Sensing skin' could monitor the health of concrete infrastructure continually and inexpensively

n the MIT laboratory, researchers tested the "sensing skin" by attaching it to the underside of a concrete beam, then applying enough force to cause tiny cracks to form in the beam under one patch of the skin. Credit: Simon Laflamme, MIT

In 2009, the American Society of Civil Engineers (ASCE) assigned the grade "D" to the overall quality of infrastructure in the U.S. and said that ongoing evaluation and maintenance of structures was one of five key areas necessary for improving that grade. Since that time, federal stimulus funds have made it possible for communities to repair some infrastructure, but the field of high-tech, affordable methods for the continual monitoring of structures remains in its infancy. Instead, most evaluation of bridges, dams, schools and other structures is still done by visual inspection, which is slow, expensive, cumbersome and in some cases, dangerous.

Each structural member of the deck is designed to take weight and transfer it through connection to the ground. The decking material, takes the weight of the people and items that are on the deck, this weight is transferred to floor joists, which in turn transfer the load to the beams which in turn transfer the load to the posts which take the load to the ground, as shown in Figure 2.

Figure 2 - Transfer of load through post and beam construction

The method employed to join the joists to the beams and the beams to the posts makes a big difference to the structural strength of the deck and as well, its longevity.

The best construction method is to have beams sitting on the posts and the joists sitting on the beams as shown in Figure 2 above. Looking at a side view of the same figure, the lumber would be notched as shown in Figure 3 below.

Figure 3 - Side view of post and beam construction showing load transfer to ground

Although the aforementioned construction method provides the best possible support, there are some variations that can be utilized. You can notch the post so that only one of the beam members is actually resting on the post and the other is bolted to the first beam. Adding a support that is well fastened to the beam increases the strength as shown in Figure 4.

1) Which of the following gives the balanced steel ratio.

2) Which of the following gives the depth of the compression block.

3) Which of the following gives the nominal moment capacity of the beam.

Solution:

Section 5.9.5.2.1 of the National Structural Code of the Philippines specifies the minimum thickness as shown on the table apply for one way construction not supporting or attached to partitions or other construction likely to be damaged by large deflections, unless computation of deflection indicates a lesser thickness can be used without adverse effects.

Minimum thickness of Non-Prestressed Beams or One Way Slabs Unless Deflection Are Computed.

		Minimum thickness, h
	Simply Supported	One end condition	Both continuous	Cantilever
Member	Members not  supporting or attached to partitions or other construction likely to be damaged by large deflections.
Solid one-way slab	L/20	L/24	L/28	L/10
Beams or ribbed one-way slabs	L/16	L/18.5	L/21	L/8

For the continuous beam shown

1) Which of the following gives the minimum thickness of beam B-1.

2) Which of the following gives the minimum thickness of beam B-2.

3) Which of the following gives the minimum thickness of beam B-3.

Solution:

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A cantilever prismatic beam is 8m. long and propped at one end. EI = 1750 N.m²

1) Which of the following gives the moment induced by a unit rotation at the propped end.

2) Which of the following gives the moment induced at the fixed end.

3) Which of the following gives the reaction at the propped support.

Solution:

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A rod is connected to a pin at A and a chord at B as shown. It holds a cylindrical drum which weighs 176N. The drum has a diameter of 1m.

1) Which of the following gives the force between the drum and the rod.

2) Which of the following gives the force in the chord BC.

3) Which of the following gives the reaction at the pin at A.

Solution:

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1) Which of the following gives the acceleration of the car

2) Which of the following gives the normal force on the front wheels.

3) Which of the following gives the minimum coefficient of friction needed so that motion is possible.

Solution:1) Tensio:

F_b = 0.60Fy

2) Compression:

For member meeting the requirements of Sect. 4.9.1.2, having an axi of symmetry in, and loaded in, the plane of their web, and compression on extreme fibers of channels bent about their major axis:

The larger value computed by Formula (4.5-6a) or (4.5-6b) and (4.5-7), as applicable (unless a higher value can be justified on the basis of a more precise analysis), but not more than 0.60Fy.

Or, when the compression flange is solid and approximately rectangular in cross-section and its area is not less than that of the tension flange:

In the foregoing:

I = distance between cross sections braced against twist or lateral displacement of the compression flange, mm. For cantilevers braced against twist only at the support, I may conservatively be taken as the actual length.

r_t =  radius of gyration of a section comprising the compression flange plus 1/3  of the compression web area, taken about an axis in the plane of the web, mm.

A_f = area of the compression flange, mm³

C_b = 1.75 + 1.05(M_1/M₂) + 0.30(M_1/M₂)² but more than 2.3, where M₁ is the smaller and M₂ the larger bending moment at the ends of the unbraced length, taken about the strong axis of the members, and where M_1/M₂ the ratio of end moments, is positive when M₁ and M₂ have the same sign (reverse curvature bending) and negative when they are of opposite sighs (single curvature bending). When the bending moment at point within an unbraced length is larger than that at both ends of this length, the value of C_b may be computed by the formula given above for frames subject to joint translation, and it shall be taken as unity for frames braced against joint translation. C_b may conservatively be taken as unity for cantilever beams.

A simply supported beam having a span of 8m. is laterally unsupported is made up of A36 steel with Fy = 248 MPa with the following properties:

b_f = 0.210m                             r_t = 0.073m.

S_x = 0.002077m³*********t_f = 0.016m.

d = 0.533m.

1) Which of the following gives the slenderness ratio above which the beam would be considered long.

2) Which of the following gives the allowable bending stress.

3) Which of the following gives the maximum uniform load that can be carried by the beam.

Solution:

1) Which of the following gives the reaction at the fixed end.

2) Which of the following gives the maximum torsional stress.

3) Which of the following gives the angle of twist of the shaft.

Solution:

Important Points from Presentation
A design engineer’s responsibility should include assuring the structural safety of the design, details, checking shop drawing.

Detailing is as important as design since proper detailing of engineering designs is an essential link in the planning and engineering process as some of the most devasting collapses in history have been caused by defective connections or DETAILING. There are many examples explained in the book” DESIGN AND CONSTRUCTION FAILURES by Dov Kaminetzky.

Detailing is very important not only for the proper execution of the structures but for the safety of the structures.

Detailing is necessary not only for the steel structures but also for the RCC members as it is the translation of all the mathematical expression’s and equation’s results.

For the RCC members for most commonly used for buildings we can divide the detailing for Slabs-with or without openings.(Rectangular,circular,non-rectangular-pyramid slab,triangular etc) balcony slab, loft slab, corner slab etc

Beams – With or without openigs.(Shallow & deep beams)
Columns – (Rectangular,l-shape,t-shape, circular,octagonal,cross shape etc)
Foundations.

Detailing for gravity loads is different from the lateral loads specially for the SEISMIC FORCES.

Apart from the detailing for the above there is a different detailing required for the Rehabilitation and strengthening of damaged structures.

We will now dwell on the DETAILING OF MEMBERS FOR THE GRAVITY AND SOME CODAL DETAILINGS AS PER IS CODE IS 13920 AND IS 4326 AS REQUIRED FOR SEISMIC FORCES.

DO’S & DONOT’S FOR DETAILING
DO’S-GENERAL
Prepare drawings properly & accurately if possible label each bar and show its shape for clarity.

Cross section of retaining wall which collapsed immediately after placing of soil backfill because ¼” rather than 1-1/4” dia. were used. Error occurred because Correct rebar dia was covered by a dimension line.

2. Prepare bar-bending schedule, if necessary.
3. Indicate proper cover-clear cover, nominal cover or effective cover to reinforcement.
4. Decide detailed location of opening/hole and supply adequate details for reinforcements around the openings.
5. Use commonly available size of bars and spirals. For a single structural member the number of different sizes of bars shall be kept minimum.
6. The grade of the steel shall be clearly stated in the drawing.
7. Deformed bars need not have hooks at their ends.
8. Show enlarged details at corners, intersections of walls, beams and column joint and at similar situations.
9. Congestion of bars should be avoided at points where members intersect and make certain that all rein. Can be properly placed.
10. In the case of bundled bars, lapped splice of bundled bars shall be made by splicing one bar at a time; such individual splices within the bundle shall be staggered.
11. Make sure that hooked and bent up bars can be placed and have adequate concrete protection.

12. Indicate all expansion, construction and contraction joints on plans and provide details for such joints.
13. The location of construction joints shall be at the point of minimum shear approximately at mid or near the mid points. It shall be formed vertically and not in a sloped manner.

DO’S – BEAMS & SLABS:
1. Where splices are provided in bars, they shall be , as far as possible, away from the sections of maximum stresses and shall be staggered.
2 Were the depth of beams exceeds 750mm in case of beams without torsion and 450mm with torsion provide face rein. as per IS456-2000.
3. Deflection in slabs/beams may be reduced by providing compression reinforcement.
4. Only closed stirrups shall be used for transverse rein. For members subjected to torsion and for members likely to be subjected to reversal of stresses as in Seismic forces.
5. To accommodate bottom bars, it is good practice to make secondary beams shallower than main beams, at least by 50mm.

Do’s –COLUMNS.
1. A reinforced column shall have at least six bars of longitudinal reinforcement for using in transverse helical reinforcement.-for CIRCULAR sections.
2. A min four bars one at each corner of the column in the case of rectangular sections.
3. Keep outer dimensions of column constant, as far as possible , for reuse of forms.
4. Preferably avoid use of 2 grades of vertical bars in the same element.

DONOT’S-GENERAL:
1. Reinforcement shall not extend across an expansion joint and the break between the sections shall be complete.
2. Flexural reinforcement preferably shall not be terminated in a tension zone.
3. Bars larger than 36mm dia. Shall not be bundled.
4. Lap splices shall be not be used for bars larger than 36mm dia. Except where welded.
5. Where dowels are provided, their diameter shall not exceed the diameter of the column bars by more than 3mm.
6. Where bent up bars are provided, their contribution towards shear resistance shall not be more than 50% of the total shear to be resisted. USE OF SINGEL BENT UP BARS(CRANKED) ARE NOT ALLOWED IN THE CASE OF EARTHQUAKE RESISTANCE STRUCTURES.

DETAILING OF SLABS WITHOUT ANY CUT OR OPENINGS.
The building plan DX-3 shows the slabs in different levels for the purpose of eliminating the inflow of rainwater into the room from the open terrace and also the sunken slab for toilet in first floor.

The building plan DX-A3 is one in which the client asked the architect to provide opening all round.

SLABS:
It is better to provide a max spacing of 200mm(8”) for main bars and 250mm(10”) in order to control the crack width and spacing.

A min. of 0.24% shall be used for the roof slabs since it is subjected to higher temperature. Variations than the floor slabs. This is required to take care of temp. differences.

It is advisable to not to use 6mm bars as main bars as this size available in the local market is of inferior not only with respect to size but also the quality since like TATA and SAIL are not producing this size of bar.

BEAMS:
A min. of 0.2% is to be provided for the compression bars in order to take care of the deflection.
The stirrups shall be min.size of 8mm in the case of lateral load resistance .
The hooks shall be bent to 135 degree.

DEVELOPMENT LENGTH OF BARS FOR A CONCRETE GRADE M20 &STEEL STRENGTH F_y=415

SLNO	BAR DIA.	TENSIONmm	COMPRESSION	REMARKS
1	8	376.0	301.0
2	10	470.0	376.0
3	12	564.0	451.0
4	16	752.0	602.0
5	20	940.0	752.0
6	22	1034.0	827.0
7	25	1175.0	940.0
8	28	1316.0	1053.0
9	32	1504.0	1203.0

APPROXIMATELY USE 50Xdia FOR TENSION

References:
1. Handbook On Concrete Reinforcement And Detailing-sp:34(s&t)-1987.
2. Manual Of Engineering & Placing Drawings For Reinforced Concrete Structures- (Aci 315-80
3. Manual Of Standard Practice –concrete Reinforcing Steel Institute.
4. Tward Board Manual For Rural Water Supply Schemes.
5. Design Principles And Detailing Of Concrete Structures. By D.S.Prakash Rao.
6. Simplified Design-rc Buildings Of Moderate Size And Height-by Portland Cement Association,Usa.
7. Design And Construction Failures By Dov Kaminetzky.
8 IS:2502-1963 Code of practice for bending and fixing of bars for concrete reinforcement.
9. IS:1893:2000.
10. IS:4326.
11. IS:456:2000
12. Reinforced hand book by Reynold.