ reinforced beams cover a range of web widths and ultimate applied uniformly distributed loads (uaudl). They are divided into:
Rectangular beams – isolated or upstand beams, beams with no flange, beams not
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homogeneous with supported slabs.
Inverted L-beams – perimeter beams with top flange one side of the web.
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T-beams – internal beams with top flange both sides of the web
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Table 3.A lists web widths for which information is provided in the charts and data.
Table 3.A
Range of in-situ beams covered in charts and data
Span type Rectangular beams Inverted L-beams T-beams Single span 300
The user must determine which form of beam is appropriate and, therefore, select which figure and table to use. From the appropriate chart(s) and data, use the maximum span and appropriate ultimate applied uniformly distributed loads (uaudl), determined in accordance with Section 8.3, to interpolate between values given in the charts and data.
The charts and data for multiple-span beams are based on a minimum of three spans, so the user is expected to make adjustments for two-span configurations. In particular, the user is expected to round up both the derived depth and loads to supports in line with modular sizing and with his or her confidence in the criteria used. A nominal depth limit of 800 mm is used in the charts and data.
Users should note that the data for slabs give ultimate load to supporting beams. These loads assume the use of elastic reaction factors of 1.0 to internal beams and 0.5 to end supports.
For internal beams acting as penultimate supports, a suitable elastic reaction factor should be applied in accordance with Section 8.3.2.
Design assumptions
Dimensions
The default dimensions are given in Table 3.B. Flange widths are in accordance with Eurocode 2[2], Cl. 5.3.2.1.
3.2.2
3.2.3
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Table 3.B
Assumed dimensions for different types of beams
Beam type Rectangular L-beam T-beam
Flange width, single span bw bw + 0.10L bw + 0.20L Flange width, continuous spans bw bw + 0.07L bw + 0.14L
Top flange thickness 100 100 100
Design
The assumptions used to derive the charts and data are detailed in Section 7.1.4. Essentially the charts and data are based on:
Moments and shears from three-span sub-frame analysis to Eurocode 2, assuming continuity
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with nominal 250 mm sq. columns above and below.
Variable actions,
■ Qk ≤ permanent actions, Gk. Substantially uniformly distributed loads.
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Quasi-permanent value of variable actions = 0.6
■ Qk (i.e. c2 = 0.6, applicable to all but storage
areas where an allowance for c2 = 0.8 should be made).
The more onerous of Expressions (6.10a) or (6.10b).
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Minimum span ≥ 0.85 x maximum span.
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End spans are considered critical. Unless subjected to more than 15% redistribution of support moments, two-span slab elements will be subject to greater support moments and shears than those assumed. Nonetheless, the sizes given in the charts and data can be used cautiously for span conditions unless support moment or shear is considered critical. In such cases two-span beams should be justified by analysis and design.
In the charts, sizes of beams are based on a single layer of reinforcement where feasible. In any case, no more than two layers of reinforcement have been considered or used.
Load factors to BS EN 1990[9], Expressions (6.10a) or (6.10b) have been employed throughout.
If the more basic Expression (6.10) is used in design, greater beam depths may be required.
In order to satisfy deflection criteria, the steel service stress, ss, has in many cases been reduced by increasing As,prov (area of steel provided) but keeping As,prov/As,req within code limitations.
Fire and durability
Fire resistance 1 hour (R60); exposure class XC1; cover to all max[15; f] + D cdev where D cdev = 10 mm.
Loads
Beam self-weight (in addition to an assumed 200 mm depth of solid slab in T- and L-beams) has been allowed for and is included in ultimate loads to supports.
Ultimate loads to supports assume reaction factors of 1.0 internally and 0.5 to ends. The user should make allowance for elastic effects, particularly at penultimate supports (see Section 8.2.2).
Concrete
This is taken as C30/37; 25 kN/m3; 20 mm aggregate.
Reinforcement
Main bars: fyk = 500 MPa. Maximum H32s top and bottom, links: minimum H8. Minimum 50 mm between top bars. No additional top cover has been allowed for bars passing at right-angles.
Reinforcement quantities are for the beams only. For T- and L-beams, density of reinforcement relates to overall depth x web width. See also Section 2.2.4.
Variations
Variations from the above assumptions and assumptions for the individual types of beam are described in the relevant data. Other assumptions made are described and discussed in Section 7, Derivation of charts and data.
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