Skin Reinforcement

DESIGN GUIDES

Introduction

ACI 318 modified the criteria for determining skin reinforcement in large beams. Currently, tables are still used with a distribution that does not correspond to what is established in current standards.

In this work, it is intended to obtain, in accordance with ACI 318 – 14, new tables to define the number of bars required on each face of beams with heights greater than 36 inches. In addition, it is intended to give some criteria to select the diameter of the skin bars, that under this new criterion the calculator is the one who must define it.

Skin Reinforcement. Need for its use

The origin of reinforced concrete beams is due to the insufficient capacity of concrete to resist traction. It is known that the tensile strength is of the order of 10 to 15% of the compressive strength. The steel reinforcement placed in the tensile zone of the concrete has the function of making up for this deficiency; therefore, there will always be cracks in the concrete in that zone.

Cracks cannot be avoided; the important thing is to ensure that there is not a single large crack but numerous small cracks and each one of them with a smaller opening. 

The code provides regulations on minimum and maximum spacing, minimum amount, coatings, etc. that ensure that the cracks at the level of the main reinforcement are within the established regulations. However, when the beam elements have a high depth, alterations to the cracks are originated that give rise to the so-called skin reinforcement, which will be the main objective of this work.

The longitudinal reinforcement to deform simultaneously with the concrete limits the width of the crack at a level where the reinforcement is located. Now, the crack extends from the tension edge to proximity to the neutral axis and within this zone the crack shows a larger opening than the one that occurs at the level of the bending steel, especially in beams with heights greater than 36 inches.

In Fig. 1 one of the possible cracks is shown with the configuration that it acquires in its height and in Fig. 2 the same crack when the skin reinforcement is placed on both faces of the beam.

The so-called skin reinforcement, they are nothing more than bars placed horizontally at equal spacing, which prevents cracks from opening further than the flexural reinforcement allows. The crack distribution would be as shown in Fig. 2.

The standard establishes that the placement of the skin reinforcement is required in beams with a depth greater than 36 inches and indicates that it is only required in tension zones, that is, in the lower central zone of simple beams and in the lower central zone and upper zone in the supports in continuous beams.

In Fig. 3 the different areas are shown where skin reinforcement is theoretically necessary.  

Considering that the areas of the different zones, as well as the required anchorage lengths, practically cover the entire area of ​​the beam, therefore, it is common practice to extend the zones and cover the entire height of the beam and the entire length of the beam, for simple and continuous beams to facilitate the design and execution.

The code clarifies that this skin reinforcement can be included in the structural design of the beam as long as the deformation compatibility analysis is considered. However, it is not at all easy to do the calculations under this condition in order to use the skin reinforcement in the flexural structural design. Now, it is reasonable to be able to use this reinforcement as part of the reinforcement due to torsion, horizontal bending due to wind or other causes.

For example, in elements subjected to torsion, this reinforcement can be considered as the part of longitudinal steel that must be placed on the lateral faces of the beam. The possibility of using this reinforcement in the indicated cases will be studied later.

Distribution of the Flexural Reinforcement Due to cracking in beams

The new code criterion for the distribution of skin reinforcement is to apply the analysis of cracking in beams and slabs in one direction for tension reinforcement, on the lateral faces of beams with high elevation.

Flexural tensile reinforcement must be adequately distributed to control cracking. When service loads lead to high stresses, visible cracks should be expected, and precautions should be taken to control cracking. (ACI 318 – 14 Section 24.3.1). This concept is now applied to skin reinforcement.

The maximum spacing of adhered reinforcement in beams for corrugated bars or wires must comply with the provisions of Table 24.3.2

.                .                  s ≤ 15 • in • ((40000 • psi)/ƒs) – 2.5 • Cc ≤ 12 • in • ((40000 • psi)/ƒs)

If fy = 60000 psi                  and assuming that fs = (2/3)fy = 40000 psi, we have:

.                .                .                         s ≤ 15 • in – 2.5 • Cc ≤ 12 • in

where Cc = Cover + φv

being        s = maximum spacing for skin reinforcement

.                 fs = stress in the skin reinforcement subjected to service loads,

.                 It is allowed to consider that fs = (2/3)fy.

.                 Cc = Net cover from the skin reinforcement to the edge of the concrete section.

.                 Cover = Net cover from between the surface of embedded reinforcement, ties, and the outer surface of the concrete.

.                 Φv = diameter of ties

It is important to clarify that the application of this formula is being done with a simplification allowed by the norm, which consists of assuming that the reinforcement tension is constant and equal to fs = (2/3)fy. 

In reality, the bars are separating from the edge, the stresses fs should decrease and the spacing of the skin reinforcement should be increasing. However, it is acceptable for spacing to be uniform, which leads to a simplification in the calculation and greater ease in execution. 

Fig. 4 shows the section and all the previous nomenclature. The skin reinforcement is assumed to cover the entire height of the beam with uniform spacing.

Design Guides

Evaluating the equation of s for the different Cc that results from the combination of the net cover plus the diameter of the stirrup reinforcement, three Tables are obtained that allow to quickly calculate the number of skin bars for each face of the beam and the maximum spacing of the bars. In the tables that are attached in the location: HELP DESIGN. SKIN REINFORCEMENT for stirrup diameters of 3/8, 1/2 and 5/8 inches and for stirrup covers on 1 1/2, 2 and 3 inch beams.

Selection of the diameter of the Skin Reinforcement

The code makes it explicit that the diameters of the skin bars are not involved in determining the spacing of this reinforcement. The code itself suggests that the diameters can be #3, #4 and #5. However, if they are used structurally, larger diameters can be considered without altering the given spacings.

Additionally, it is indicated that said skin reinforcement can be used as part of the structural design, provided that a deformation compatibility analysis is carried out.

It has already been mentioned that this analysis extremely complicates the calculations, and some indications will be given which will allow us to select the diameter of the skin bars without having to resort to cumbersome calculations.

Case 1. When torsion requires longitudinal reinforcement.

In this case, the longitudinal torsion reinforcement Al is divided into four parts. Each side face will require Al/4, so we have:

Areq := ((Al/4)/No. bars)

with which the diameter of the skin reinforcement on each side is defined. 

If it is larger than bar #5, the diameter of the bar can be increased with the same spacing or the number of bars can be increased to meet the total required area and the spacing can be decreased.

Case 2. When an edge beam is subjected to lateral bending

In this case, in addition to the possibility of torsion, the beam may be subject to lateral bending due to wind loads on the walls. When there is no slab at that level, the lateral load generates bending and therefore requires reinforcement on both sides because the wind can act in both directions, that is, positive pressure or suction.

The reinforcement calculated for the extreme condition Al is distributed in its entirety among the number of bars of a face, then:

Areq := (Al/No. bars)

with which the diameter of the skin reinforcement on each side is defined. 

If it is larger than bar #5, the diameter of the bar can be increased with the same spacing or the number of bars can be increased to meet the total required area and the spacing can be decreased.

Case 3. When an edge beam is subjected to tension due to the effect of horizontal diaphragm 

The effect of a horizontal diaphragm can generate longitudinal stresses in an edge beam such that a certain amount of total reinforcement Al is required, the same as in the case of longitudinal reinforcement due to torsion.

When we obtain the reinforcement due to the effect of the diaphragm as a tensor of a lower chord of a frame, Al, we proceed in the same manner as in torsion, we divide by 4 and placed on each side. From there, we obtain the necessary diameter or the necessary increase, in the same way as Case 1. 

Case 4. Other situations. 

Various situations exist where the skin reinforcement may be chosen to cover structural design problems with such bars. These include, for example, the effect of concentrated horizontal loads due to the support of metal canopies, the effect of the wind on stair beams, the upper closing beam on high parapets, etc.

In all these cases the procedure is the same, obtain the required reinforcement and distribute the required reinforcement to the number of skin bars on each side of the beam.

Conclusions and recommendations

• a) Luckily, the beams with depths greater than 36” are not as common and it can be assumed that the skin reinforcement is not needed for other structural functions.  

Except in exceptional cases, this skin reinforcement does not need to be considered in the bending analysis of the beam.

A modification after the beam has been built, a change in loads, or another emergency situation may require a more complex analysis. Manually, this results in a design that requires a trial-and-error process to obtain the capacity of the section. 

When necessary, we recommend doing the calculations considering the section of the beam as if it were a column without slenderness. The programs allow locating the upper and lower bars, as well as the skin reinforcement, so that a load Pu (any small value) is applied to the column, since it does not intervene in the calculation. Once the interaction diagram is obtained, the maximum capacity of the beam will be the value of Mu for Pu = 0, which considers all the skin bars, including the compressed bars.

• b) In the event that the reinforcement is not considered structural, the splicing of the skin reinforcement must be located outside the tension zones.
• c) When an air conditioning duct passes through or for another reason, this skin reinforcement can be interrupted if it is in the areas of compression.
• d) In the case of wall-beams (long-depth beams), the minimum horizontal reinforcement established for these cases must prevail. However, it is advisable to check that the skin reinforcement is inferior, if it is not, use the latter.

This concludes the article, thank you very much for your attention.

If you have had experiences related to this work, please tell me about it so that I can include it as part of the conclusions. Also, I would like to know if the tables help you in the design or if you need any other information.

Ernesto F. Valdes Avellaneda

SKIN REINFORCEMENT
No. of bars on each face for the range of h indicated. COVER = 1 1/2″
Max. Esp.	10 1/4″	10″	9 5/8″
No. of bars	Tie Diam. 3/8″	Tie Diam. 1/2″	Tie Diam. 5/8″
3	36″ < h ≤ 45″	36″ < h ≤ 44″	36″ < h ≤ 43″
4	45″ < h ≤ 56″	44″ < h ≤ 54″	43″ < h ≤ 53″
5	56″ < h ≤ 67″	54″ < h ≤ 64″	53″ < h ≤ 63″
6	67″ < h ≤ 76″	64″ < h ≤ 74″	63″ < h ≤ 72″
7	76″ < h ≤ 86″	74″ < h ≤ 84″	72″ < h ≤ 82″
8	86″ < h ≤ 96″	84″ < h ≤ 94″	82″ < h ≤ 92″

SKIN REINFORCEMENT
No. of bars on each face for the range of h indicated. COVER = 2″
Max. Esp.	9″	8 3/4″	8 1/2″
No. of bars	Tie Diam. 3/8″	Tie Diam. 1/2″	Tie Diam. 5/8″
3	36″ < h ≤ 41″	36″ < h ≤ 40″	36″ < h ≤ 39″
4	41″ < h ≤ 50″	40″ < h ≤ 49″	39″ < h ≤ 48″
5	50″ < h ≤ 59″	49″ < h ≤ 58″	48″ < h ≤ 56″
6	59″ < h ≤ 68″	58″ < h ≤ 67″	56″ < h ≤ 65″
7	68″ < h ≤ 78″	67″ < h ≤ 75″	65″ < h ≤ 73″
8	78″ < h ≤ 86″	75″ < h ≤ 84″	73″ < h ≤ 82″

SKIN REINFORCEMENT
No. of bars on each face for the range of h indicated. COVER = 3″
Max. Esp.	6 1/2″	6 1/4″	6″
No. of bars	Tie Diam. 3/8″	Tie Diam. 1/2″	Tie Diam. 5/8″
3	36″ < h ≤ 40″	36″ < h ≤ 39″	36″ < h ≤ 37″
4	40″ < h ≤ 46″	39″ < h ≤ 45″	37″ < h ≤ 43″
5	46″ < h ≤ 53″	45″ < h ≤ 51″	43″ < h ≤ 49″
6	53″ < h ≤ 60″	51″ < h ≤ 57″	49″ < h ≤ 55″
7	60″ < h ≤ 66″	57″ < h ≤ 64″	55″ < h ≤ 61″
8	66″ < h ≤ 73″	64″ < h ≤ 70″	61″ < h ≤ 67″

Translation By: Ivet Llerena (Eastern Engineering Group)

© 2021 This article was written by Ernesto Valdes and published by Eastern Engineering Group. All rights reserved.