Basement Retaining Wall Design – Continued – Reinforcement Details

Draft – Comments Welcome

outline

1. Minimum supply
2. The biggest supplement
3. Progress Length
4. Embedment of V.S. on the footing
5. V.S. at Top of Wall
6. Reinforcement Splices and Continuity
7. Summary
8. References
• 1. Minimum support

  The minimum vertical reinforcement ratio for a wall is generally 0.0012 (Gr. 60 bars no larger than #5); and this is for the compression member of the wall. For bending is the least support…

  … (3 f ‘_c / f_y) b d … but not less than 200 b d / f_y

  to our cause;

  … 3 f ‘ _c is 3 √3000 = 164 … so 200 governs, giving us …

  … 200 (psi) b d / 60,000 psi = 0.0033 b d.

  With our final design finished at 0.0044…we are good.

  The horizontal steel already satisfies the wall at least, and we are not counting on it for bending (although we know there will be some double action), so we are good in the horizontal direction.

  2. Maximum supply

  Our wall acts primarily as a flexural member (holding backfill soil). Hence, the maximum amount of reinforcement will be such that the stresses in the steel will be at least 0.004 when we reach the flexural wall. This shall be provided with a reinforcement ratio not greater than 0.0155 for 3000 psi concrete and 60,000 psi steel (from effective depth d).

  Our complement ratio is 0.0044 (also from d), so, good!

  A minimum deflection of 0.004 in steel in a flexural member will generate ductile behavior at failure. Whether the 0.004 method of throwing is really applicable to the wall below the retaining wall can be argued, but because it meets the limit. surely it is not worth arguing about.

  3. Development Length

  We must ensure that there is a sufficient contact area between the concrete and the support, so that the support can be used; this is achieved by specifying a certain “length” of the contact (progression length, l_d).

  From the ACI Code, Section 12.2.3;

  l_d = { (3/40) (f_y /√f ‘c) (α β γ λ ) / [(c + k_{t r} sub>) / d_b ] } d_b

  We will cover evolution in more detail in another lesson. For the particular situation at hand we will use α = 1.3 for horizontal reinforcement (more than 12 m. or fresh concrete placed below) and 1.0 for vertical reinforcement; β = 1.0; γ = 0.8; λ = 1.0; c = 1-13/16 in. (smaller distance of the center of the bar to the face of the concrete or half the bar c.t.c spacing), and k _{t r} = 0 .

  The ratio (c + k_{t r}) / d_b in this example becomes 2.9 (using #5 bars) but cannot be taken greater than 2.5, so

  l_d = { (3/40) (60,000 psi /√3000 psi) (1.0 or 1.3)(0.8) / [ 2.5] } d_b

  … = 26.3 d_b or 34.2 d_b … (for loci’). Sometimes in the “diameters” … in this case the lengths of the progression are specified, namely 26 and 34.)

  So, for #5 horizontal reinforcement … l_d = 21.4 in.

  and for supplementary spelling … 16.4 in.

  The code allows us to reduce the length of the development by providing our “excess” supplement. We previously found that the ratio of our supplemental supplement to what was compared to 0.81, so that the length of development can reduce to 0.81 x 16.4 = 13.3. In this way we have to leave at 16 in. (16.0).

  So recapitulating;

  Progress lengths are (#5 bar): 16 in. to V.S. and 22 in. for H.S

  Progress lengths as may not mean much to a contractor (say, in Construction Documents), but it’s not done yet!

  Note that we do not have ‘tracking’. But in this example let’s say it is reinforced with #5 longitudinal bars (also Gr. 60) and no transverse bars are required. In addition, it is assumed that the longitudinal reinforcement of the foundation is less than 12 in the fresh concrete cast below. (You’ll see where this is leading a little later.)

  Now check to make sure the bars (vertical subduction tension) are “developed”. The maximum slope occurs at 4.8 ft from the top of the wall. In order to develop support, at least the length of the development should extend in both directions from where it is needed. With the bars extending over 4 ft up and at least 3 ft down, we’re good!

  But there are a few things that need to be checked: where the ‘end support’ is at the top and bottom of the wall.

  As we move away from the critical point, the amount of length available for development decreases faster than the need for it, so we better make sure we don’t get to the point where we need more than is available and so we tend to. from rebar to draw This could potentially happen at the ends of a “simply supported” beam. This is definitely not done, satisfying Equation (12-3) of the ACI code (Section 12.11.3). I call it ‘kick pullout’.

  Equation (12-3) is … (slightly re-written)

  … l_d ≤ (1.0 or 1.3) M_n / V_u + l _a … (ACI 12-3).

  where

  M_n = moment of nominal strength assuming all steel is developed at f_y,

  V_u = factored in shear section,

  l _a = the anchorage of the center of support of the past, and

  where 1.3 represents an increase of 30%, if the reinforcement is closed by a compressive reaction; otherwise use 1.0.

  4. AMENDMENT V.S. On the foot

  Section 12.11.3 provides that Eq. 12-3 need not be satisfied if the reinforcement ends outside the center of support with the flag hook. For this matter we will use 4 in. the bottom of the basement as a “support” at the bottom of the wall, and require that the support be nailed to the foot of the standard hook. The length of the minimum bar development hook is 8 diameter (in our case 5 in.) or 6 in. so,

  … V.S.: … embedded 6 in. in the foot with a sign (90º) hook.

  NOTE: For cell board to support the side of the wall, the concrete board must be spread against the concrete core of the wall. This will require the removal of the bottom 4 in. (or so) of the ICF in the interior after the wall became hard.

  5. The termination V.S. at Wall Top‘s

  In the absence (as seen in the video) of clear provisions to handle the development of the vertical reinforcement at the top of the wall (when it is laterally supported by laying wood system), I provide the following:

  Add a #5 horizontal bar to the top 12 in. walls and ‘consider’ the top 12 in. of the wall as a `step beam’ support.

  Terminate V.S 3 in. from the top of the wall The centerline of the bearing is thus 6 in. from the top and l _a so 3 in.

  Regarding ACI Equation 122-3;

  from Part 1, … φ M_n = 52,800 lb-in., and V _top = 694 lb.

  Yes,

  M_n= 52,800 lb-in. / 0.9 = 58,600 lb-in., … and V_u = 1.6 (694 lb) = 1110 lb.

  Therefore, Eq. 12-3 …

  Is l_d ≤ (1.0) M_n / V_u + l_a ???…

  Is it 16 in. ≤ 58,600 lb-in. /1110 lb. + 3 in. = 53 in. + 3 in. = 56 in.???

  Yes, it is just; pullout should not be a problem.

  V.S. 3 in. from the top of the wall

  6. Supplemental Splices and Continuity

  We will fix the V.S splices at the roots of the interface wall with the standard brackets at the foot. Thus, the hooks should be driven in place at the foot of the construction with the rest of the ‘straight’ V.S bars placed below in the wall construction. All supplements will be spliced ​​at this point, so we’ll use a Class B splice from here on. But since we are at the end of the simple support condition, theoretically require nothing in the supplement, so the minimum development length is 12 in. Argument and thus Class B splice length. 1.3 (12 ex.) = 15.6 in. (use 16 in.). Yes, … V.S toros lap This will usually fit using 16 + 6 = 22 in. long (flag) poles fixed to the hook at the foot 6 in. and protruded 16 in. Straight wall pieces of rebar (installed behind the foot) so they will be open to the bottom of the wall (top of foot) and the pocket with the longer legs of the vertical brackets.

  For horizontal support we will use Class B splices, for which the splice length is 1.3 l_d = 21.4 in. = 27.8 in. use 28 in.).

  For the longitudinal reinforcement in the foundation, we could use a location factor of 1.0 bar from the splice length of the blade (Class B of 27.8 / 1.3 = 21.4 in. 22 in.). A good explanation involves being clear and “economical”, but it also recognizes that too much information (or options) can lead to confusion. Hence, it is prudent to specify a single horizontal rebar splice length (28 in.). (And in some cases one splicing length can be wise horizontal and vertical threads.

  For the integrity of the entire foundation, the horizontal and longitudinal reinforcement must be continuous around the entire foundation.

  7. Summary

  We are now ready to summarize the retaining wall plan.

  Concrete 28-Day Compressive Strength: 2000 psi;

  Support: Grade 60 ksi, deformed;

  Max wall height: 8′-4″

  Wall Core Thickness: 6-1/4 in.

  V.S.: #5 @ 16 in. o.c. located 1-13/16 m. from the inside face (inboard tab of ICF); are terminated by V.S. 3 in. from the top of the wall; immersed in V.S. 6 in. V.S can be split at the bottom of the wall (16 m splice length);

  H.S.: #5 @ 16-3/4 in. provide 2 #5 bars within the top 12 in. of the wall

  Some of this information can be found on the General Specifications page, while other information could be explained by dimensions, others noted in the text. I often mark information with “general” requirements. The exact ‘location’ of the two H.S vectors at the top is perhaps somewhat dictated by the ‘tabs’ of the specific ICF system.

  8. Notes

  Building Code Requirements for Structural Concrete, ACI 318, American Concrete Institute, P.O. Box 9094, Farmington Hills, Michigan, 48333.

  International Building Code, International Council, 4051 West Flossmoor Road, Country Club Hills, NE 60478