nbcc-loadcombos[1]

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1 Loads and Load Combinations for NBCC Prepared by Dr Michael Bartlett, P.Eng University of Western Ontario Presented with minor modifications by Dr Robert Sexsmith, P.Eng. University of British Columbia CSCE Workshop – NBCC Loads and Seismic Design 2004 June 10, Calgary AB Outline Companion Action Principle Review of NBCC 1995 Snow Load Factor Return Period on Environmental Loads NBCC 2004 Provisions Dead Loads & Load Effects Load Combinations Impact on Design Load Effects Summary

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Page 1: NBCC-loadcombos[1]

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Loads and Load Combinations for NBCCPrepared by

Dr Michael Bartlett, P.Eng

University of Western Ontario

Presented with minor modifications by

Dr Robert Sexsmith, P.Eng.

University of British Columbia

CSCE Workshop – NBCC Loads and Seismic Design

2004 June 10, Calgary AB

Outline• Companion Action Principle

• Review of NBCC 1995– Snow Load Factor– Return Period on Environmental Loads

• NBCC 2004 Provisions– Dead Loads & Load Effects – Load Combinations– Impact on Design Load Effects

• Summary

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NBCC 1995 Format

�R > �D D + ������L L + �Q Q + �T T�where � = load combination factor

� = importance factor

• NBCC 1995 Load Combinations:1.25 D + 1.5 L1.25 D + 1.5 Q (wind)1.25 D + 0.7 {1.5 L + 1.5 Q (wind)}

= 1.25 D + 1.05 L + 1.05 Q (wind)

note that snow is (was!) included with live

Reminder

• These are combinations of EFFECTS– ie Axial force in a column, moment in part of a frame,

etc

A – we can apply to the structure the combined factored load and find the resulting effect

OR

B- we can apply unfactored loads of each type, and then calculate the combined factored effect. This is easier to automate, but only valid for linear structures.

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Time History of Loading

time

Load

DeadRenovation

SustainedLive

Snow

TransientLive/Wind

Maximum Load

Turkstra’s Rule (early ’80s)

Worst case of combined transient loads occurs when:

• one load, the principal action, is its extreme value

• other loads, the companion actions, are the largest that would be expected while the principal action has its extreme value

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Companion Action Format

�R > �D D + �i Si + ���ik Sk , i k

where Si = principal actionSk = companion actions

Typical Load Combinations:

1.25 D + 1.5 L + 0.4 W (wind)

1.25 D + 1.4 W (wind) + 0.5 L

Companion Action Format

• Better represents the situation of one extreme event with the other loads that may be acting

• Permits logical extensions for special cases

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Designer can Envisage Hazards

• Correlation of transient loads explicitly considered

• Can you imagine a structure where simultaneous maximum values of transient loads are:

– unlikely?– expected?

1995 NBCC Reliability Indices

0

1

2

3

4

0 1 2 3 4 5

Nominal variable load/nominal dead load

Rel

iab

ility

ind

ex (

50-y

ear)

D + W

D + L

D + S

• reliability for snow load deficient?

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2000/2001 Failures: Sarnia Mall

Source: Globe and Mail 2000 December 09

Collapse

Return Period for Environmental Loads

• NBCC 1995 specifies:– 30 years for specified Snow, Wind– 10 years for Wind for Deflections– 100 years for wind on Important Structures

• Use 50 year or 500 year return periods (only) for 2004 NBCC?

• Ratio n-yr/30-yr depends coefficient of variation of annual maximum load

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Specified Load Return Period

0

0.5

1

1.5

2

0.00 0.05 0.10 0.15 0.20 0.25 0.30COV of Maximum Annual Wind Velocity

n-y

ear

pre

ssu

re /

30-y

ear

pre

ssu

re

90% of values in this range

mean1-in-10

1-in-501-in-100

1-in-500

Note: if we used 1/500 values we could drop the load factor

NBCC 2004 Calibration Process

1. Reliability indices for 1995 NBCC

2. Preliminary load combinations for 50-yr, 500-yr loads by Bartlett, Hong & Zhou

• review by Part 4 Task Group on Snow & Wind Loads

• review by Part 4 Standing Committee

3. Revised load combinations, 50-yr loads• review by Task Group and Part 4

committee• public review

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Proposed 1.2 D Criticized

• History: 1.3 proposed for 1975 NBCC reduced to 1.25 to maintain same ratio of dead/live load factor as in ACI 318-71

• Specific concerns:– floor thickness variability– dead load of soil & landscaping– tributary area computation

2000 Survey:Concrete Floor Thickness

• Marked variability for– Cast-in-place toppings on precast– Cover slabs in unshored composite

construction (no specified tolerances?)• “Uncertain D” with load factor of 1.5

considered but not adopted

• Make allowances for extra dead load• Consider deflections of supporting members

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Tributary Areas in NBCC

Commentary G:lines of zero shearhalfway betweencolumn lines

• Safe for corners• Unsafe for 1st

interior columns

a (typ)0.5a0.55a

0.55a

0.5a

Note: problem with low D factor when “approximations” are used

2004 NBCC Combinations

1.4 D1.25 D + 1.5 L + (0.4 W or 0.5 S)1.25 D + 1.4 W + (0.5 L or 0.5 S)1.25 D + 1.5 S + (0.5 L or 0.4 W)0.9 D + (1.5 L or 1.4 W or 1.5 S)

Add to all combinations:P = prestressH = horizontal earth pressuresT = restrained deformations (safety)

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50-yr Wind & Snow Specified

• typically ~10% greater than 30-yr values

• snow load factor initially 1.7, implies a 25% increase in factored load, deemed too big.

• modify for importance categories based on use & occupancy

• reduce for SLS checks

Importance Factors for S, W

Importance Ultimate Serviceability Category (Snow or Wind) Snow Wind

Low 0.8 0.9 0.75

Normal 1.0

High 1.15

Post Disaster 1.25 0.9 0.75

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Impact: Single Transient Load

0.98

1.00

1.02

1.04

1.06

1.08

1.10

0 1 2 3 4 5

Transient-to-dead load ratio

Pro

pose

d/N

BC

C 1

995

D+W

D+S

D+L

Up to 8% increase for high

snow/dead ratio

Up to 2% increase for high wind/dead ratio

Uncoupling Snow & Live

• Members resisting low D, high L (use + occupancy), high S require less resistance.

• Logical consequence of considering Live and Snow as independent

• Similar format adopted in ASCE-7 based on load combinations derived in 1980

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Impact: D+L+S

0.7

0.8

0.9

1.0

1.1

1.2

0 1 2 3 4 5

Live-to-dead load ratio, L/D

Pro

po

sed

/199

5 N

BC

C

S/D = 0.250.5

1.0

2.0

4.0

Impact: D+L+W

0.7

0.8

0.9

1.0

1.1

1.2

0 1 2 3 4 5

Live-to-dead load ratio, L/D

Pro

po

sed

/199

5 N

BC

C

W/D = 4.0

2.01.00.25 0.5

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Impact: D+W+S

0.7

0.8

0.9

1.0

1.1

1.2

0 1 2 3 4 5

Snow-to-dead load ratio

Pro

po

sed

/199

5 N

BC

C

W/D = 4.02.01.00.50.25

Summary

1. Companion action load combination format proposed for NBCC 2004:

• more realistic representation

• permits logical decisions for unusual cases• little difference for many members• consistent with other international

standards (ACI 318, AISC LRFD, etc.)

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2. Dead loads:• make allowance for extra thickness of thin

toppings• tributary areas for first interior columns

3. Snow loads are no longer classified with live loads due to use and occupancy.

• less resistance needed for members carrying snow and live loads

4. Only 50 year environmental loads specified:

• increases specified loads by ~ 10%• additional increases for important and

post-disaster buildings• load factors less than 1.0 reduce specified

loads for serviceability checks.

5. New load combinations give similar demands to NBCC 1995:

• less demand due to snow & live loads

• more demand due to snow only

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6. New load combinations have been reviewed by various committees

• Additional references: papers by Bartlett, Hong & Zhou in Canadian Journal of Civil Engineering

Acknowledgements• National Research Council of Canada

• Natural Sciences & Engineering Research Council of Canada

• NBC Part 4 Task Group on Snow and Wind Loads (D. E. Allen, Chair)

• Canadian Meteorological Centre• Steel Structures Education Foundation

• J. G. MacGregor• Michael Bartlett (who prepared this

presentation and permitted its use for this CSCE short course)