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Lecture 16 – Design(T-Beams)PowerPoint Presentation

Lecture 16 – Design(T-Beams)

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Lecture 16 – Design(T-Beams)

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Lecture 16 – Design(T-Beams)

February 21, 2003

CVEN 444

- Design of T-Beams
- Known section dimensions

Assume that the material properties, loads, and span length are all known. Estimate the dimensions of self-weight using the following rules of thumb:

a.The depth, h, may be taken as approximate 8 to 10 % of the span (1in deep per foot of span) and estimate the width, b, as about one-half of h.

b.The weight of a rectangular beam will be about 15 % of the superimposed loads (dead, live, etc.). Assume b is about one-half of h.

Immediate values of h and b from these two procedures should be selected. Calculate self-weight and Mu.

- Calculate controlling value for the design moment, Mu.
- Assume that resulting section will be tension controlled, et 0.005 so that can take f = 0.9.

- Calculate d, since h is known

For one layer of reinforcement. For two layers of reinforcement.

- Determine the effective width of the flange, beff
- Check whether the required nominal moment capacity can be provided with compression in the flange alone.
and

If Need to utilize web below flanges. Go to step 4.

If Use design procedure for rectangular beams with b = beff , (d -a/2) = 0.95d

Note:f = 0.9 for flexure without axial load (ACI 318-02 Sec. 9.3)

Find nominal moment capacity provided by overhanging flanges alone (not including web width)

For a T shaped section:

and

- Find nominal moment capacity that must be provided by the web.

- Calculate depth of the compression block, by solving the following equation for a.

- Find required reinforcement area, As (req’d)

- Select reinforcing bars so As (provided) As (req’d). Confirm that the bars will fit within the cross-section. It may be necessary to change bar sizes to fit the steel in one layer or even to go to two layers of steel.

- Calculate the actual Mn for the section dimensions and reinforcement selected. Check strength f Mn Mu (keep over-design within 10 %)

- Check whether As provided is within allowable limits.
As (provided) As (min)

Calculate the minimum amount of steel

ACI 318 Section 10.6.6

Must distribute flexural tension steel over effective flange width, be (tension)

ACI 318 Section 10.6.6

When be (comp) > l/10 some longitudinal reinforcement shall be provided in outer portions of flange.

ACI 318 Section 10.6.6

For l use centerline dimensions when adjacent spans for - M @ support are not equal, use average l to calculate be (tension)

One scenario when be (tension) < be (compression)

Do a preliminary geometric size based on the following:

1

2

3

4

A reasonable value for k in terms

Effective flange width based on ACI guidelines.

Desired ration of b and d. b = 0.5 – 0.65 d

Depth of the flange based on design of the slab.

Assume

T-Beam with unknown dimensions, hf = 6 in.(slab)

fc = 4500 psi & fy = 60 ksi.

Three spans continuous beam, simply supported on walls. Spans are (25ft, 30 ft. and 25 ft.) The beam spacing is 14 ft center to center

Using estimated values h = 26 in. , b =16 in.

Max + Mu = 300 k-ft

Max - Mu = 435 k-ft

Calculate the – moment term, where the bottom section is in compression. Max –Mu = 435 k-ft.

Determine the b1 term for the concrete

Calculate a desired k = c/d

Determine the Ru term for the concrete

From the design of the

Determine the Ru term for the concrete

The nominal moment is defined as

The bd2 value will for design is

Assume that the b = 0.65d so that

Determine the value for b

Determine h assuming a single layer of reinforcement

Check to see if the estimate will work

Over-designed by 10.9 % so it will work but we would need to go back an recalculate the weight

Calculate the actual value for k

Use the quadratic formula

Calculate the actual value for k’

Calculate the actual value for k

Calculate the As required for the beam

Calculate the actual value for As

The flange is in tension so the reinforcement, the beff in tension must be computed according to ACI 10.6.6

The size of the flange in compression is from 8.10.2 of the ACI code, the beff in compression

The size of the flange in compression is from 8.10.2 of the ACI code, the beff in compression

Use 82 in. for the compression flange.

The flange is in tension so the reinforcement, the beff in tension must be computed according to ACI 10.6.6

Use 33 in. for beff in tension.

Select the steel for the reinforcement at least 0.5 As needs to be in the beff of the beam. Use 3 #8 bars and 8 # 5 bars, (which will give you 4 on each side.)

The As = 4.85 in2 > 4.84 in2 OK!

Check to the value for d

The d will work.

Calculate the new a using the equilibrium equations.

Check the strain condition for the beam

Use f = 0.9

Calculate the moment capacity of the beam

Ultimate moment capacity of the beam.

Calculate the minimum amount of steel

Amin = 2.26 in2 < 4.85 in2 OK

Summary of the beam with M 435 k-ft.

Calculate the + moment term, where the bottom section is in compression. Max +Mu = 300 k-ft.

Calculate the moment capacity of the beam with d= 22.5 in. and hf = 6 in.

The minimum amount of steel is

Use As = 6.19 in2

So the beam can be designed as a singly reinforced beam with the minimum amount of steel 5.91 in2.

Compute the moment capacity of the beam with the minimum amount of steel 6.19 in2.

Use 8 #8 bars As = 6.32 in2. Check to see that the steel will fit. It will not be within 10% of the ultimate moment capacity of the beam. However, the minimum amount steel will preside.

Problem 5.13