ENG1001 Lecture Notes - Lecture 1: Net Force, Structural Load
30 May 2018
School
Department
Course
Professor
!"#$
%&'()
*##+,$-.
Point&(transv&
force)
UDL&(transv&
force)
Applied&
moments
/+*00
%1*20$.
Dead
Live
3',4561'41$457*5$- +'*-
=&a&load&that&can&be&considered&to&be&acting&@&a&single&pt
(eg.&person&sitting&on&a&beam)
89: %24,;'7<+")-,057,=25$-)+'*-.
=&a&load&w/&constant&magnitude&along a&structural&memb
Most&commonly&self-weight
(eg.&a&shelf&supporting&food&cans&along&a&length&of&500mm)
Unit&=&kg/m&[mass]&or&kN/m&[wt]
*take&g&=&10m/s2
>##+,$-)<'<$45
No&net&force&applied&to&beam,&as&2&horiz&forces&cancel&out
=&trying&to&rotate&beam&abt&pt of&application
Either&a&bending or&twisting actn,&commonly&caused&by&another&elem&of&struc
For&2D,&rep&by&curved&arrow
For&3D,&rep&by&double-headed&arrow
Direction&gives&dir&of&applied&moment,&acc&to&RH&screw&rule
Select&axis&you&are&summing&applied&moments&abt&(i.e.&the&axis&abt&
which&you&are&checking&for&rotational&eq)
1.
Wrap&RH&arnd&this&axis,&w/&thumb&along&+ve&x2.
Direction&defined&by&fingers&=&+ve&applied&moment&direction3.
9$*- +'*-
=&self-weight.&Basically&how&much&the&struc&weighs,&permanent&fixtures
Vertical,&permanent
Calculate&vol&of&all&materials&used&in&bridge1.
Multiply&by&density&of&materials2.
*must&design&to&know&weight!
All&design&must&start&w/&est&of&size,&shape,&density&of&struc&to&est&dead&load
BUT material&density&can&be&variable!&So&we&need…
/&*7*15$7,05,1 -$*-)+'*-
=&a&value&of&material&density&chosen&to&only&have&5%&chance&of&exceeding
Normal&(Bell)&Dist&Curve
Symm&abt&mean/avg•
Std&deviation&used&to&indicate°ree&of&spread&of&values&frm&mean•
?@Aof&values&occur&within&mean&+-sd•
90%&of&values&occur&within&mean&+-1.650
Accepted&prac&to&choose&value&of&density&that&only&has 5% chance&of&
being&exceeded&(ie&mean&+-1.650)
○
On&avg,&1&design&in&20&will&have&material&density&<&actl&material
○
•
/&*7*15$7,05,1)+'*-0 =&all&unfactored&loads,&have&
5%&chance&of&being&exceeded
:'*-);*15'70 =&added&to&characteristic& loads&to&
increase&safety
:,<,5)05*5$)-$0,B4
(interchangeable&with&"force")
Vertical&forces&resulting&frm&gravity
C7*D,5" =&gives&weight&to&objects
○
Actually&an&acceleration&of&9.81m/s2
○
Unit:&N&(kgm/s2)
○
•
E$D,1$*=,+,5" +,<,5)05*5$
<&1.0&>>>&load&factor for&limit&state&of&deflection
User&must&be&comfortable&with&performance&(defl)&of&struc
Increase&chance&of&design&load&being&exceeded&frm&1&
in&20&to&1&in&5/10
1.0G&+&0.7Q
Deflection•
Vibration•
Fatigue&checks&(including&reparable&damage&due&to&
fatigue)
•
Corrosion•
Fire•
F&")1&*7*15$7,05,1)+'*-0),<#5G
Structures&may&be&exposed&to&a&wide&range&of&loading&conditions,&some&of&
which&may&be&extreme&(eg.&earthquakes&and&fires)&and&all&have&a°ree&of&
uncertainty.
SO&we&design&for&&,B&$7H$4-6$I57$<$ loads,¬&avg&loads.
ALL&LOADS&INVOLVE&UNCERTAINTY,&impossible&to&anticipate&all&possible&
variations&of&load.
For&eg,&dead&loads&may&vary&due&to:
Over/under-est&densities•
Inconsistent&materials&used&on-site•
Variation&in&constructn&tolerances&and&workmanship•
~~~
:,<,5)05*5$0
=&conditions&that&could&cause&struc&to&fail
=&describes&when&structure&is&on&the&verge&of&becoming&unfit&for&use,&as&a&
result&of:
Collapse&>>&severe&conseq,&extreme&load&events•
Excessive&deflection&>>&less&severe&conseq,&avg&load&events•
Disruptive&vibration&>>¬&severe&conseq,&load&events•
Fire&resistance&>>&severe&conseq,&load&events•
*since&seriousness&of&consequences&for&each&condition&varies,&acceptable&risk&
of&exceeding&limit&state&varies&too
Variable&risk&achieved&by&using&diff&load&factors&for&diff&limit&states
:'*-);*15'70
Applied&to&characteristic&loads&to&increase&safety,&calling&them:
Design&loads (load&factors&*&characteristic&loads)
(two&criteria&in&Limit&State&Design&method)
8+5,<*5$ +,<,5)05*5$
1.2-2&>>>&load&factor for&limit&state&of&collapse
Structure&must¬&collapse&when&subjected&to&peak&design&load&designed&for
Reduces&chance&of&design&load&being&exceeded&frm&1&in&20&to&1&in&
100,000
1.2G&+&1.5Q
Strength&(yield,&buckling)•
Stability&against&overturning&and&sway•
Fracture&due&to&fatigue•
Plastic&collapse•
Brittle&fracture•
:,D$ +'*-
=&includes&any&temporary/transient&loads&that&act&on&a&struc&(eg.&ppl,&
furniture,&vehicles)
/&*7*15$7,05,1 +,D$)+'*-
=&a&value&of&material&density&chosen&to&only&have&5%&chance&of&being&
exceeded&once&in&50&years
Accepted&prac&to&choose&value&of&density&that&only&has 5% chance&of&being&
exceeded&once&in&50&years
(50&years&=&design&life&of&most&buildings)
:,D$)+'*-)#7'=*=,+,5")-$40,5")127D$ spreads&further
Live&load&values&will&have&LARGE&variation
Std&dev&higher
Eg.&
Domestic&houses:&1.5kPa&(150kg/m2)
Carparks:&3.0kPa&(300kg/m2)
Offices:&3.0kPa&(300kg/m2)
Grandstands:&5.0kPa&(500kg/m2)
Week$1:$Loads
Saturday,&22&July&2017
16:17
!"#$
%&'()
*##+,$-.
Point&(transv&
force)
UDL&(transv&
force)
Applied&
moments
/+*00
%1*20$.
Dead Live
3',4561'41$457*5$- +'*-
=&a&load&that&can&be&considered&to&be&acting&@&a&single&pt
(eg.&person&sitting&on&a&beam)
89: %24,;'7<+")-,057,=25$-)+'*-.
=&a&load&w/&constant&magnitude&along a&structural&memb
Most&commonly&self-weight
(eg.&a&shelf&supporting&food&cans&along&a&length&of&500mm)
Unit&=&kg/m&[mass]&or&kN/m&[wt]
*take&g&=&10m/s2
>##+,$-)<'<$45
No&net&force&applied&to&beam,&as&2&horiz&forces&cancel&out
=&trying&to&rotate&beam&abt&pt of&application
Either&a&bending or&twisting actn,&commonly&caused&by&another&elem&of&struc
For&2D,&rep&by&curved&arrow
For&3D,&rep&by&double-headed&arrow
Direction&gives&dir&of&applied&moment,&acc&to&RH&screw&rule
Select&axis&you&are&summing&applied&moments&abt&(i.e.&the&axis&abt&
which&you&are&checking&for&rotational&eq)
1.
Wrap&RH&arnd&this&axis,&w/&thumb&along&+ve&x
2.
Direction&defined&by&fingers&=&+ve&applied&moment&direction
3.
9$*- +'*-
=&self-weight.&Basically&how&much&the&struc&weighs,&permanent&fixtures
Vertical,&permanent
Calculate&vol&of&all&materials&used&in&bridge1.
Multiply&by&density&of&materials2.
*must&design&to&know&weight!
All&design&must&start&w/&est&of&size,&shape,&density&of&struc&to&est&dead&load
BUT material&density&can&be&variable!&So&we&need…
/&*7*15$7,05,1 -$*-)+'*-
=&a&value&of&material&density&chosen&to&only&have&5%&chance&of&exceeding
Normal&(Bell)&Dist&Curve
Symm&abt&mean/avg•
Std&deviation&used&to&indicate°ree&of&spread&of&values&frm&mean•
?@Aof&values&occur&within&mean&+-sd•
90%&of&values&occur&within&mean&+-1.650
Accepted&prac&to&choose&value&of&density&that&only&has 5% chance&of&
being&exceeded&(ie&mean&+-1.650)
○
On&avg,&1&design&in&20&will&have&material&density&<&actl&material
○
•
/&*7*15$7,05,1)+'*-0 =&all&unfactored&loads,&have&
5%&chance&of&being&exceeded
:'*-);*15'70 =&added&to&characteristic& loads&to&
increase&safety
:,<,5)05*5$)-$0,B4
(interchangeable&with&"force")
Vertical&forces&resulting&frm&gravity
C7*D,5" =&gives&weight&to&objects
○
Actually&an&acceleration&of&9.81m/s2
○
Unit:&N&(kgm/s2)
○
•
E$D,1$*=,+,5" +,<,5)05*5$
<&1.0&>>>&load&factor for&limit&state&of&deflection
User&must&be&comfortable&with&performance&(defl)&of&struc
Increase&chance&of&design&load&being&exceeded&frm&1&
in&20&to&1&in&5/10
1.0G&+&0.7Q
Deflection•
Vibration•
Fatigue&checks&(including&reparable&damage&due&to&
fatigue)
•
Corrosion•
Fire•
F&")1&*7*15$7,05,1)+'*-0),<#5G
Structures&may&be&exposed&to&a&wide&range&of&loading&conditions,&some&of&
which&may&be&extreme&(eg.&earthquakes&and&fires)&and&all&have&a°ree&of&
uncertainty.
SO&we&design&for&&,B&$7H$4-6$I57$<$ loads,¬&avg&loads.
ALL&LOADS&INVOLVE&UNCERTAINTY,&impossible&to&anticipate&all&possible&
variations&of&load.
For&eg,&dead&loads&may&vary&due&to:
Over/under-est&densities•
Inconsistent&materials&used&on-site•
Variation&in&constructn&tolerances&and&workmanship•
~~~
:,<,5)05*5$0
=&conditions&that&could&cause&struc&to&fail
=&describes&when&structure&is&on&the&verge&of&becoming&unfit&for&use,&as&a&
result&of:
Collapse&>>&severe&conseq,&extreme&load&events•
Excessive&deflection&>>&less&severe&conseq,&avg&load&events•
Disruptive&vibration&>>¬&severe&conseq,&load&events•
Fire&resistance&>>&severe&conseq,&load&events•
*since&seriousness&of&consequences&for&each&condition&varies,&acceptable&risk&
of&exceeding&limit&state&varies&too
Variable&risk&achieved&by&using&diff&load&factors&for&diff&limit&states
:'*-);*15'70
Applied&to&characteristic&loads&to&increase&safety,&calling&them:
Design&loads (load&factors&*&characteristic&loads)
(two&criteria&in&Limit&State&Design&method)
8+5,<*5$ +,<,5)05*5$
1.2-2&>>>&load&factor for&limit&state&of&collapse
Structure&must¬&collapse&when&subjected&to&peak&design&load&designed&for
Reduces&chance&of&design&load&being&exceeded&frm&1&in&20&to&1&in&
100,000
1.2G&+&1.5Q
Strength&(yield,&buckling)•
Stability&against&overturning&and&sway•
Fracture&due&to&fatigue•
Plastic&collapse•
Brittle&fracture•
:,D$ +'*-
=&includes&any&temporary/transient&loads&that&act&on&a&struc&(eg.&ppl,&
furniture,&vehicles)
/&*7*15$7,05,1 +,D$)+'*-
=&a&value&of&material&density&chosen&to&only&have&5%&chance&of&being&
exceeded&once&in&50&years
Accepted&prac&to&choose&value&of&density&that&only&has 5% chance&of&being&
exceeded&once&in&50&years
(50&years&=&design&life&of&most&buildings)
:,D$)+'*-)#7'=*=,+,5")-$40,5")127D$ spreads&further
Live&load&values&will&have&LARGE&variation
Std&dev&higher
Eg.&
Domestic&houses:&1.5kPa&(150kg/m2)
Carparks:&3.0kPa&(300kg/m2)
Offices:&3.0kPa&(300kg/m2)
Grandstands:&5.0kPa&(500kg/m2)
Week$1:$Loads
Saturday,&22&July&2017 16:17
Document Summary
Load factors = added to characteristic loads to increase safety. = a load that can be considered to be acting @ a single pt (eg. person sitting on a beam) = a load w/ constant magnitude along a structural memb. Most commonly self-weight (eg. a shelf supporting food cans along a length of 500mm) No net force applied to beam, as 2 horiz forces cancel out. = trying to rotate beam abt pt of application. Either a bending or twisting actn, commonly caused by another elem of struc. Direction gives dir of applied moment, acc to rh screw rule. Select axis you are summing applied moments abt (i. e. the axis abt which you are checking for rotational eq) Wrap rh arnd this axis, w/ thumb along +ve x. Direction defined by fingers = +ve applied moment direction. Basically how much the struc weighs, permanent fixtures. Calculate vol of all materials used in bridge.
