Sizing of Pile Caps(Part-2)

*Sizing of Pile Caps(Part-2)*
Pile cap ဆိုတာ Single Footing နဲ႕
သိပ္မကြာလွပါဘူး ... ကြာသြားတာက

‪#‎Difference‬:1
Single/isolated footing ေအာက္မွာ
အမာခံ Sufficient Soil Bearing ရွိျပီး
အ့ဲဒီ Soil pressures က footing ကို
Uniformly distributed Loadအျဖစ္
ညီညီညာညာ uplift လုပ္ေပးပါတယ္
Cap ရဲ႕ေအာက္မွာက Pile တစ္ခု(သို႕)
Group လိုက္ ရွိျပီး အ့ဲဒီ Pile က ျပန္ျပီး
ေထာက္ကန္ေပးတ့ဲ Reaction ေတြက
Cap ကို Concentrated point load
အျဖစ္ သီးျခားစီ uplift ျပန္တြန္းပါတယ္
#Difference:2
Cap ေအာက္က Bearing Capacity က
Single Footing ထက္ပိုမ်ားတတ္ပါတယ္
Cap ရဲ႕ေအာက္က ေထာက္ထားတ့ဲ
Pile အားလံုးရဲ႕ Capacity Reactions
ေတြကို Cap ရဲ႕ Area နဲ႕ျပန္စားၾကည့္ရင္
ဥပမာ - DL+LL = 450 tons
9'x9' pile cap
number of piles, n = 9
one pile capacity = 30 tons
Pile reactions = 9x50 = 450 tons
Upward force = Downward force
So, 9 piles can resist DL+LL.
for comparison only,
Q = 9x50/ (9x9) = 5.55 tsf
9'x9' square footing
Q = DL+LL/ Area
Q = 450/ (9x9) = 5.55 tsf
Point to remember-
‪#‎ေရာသြားမွာစိုးလို႕‬ Shallow
Foundation ျဖစ္တ့ဲ Square footing
Design လုပ္တ့ဲ foundation depth
level မွာ 5.55 tsf ရဖို႕ခဲယဥ္းပါတယ္
အ့ဲဒီ Depth မွာ 5.55 tsf ရခ့ဲရင္ ေတာ့ Deep foundation ျဖစ္တ့ဲ Pile မလိုပဲ
9'x9' square footing နဲ႕တင္ ရပါျပီ)
Q = 1 tsf ရတယ္ဆိုပါေတာ့
Area = DL+LL/ Q
Area = 270/ 1 = 270 Sft
L x B = 16.4' x 16.4'
Pile 9 လံုးခံထားတ့ဲ 9'x9' cap တစ္ခုက
DL+LL = 270 tons ခံႏိုင္ တ့ဲအခ်ိန္မွာ
DL+LL = 270 tons ေလာက္ကိုခံႏိုင္ ဖို႕
16.4'x16.4' ေလာက္ၾကီးတ့ဲ Square
Footing လိုလိမ့္မယ္လို႕ေကာက္ခ်က္ခ်ပါတယ္
Note:1
အ့ဲဒါေၾကာင့္ Cap Area က Single
Footing ထက္စာရင္ ငယ္ပါလိမ့္မယ္
Note:2
Cap ရဲ႕ Depth/ Thickness က ေတာ့ ပံုမွန္ Footing ထက္ပိုထူလာတတ္ပါတယ္
Pile reaction ေတြက Point load
အျဖစ္သက္ေရာက္တာက တစ္ေၾကာင္း
Comparison လုပ္ၾကည့္တ့ဲအခါ
Uplift Soil Pressures တန္ဖိုးကလဲ
ပိုမ်ားလာတတ္တာတစ္ေၾကာင္းမို႕လို႕ပါ
Pile Cap အ ေၾကာင္းထပ္တင္ေပးပါမယ္
Design Basis ဗဟုသုတရၾကပါေစဗ်ာ
မွ်ေဝတယ္ဆိုတာ မဂၤလာတစ္ပါးပါ
Aung Myat Thu(TU-Taunggyi)
Millimetre-Training and Design Group
9:55PM, 11.11.2014(Thuesday)
Reference:p.g.586, Chapter(16)
Design of Concrete Structures
14th Edition, Arthur H.Nilson
16.10-Pile Caps
Piles are generally arranged in
groups or clusters, one under
each column.
The group is capped by a spread
footing or cap that distributes
the column load to all piles in
the group.
These pile caps are in most ways
very similar to footings on soil,
except for two features.
#Difference:1
For one, reactions on caps act
as concentrated loads at the
individual piles, rather than as
distributed pressures.
#Difference:2
For another,if the total of all pile
reactions in a cluster is divided
by the area of footing to obtain
an equivalent uniform pressure
(for purposes of comparison only),
it is found that this equivalent
pressure is considerably higher
in pile caps than for spread footings.
This mean that moments,and
particularly ‪#‎shears‬ are also
correspondingly larger, which
requires ‪#‎greater‬ depths than
for a spread footing of similar
horizontal dimensions.