CE 415 Design of steel structures

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CE 318

STRUCTURE ANALYSIS AND DESIGN II LAB

LESSON 2 SLAB DESIGN

SEMESTER: SUMMER 2021

COURSE TEACHER: SAURAV BARUA CONTACT NO: +8801715334075

EMAIL: [email protected]

copyright @Saurav Barua 1

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LECTURE PLAN

2 copyright @Saurav Barua

Slab load calculation

Moment calculation using ACI coefficient

Reinforcement calculation

Slab reinforcement detailing

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SLAB OUTLINE

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SLAB LOAD CALCULATION

• Consider S1 slab first.

• Assume beams =12” x 18” (beam width is 12” as per BNBC 2015)

• And beam depth is equal to span in inch. (Thumb rule!!)

• Since maximum span is 18’ so we assume beam depth 18”

• Some over design will increase cost, however make the building safe structurally.

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SLAB LOAD CALCULATION

• Dead load calculation

Slab 5” thick weight = 5/12 x 0.15 = 0.0625 ksf. RCC unit weight = 150 psf = 0.15 ksf Floor finish (FF) weight = 25 psf = 0.025 ksf (BNBC 2015)

Wall (exterior + interior) weight = 80 psf = 0.08 ksf Slab is 48’x32’

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LOAD CALCULATION

• Total DL = slab weight + FF weight + wall weight

= 0.17 ksf

Live load = 40 psf (ACI code 318, BNBC 2015)

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LOAD CALCULATION

• LL = 40 psf = 0.04 ksf

Factored DL, w_DL = 1.4 x DL = 1.4 x 0.17 = 0.24 ksf Factored LL, w_LL = 1.7 x LL = 0.07 ksf

Total factored Load, w_T = 1.4 DL + 1.7 LL ≈ 0.31 ksf

(Use old factor value to keep structure safe side!! Workmanship issues!!)

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ACI COEFFICIENT

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ACI COEFFICIENT

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ACI COEFFICIENT

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MOMENT CALCULATION

• Our slab is case 8, we have beams all around in each slab.

• We have one discontinuous edge in long side.

(Explain all cases in lectures!!)

• a = short direction and b = long direction

• m = l_a/l_b = 16/18 = 0.89

• Need interpolation (Explain in video lecture!!)

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MOMENT CALCULATION

• C_a,-ve= 0.043 and C_b,-ve= 0.052

• C_a,DL+ve= 0.025 and C_b,DL+ve= 0.019

• C_a,DL+ve= 0.035 and C_b,DL+ve= 0.024

• Moment, M = C x w x l²

M_a,-ve= 0.043 x 0.31 x 16² = 3.41 k-ft/ft (Explain!!)

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MOMENT CALCULATION

M_b,-ve= 0.052 x 0.31 x 18² = 5.22 k-ft/ft

M_a,+ve= 0.025 x 0.24 x 18² + 0.035 x 0.07 x 18² = 2.74 k-ft/ft (Explain!!) M_b,+ve= 0.019 x 0.24 x 16² + 0.024 x 0.07x 16² = 1.6 k-ft/ft (Explain!!)

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REINFORCEMENT CALCULATION

As_a,-ve= M_a,-ve/Ø fy (d – a/2) = 5.22 x 12/(0.9x 60x(4-1/2) = 0.33 in²/ft Clear cover for slab = 1”, so d = 1”. Assume, a = 1”

a = As_a,-vef_y/0.85 f_c’ b = .33x 60/ 0.85x3x12 = 0.65 Trial again, using a = 0.65

As_a,-ve= 0.32 in²/ft (it is close to previous one, so no further trial)

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EFFECTIVE DEPTH CHECK

Steel ratio ρ = 0.75 ρ_b = 0.75 x α x β x fc’/fy x 87/(87+fy) [nelson book]

= 0.75 x 0.85 x 0.85 x 3/60 x 87/(87+60) = 0.016 d_req = ^𝑀^𝑢

Øρ^𝑓𝑦𝑏(1−0.59ρ^𝑓𝑦/𝑓_𝑐′ = √( 5.22 x 12/0.9x0.016x60x12x(1-.59x0.016x60/3)

= 2.73” <4” (d_provided= 5”-1” = 4”) ok

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REINFORCEMENT

As_b,-ve= 0.32 in²/ft

Similarly, for As_a,-ve= 0.21 in²/ft

• As_a,+ve= 0.17 in²/ft

• As_b,+ve= 0.1 in²/ft

Minimum rebar for temperature As_temp= 0.0018 x b x h = 0.0018 x 12 x 5 = 0.11 in²/ft (ACI code)

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REINFORCEMENT

Provide, Ø10mm bar for short direction (a-side)

Spacing, s = 0.11 x 12/0.17 ≈ 7.5”c/c [explained in lecture!!]

For –ve rebar at top, alt ckd, steel = 0.17/2 = 0.085 to the top , so required rebar at top = 0.21- .85 = 0.13 in². so provide 2- Ø10mm extra top rebar

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REINFORCEMENT

Provide, Ø10mm bar for long direction (b- side)

Minimum temperature rebar is governed, As = 0.11 in²/ft

Spacing, s = 0.11 x 12/0.11 ≈ 12”c/c [explained in lecture!!]

However maximum slab rebar spacing can be 2h = 2 x 5 = 10 in” c/c (ACI code) Provide Ø10mm @ 10”c/c for long direction positive moment.

Asprovide(b+ve) = 0.11x 12/ 10 = 0.13 in²/ft

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REINFORCEMENT

• For –ve rebar at top, alt ckd, steel = 0.13/2 = 0.065 to the top , so required rebar at top = 0.32-.065 = 0.26 in². so provide 2-Ø12mm extra top rebar (Explanation 2 x 0.2 = 0.4 in²)

• Corner reinforcement at top and bottom should be same Maximum positive moment rebar and extended up to l/5 distance of long direction. l = 16’ (span at edge) [explained in lecture!!]

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REINFORCEMENT

Since span of other slab S2 is almost same as S1.

For moment calculation, it will be case 4.

So we can keep the reinforcement spacing same for entire slab for fabrication simplicity!!

Short direction +ve rebar: Ø10mm bar @ 7.5” c/c alt ckd

Short direction +ve rebar: 2-nos. Ø10mm bar in between alt ckd Long direction +ve rebar: Ø10mm bar @ 10” c/c alt ckd

Long direction +ve rebar: 2-nos. Ø12mm bar in between alt ckd

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REINFORCEMENT

Maximum spacing for slab = 2h = 2 x5 =10” c/c (ACI code) So, our design is ok

Corner rebar: Ø10mm bar @ 7.5” c/c, up to 16/5 = 3’-2” at corner

 Extra top rebar length l/5 at exterior and l/7 at interior for slab

 Short direction bar at top layer [explained in lecture!!]

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DETAILING

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DETAILING

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