Value Engineering for Economy

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Support of cast stone panels require due consideration to all applied forces. These forces include dead load (self weight) and live loads (wind and seismic). Often the most critical loading will be a combination of dead and live loads using the greater value of wind or seismic forces (if required by local building codes). Keep in mind that dead loads are often applied with a certain amount of eccentricity, which must be taken into account. Under special conditions, temperature and volume changes can cause significant forces, which need to be considered. The following table shows the required formula necessary to determine the design wind pressure.

Wind Loading*
Design wind pressure = Ce x Cq x qs x I

Ce, Combined Height, Exposure and Gust Factor Coefficient

Height Above Average Level of Adjoining Ground, in Feet

 Exposure C(1) Exposure B(2)
0-20 1.2 0.7
20-40 1.3 0.8
40-60 1.5 1.0
60-100 1.6 1.1
100-150 1.8 1.3
150-200 1.9 1.4
200-300 2.1 1.6
300-400 2.2 1.8

*Source: Uniform Building Code, 1988 Edition
1: Most severe, terrain flat and generally open
2: Terrain with surface irregularities 20 feet or more in height

Cq, Pressure Coefficients

Structure or Part Thereof

 Description Cq Factor

Elements & Components:

Wall Elements:

  
 

  All Structures

1.2 Inward
 

  Enclosed Structures

1.1 Outward
 

  Open Structures

1.6 Outward
 

  Parapets

1.3 Inward or Outward

Local Areas at Discontinuities

Wall Corners

2.0 Outward
 

Canopies or overhangs at eaves of rakes

2.8 Outward

qs, Wind Stagnation Pressure at Standard Heights of 30 Ft.

Basic Wind Speed (mph) (1)

 70 80 90 100 110 120 130
Pressure, Qs (psf) 13 17 21 26 31 37 44

1: Refer UBC, Wind Speed Map

 I, Importance Factor

I =

1.15 For Essential Facilities
I =

1.00 For All Other Buildings

Anchorage of cast stone panels to the supporting structure is typically based on the allowable working stress of the anchor material. The working stress for steel is recommended by Structural Steel Designer’s Handbook. The typical working stresses for the cast stone are per the American Concrete Institute for normal weight concrete.

Working Stress for Steel
 
Steel Type Fy psi Ft (1) psi Fb (1) psi Fv psi
ASTM A36 36,000 18,000 18,000 14,000
AISI 302 40,000 18,000 18,000 14,000
AISI 304 42,000 18,000 18,000 14,000

1: Stresses are less than normally allowable to reduce effects of deflection.

Typical Working Stress for Cast Stone
 
Compressive Strength, f'c (psi) Modulus of Rupture, f 'c (psi)
5,000 7.5√f'c = 530
6,000 7.5√f'c = 580
7,000 7.5√f'c = 625

Two types of anchors support cast stone panels.  They are restraint anchors and support anchors.  Support anchors generally carry the self weight of the structure although, they can also function as a combination of support and restraint anchor.   Restraint anchors should be designed for live loads only (ie. wind or seismic forces.  A typical restraint anchor used would be a steel strap.  The following table shows allowable capacities of restraint anchors for two typical straps of various lengths.

Restraint Anchors (A36 Steel)
 
Strap Size Unsupported Length (in) Allowable Tensile Cap. (k) Allowable Compressive Cap. (K)
1/8" x 1" 2 2.7 2.23
3 2.7 1.87
4 2.7 1.44
3/16" x 1" 2 4.0 3.64
3 4.0 3.35
4 4.0 3.00
 

Restraint Anchors (Stainless Steel)
 
Strap Size Unsupported Length (in) Allowable Tensile Cap. (k) Allowable Compressive Cap. (K)
1/8" x 1" 2 2.0 1.62
3 2.0 1.62
4 2.0 0.87
3/16" x 1" 2 3.0 2.4
3 3.0 2.4
4 3.0 2.4

The following table shows the allowable shear capacities for stainless steel dowel pins of various diameters.

Pin Diam. (in) Allowable Shear Cap. (#)
1/4 491
5/16 767
3/8 1,104
7/16 1,503
1/2 1,963
9/16 2,485
5/8 3,068
11/16 3,712
3/4 4,418
13/16 5,185
7/8 6,013
15/16 6,903

The table values list only capacity of the metal anchors.  In most cases capacity of the Cast Stone material will be the limiting design factor.  For specific information pertaining to your project, consult your project engineer of record.