Beam reinforcement details shall be prepared in accordance with relevant standards for detailing methods. In addition to the rules of conduct, several guidelines were developed that we can consult when detailing beam reinforcement.
In this article, we will focus on the standard method of beam reinforcement details.
We first look at the detailing rules set out in BS 8110 Part 1.
When creating reinforcement plans, the following list of requirements should be checked.
- Minimum tensile reinforcement ratio – 100As/Ac = 0.13 for a rectangular section if f j = 460N/mm 2
- Minimum pressure reinforcement – 100As/Ac = 0.4 (if such reinforcement is required for the ultimate limit state; fy = 460 N/mm 2 )
- The maximum area of the beam reinforcement – tension or compression must not exceed R/F 4% Ac
- Fasteners to contain beam compression reinforcement – at least one-quarter the size of the largest compression bar or 6 mm, whichever is greater (if compression reinforcement is required for load capacity level sizing)
- Maximum distance between links – 12 times the size of the smallest pressure bar
- Arrangement of flanges to accommodate beam compression reinforcement – No bar within the pressure zone should be more than 150 mm from the retained bar. (if compression reinforcement is required to design the load capacity level)
- Beam side reinforcement – Provide if depth exceeds 750mm and provide 2/3 of the height measured from the tension surface. The maximum distance must not exceed 250 mm. Minimum uncovered diameter less than √(see b b/f j ) Where is b is the spacing between bars and b is the section width or 500 mm if b is greater than 500 mm.
- Clearances between tension rods – No longitudinal tension rod is more than 150 mm from the vertical leg of the connector.
- The maximum number of reinforcement in a layer, including tension overlaps – must not exceed the sum of the sizes of reinforcement in a given layer 40% of the section width at that level.
The above detailing rules must be strictly followed in preparing the beam reinforcement details.
Additionally, there are other requirements in the Standard Method for Structural Concrete Detailing by IStructE.
Typical link arrangement
Arrangement of links in cross section
Typical reinforcement shortening in continuous beam reinforcement details
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- Provides 100% of the required upper steel area (Hogging R/F) up to a length of 0.15 l (≥∅ 45), where l is the effective span
- Add another 60% up to 0.25l from the front of the support
- Furthermore, 20% of the required prestressing steel area must be provided as suspension rods.
- Provides maximum 30% sagging reinforcement support.
In general, it is a little difficult to shorten the reinforcement according to the detailing rules mentioned above. The simplest method is to install a 0.25x gap and then install hanging rods (20%).
The typical detailing rule cannot be applied to all situations. The most appropriate method is to detail the reinforcement according to the variation in the bending moment.
The guidance given in the article Shortening of beam reinforcements For the upper reinforcement shortening method, the following approach could be used.
Anchors in Beam Reinforcement Details
There are two types of anchors, namely tension anchor length and compression anchor length. Tensile anchorage length is most critical in strengthening beams for columns. The following order must be respected.
The length of the tension anchor is calculated from the tension rod. The support rods serve here to reinforce the supports.
If the upper beam reinforcement must continue below the bottom of the beam to maintain the tension anchorage length, the column construction joint must be lowered from the bottom of the beam or U-bars must be provided while the column construction joint Column construction remains level with the bottom of the beam.
The Nominal Anchor Connection Stress article could be used to calculate the anchor length.
Reinforcement of the beam suspension in the connection between beams
When one beam is supported by another, the vertical force is exerted by the suspended beam on the supporting beam.
It is necessary to provide reinforcement that can absorb this axial force. There are different guidelines for calculating the area of reinforcement.
The article Hagger Rebars The method for calculating the reinforcement area can be read through a worked example.
