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[vikram.jeet]

Two way shear as result of moments :

A personal perception :

Taking a hypothetical example of a shed footing - ( wind = 100 kg/m2 )

Size of shed bay (inner) = 18.0m � 4.5 m � 6.0 m ht cladded shed
Uplift force = (0.8+0.5) x100� 4.5x18.9/2 = 6.4 t
DL = 15 x4.5x18.9/2 = 0.6 t
Net Uplift force = 6.4 - 0.6 = 5.8 t
Wind moment on wall cladding = 100*1.2*4.5*6.0*(6/2+1.5) = 14580 kgm

基础的大小= 3.0 x3.0m mfoundation thk��250 1.5depth
Rcc pedestal ( supporting steel col) = 600 x 450
Uniaxial Moment due to wind = 14.58 tm
Tensile force = 6.4 t

Footing eff depth = 25 - 5- 0.6= 19.4cm

Shear two way to be chkd at d/2 from face of column pedestal
Perimeter dimensions L=60+ 19.4 = 79.4 cm , B= 45 + 19.4 = 64.4 cm

Area for shear = [( 79.4) +(64.4 )]*2= 287.6 cm

Shear Stress due to uplift force = 5800 /( 287.6*19.4) = 1.04 kg/cm2
This stress is allaround

Shearstress due to Moment:

On front and back faces area = ( 64.4 )*19,4 = 1249.4 cm2
Moment balanced by shear stress s = 1294.4* ( 79.4) *s = 102775*s kgcm

On sides area each side = 0.5*19.2 (79.4)/2*2= 770.2 cm2
Moment balanced = 770.2*s *2 * (79.4/2)*2/3 = 40769 *s kg/cm2

Shear Stress s = (14.58*100000 )/ (102775 +40769)= 10.15 kg/cm2

Total two way shear stress = 1.04+ 10.15= 11.19 kg/cm2
Allowable stress = 0.16* 20^0.5 = 0.715 n/mm2 ~ 7.15 kg/cm2

Hence there is need to Increase thickness of footing / or column pedestal dimensions due to two wayshear check , even though Flexural requirements of BM and one way shear are OK for footing .

[vikram.jeet]

Two way Shear check as result of moments :

A personal perception :

Hypothetical Example - A 1.75 m cantilever beam at mid landing comming out from 200th rcc wall ( eff thk = 20 - 2.5- 0.5 = 17 cm
Stair span = 6.0m

Stairs loading DL+LL= 1400 kg/m2

Loading on beam 230x450mm : self = 260 kg/m
Stairs 1400x 6.0/2= 4200 kg/m
End Railing and base = say 300 kg/m

Total udl on beam = 260+4200+300= 4760 kg/m

Cantilever BM = 4.76x 1.75^2 /2 = 7.29 tm

Two way shear area dimensions:
L= 45 + 17= 62 cm
B= 23 + 17= 40 cm

Moment resisted by top and bottom faces = 40x17 * 62*s = 42160 *s kg cm

Moment resisted by side faces :
Area both sides = (0.5x 31x17x2 ) x2 = 1054 cm2
Moment resisted = 1054x (62/2 )x2/3 x s= 21783 *s kgcm

Shear Stress s = 7.27x100000 / ( 42160+21783) = 11.37 kg/cm2

Allowable two way stress forM20 = 0.16x 20^0.5 = 0.715 n/mm2 ~7.15kg/cm2
Hence there is need to increase thickness of rcc wall / Beam size to keep two way stress within permitted limit .

[vikram.jeet]

Continuing posts on two way shear check :

Two way shear check : For footing having column under compression and moment

Generally it is seen that compressive load on column at its base is quite large compared to gravity Moment and check for Two way shear for footing is done on compressive load only . But associated Moment will enhance the shear stress ( on front face of two way perimeter ) and , I feel , it needs to be considered for working out gross shear stress.

Under wind / EQ conditions , two way shear due to the moments on Footing from columns need to be addressed in similar way , but with higher permitted stresses .


Still a learner in this vast str engg field ( A sea) , request experts / purists of str engg to guide regarding this perception in all above postings on two way shear .

[vikram.jeet]

HYDROPOWER STRUCTURES:

Structural Engineering is inherent in Hydropower Structures. But knowledge of Geotechnical , Rock Mechanics as well as Geology is also needed to understand Nature's response / loading on Structures, . Most of Structures in Hydropower projects are Nature related . Geologists at field and design office provide field data values to engineers of various disciplines viz Geotechnical / Rock Mechanics / Structural engineers .

Engineers design the excavaton and support system for Cut slopes , tunnel excavations , Large Cavern excavations and all other structures. Geologists , Structural & Geotechnical engineers , Rock Mechanics engineers along with Hydrologists , Hydraulic design specialists , Surveyors - all have to work as a team for mega Hydropower Projects in Hilly terrains with project sprawled in 25 to 50km or more .

( to be continued)

[vikram.jeet]

HYDROPOWER STRUCTURES IN HEP :



O) INFRASTRUCTURE DEVELOPMENT WORKS /STRUCTURES AT START OF PROJECT :

BRIDGES , ACCESS ROADS , RETAINING STRUCTURES , SLOPE PROTECTION , STORES , STOCKING YARDS ,WORKERS CAMPS AT VARIOUS LOCATIONS , CENTRALISED OFFICE AND CAMP , WORKSHOPS , LABORATORIES , DUMPING SITES , CRUSHING PLANT SITES , SECURITY TOWERS

A) RIVER DIVERSION STRUCTURES

DIVERSION STRUCTURES FACILITATING CONSTRUCTION OF HEADWORKS-
DIVERSION TUNNEL , DIVERSION CHANNEL , UPSTREAM COFFERDAM , DOWN STREAM COFFERDAM

B) HEAD WORKS - DAMS , BARRAGES , WEIRS , BRIDGE , GATES

C) INTAKE & SEDIMENTATION STRUCTURES

INTAKE WORKS - BAR SCREENS , INTAKE TUNNELS , SEDIMENTATION CHAMBERS , FLUSHING DUCTS , FLUSHING DUCT OUTFALL STRUCTURE, GATE CHAMBER

D) .WATER CONDUCTOR SYSTEM

WATER CONVEYANCE TUNNEL - HEAD RACE RACE TUNNEL (CC/RCC LINED)

BALANCING STRUCTURE AS RESULT OF SURGES WHILE OPENING & CLOSURE OF VALVES --- UPSTREAM SURGE SHAFT (VERTICAL SHAFT)

HIGH PRESSURE CONVEYANCE - STEEL PRESSURE SHAFT WITH SURGE SHAFT (& VALVE CHAMBER ) NEAR ITS U/S AND PENSTOCKS (TO TURBINES IN PH) ON D/S ,
( PRESSURE SHAFT - IT IS GENERALLY VERTICAL BUT SOMETIMES INCLINED PRESSURE SHAFT AS PER TOPOGRAPHY )

E) POWER HOUSE STRUCTURES

UNDERGROUND DEVELOPMENTS - POWER HOUSE CAVERN (MACHINE HALL) , TRANSFORMER HALL CAVERN , BUS BAY TUNNELS , CONTROL BLOCK (UG), ( DOWNSTREAM SURGE CHAMBER IN CASE OF FRANCIS TURBINES, FOR PELTON TURBINES THIS CHAMBER NOT REQUIRED) , MAIN ACCESS TUNNEL (MAT) , CABLE AND VENTILATION TUNNEL ,

OPEN PH DEVELOPMENTS - MACHINE HALL , TRANSFORMER HALL , CONTROL BLOCK , BUS DUCTS ,

(M/C HALL - MULTI LEVEL WITH ERECTION BAY , GENERATORS FLOOR , M/C (TURBINES) FLOOR , UNIT TAIL RACE AT BOTTOM MOST ,GANTRY CRANE

(TRANSFORMER HALL - TWO LEVELS ,LOWER LEVEL HOUSING TRANSFORMERS ,UPPER HOUSING PANELS)

MACHINES (TURBINES) - FRANCIS LOW TO MEDIUM HEAD & HIGH DISCHARGE ; PELTON IS HIGH HEAD & LOW DISCHARGE TURBINE ,

F) TAIL WORKS STRUCTURES

- TAIL RACE TUNNEL ( FOR UG PH) , TAIL RACE CHANNEL( OPEN PH) . OUTFALL STRUCTURE

G) OPEN POTHEAD YARD (FOR UG PH) , SWITCHYARD (FOR OPEN PH)

H) VARIOUS CONSTRUCTION TUNNELS ( ADITS) - SOME ARE PERMANENT ACCESS , OTHERS WITH WATER CONDUCTOR SYSTEM PLUGGED , ADIT PLUGS


J) TRANSMISSION TOWER ( LAST STRUCTURE OF HEP ) TO TRANFER POWER TO POWER GRID OF STATE GOVT

[vikram.jeet]

HYDROPOWER STRUCTURES :

更多细节请参见页面号39 TO 45 UNDER THIS POST SUBJECT ' SOME GENERAL VIEW'.

[vikram.jeet]

PROFESSION OF STRUCTURAL ENGINEERING
(As penned by an Essay Writer )

Structural Engineering is a field concerned with doing RESEARCH , MAKING PLANS , DESIGNING , CONSTRUCTING , INSPECTING , MONITORING , REHABILITATING , DEMOLISHING of both Temporary and Permanent Structures along with their COMPONENTS and STRUCTURAL SYSTEMS.

It also considers various aspects of Structures like ECONOMIC , TECHNICAL , AESTHETIC , ENVIRONMENTAL and SOCIAL aspects.

This is an IMAGINATIVE PPROFESSION which makes significant contributions to INDUSTRY , INFRASTRUCTURE as well as DEVELOPMENT of RESIDENTIAL , BUSINESS , RECREATIONAL BUILDINGS.

[vikram.jeet]

Wisdom from Structural engineer :

It is seen that many a times,, Structural engineer got rigid in provision of steel strictly as per what is calculated by him,, though it is good practice,, but certain situations demand wisdom from him beyond calculations.

在以下情况下SE需要利比亚问题ral in provision of steel :
I) Cantilever Slabs - Site , even in good supervision works,, the possibility of sinking down of top bars is very much there and little higher steel provision beyond calcs help.

(ii) Cantilever Beams : Though there is no chance of sinking of reinf,, little higher steel help in any unforseen loadings,, e.g Balconies -- provided with planters in big Gamlas,, Also chances of covering Balconies to make small room,, by constructing half brick walls, especially in Society / Development Authority Flats and so on.

(iii) + ve Moment reinf in all Beams :
Since in computer age,, the analysis is not being carried on additional cases,,ie with Alternate span loaded cases,, , The +ve moment indicated from analysis is All span loaded simultaneously case. Secondly in beams especially end spans of beams, the reinf to be provided beyond calcs in view of fact that support moment from end support may not be same due to possibility of non provision of required Ld embedment of beam top reinf at end support column. Even otherwise +ve span moment is ( WL^2/8 less supports mo contribution) ,, the support moments are dependent on joints compatibility ,, the net moment at span, if liberal,, may take care of approximations involved in compatibility /analysis.

(to be continued)

[Dr. N. Subramanian]

In cantilevers(beams or slabs), we normally calculate the short
Term deflections. Due to the Creep of concrete, there will be long term deflections, which may be about twice the short term deflections. Hence it is better to check the section for long term deflection.

[vikram.jeet]