Transcript ADVANCED STEEL DESIGN

Class 4
Applying Loads to
Buildings
Loads to study
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Dead loads
Live loads
Wind loads
Flood loads
Seismic loads
Snow loads
Rain loads
Extraordinary loads
Tributary area concept
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Tributary area concept
 Area to which the load is applied
 Done progressively through building from
top to bottom
 Required to size specific members
 Loads collected until soil support is
determined
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Examples
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Our First Analysis –
Overall Building Stability
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Uplift
Sliding
Overturning
We do this to make sure the overall
building will be stable before we concern
ourselves with the details
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Uplift
 Vertical load path
 Does weight of building and elements
exceed uplift force (primarily wind)?
 Not exceeding does not suggest failure –
it suggests that specific connections must
be made to hold the element and/or
building in place
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Uplift design process
 Dead load(0.6) ≥ Σ uplift forces (vertical)
(ASD load combination for wind)
 Calculate vertical component of wind
since wind is applied normal to the
surface
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Example building
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Example
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 Need to look at
 building/foundation connection
 building with foundation attached
 Can not take full value of weight per
ASCE 7 (see Chap 2)
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Sliding
 Weight of building and friction create the
resistance to sliding
 Again, can only use 0.6 x dead load (for wind)
 Friction occurs between the building and the
foundation or the entire building and the soil
 Must determine the frictional resistance factor
Φ
 Frictional resistance = ((tan Φ x N) + passive
soil resistance))
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Example
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 Again, must take a look at:
 Building to foundation connection
 Building with foundation attached at the
foundation/soil interface
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Overturning
 Apply overturning moments about a pivot
point
 Resisting moment can only use 0.6 x
dead load (for wind)
 Overturning forces must be split into
horizontal and vertical components and
multiplied by appropriate moment arm
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Example
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 Most likely failure mode is connection of
building to foundation
 Not likely the entire foundation will
overturn
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Worked example
 Work the CCM problem with crawlspace
foundation
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Dead loads
 ASCE 7 – Chapter 3
 Commentary Table C3-1
 Must determine building weight 1 of 2
ways
 Individual components unit weight x total
number of units in building summed or
 System unit weight x area of systems
summed
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Dead weight
 Review the methods by using the weights
from Chap 3 ASCE 7
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Examples of material
weights
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Live loads
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ASCE Chapter 4 and Table 4-1
Concentrated loads
Loads on handrails, etc
Live load reduction
Common live loads
MINIMUMS!
Read Chapter C4 in ASCE 7
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Table from IBC & Chap 4
ASCE 7
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Building code min. loads
 Residential –
 1st floor – 40 psf
 Bedrooms – 30 psf
 Balconies/decks – 60 psf
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Snow loads – Chap 7
ASCE 7
 Flat roof snow load
 Sloped roof snow load
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Definitions
 pg – ground snow load from map
 ce – exposure factor
 ct – thermal factor
 cs – slope factor
 γ – snow density pcf
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ASCE 7 pg map
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Snow design
considerations
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Cold roof
Curved roof
Warm roof
Unbalanced snow loads
Snow drifts
Sliding snow
Rain on snow
Ponding on flat roofs
Roof deflections
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Review ASCE 7 cases
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Figure 7-2
Figure 7-3
Figure 7-9
Tables 7-1,2,3,4
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Rain loads – Chap 8 ASCE
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 Only applied when rain can be ‘collected’
 Flat roof – most difficult design situation
 Very shallow sloped roof when gutters or
drains can back up
 γ = 62.4 lb/ft3
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Homework 3
 Read Chapters 4, 7, 8 ASCE and
associated commentary
 Homework 3 on web site – due in one
week
 Due 9/19/07
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