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Building Code Complexity

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This email is submitted for general comments on building code complexity.
(Comments on specific code requirements are also welcome.) 

I have been developing design requirements for a single story pre-engineered
metal building for a water treatment plant in New England. The structure is
intended to be a concentrically braced steel frame for lateral loads,
although rigid moment frames will be used to resist vertical loads. The
lateral bracing is intended to avoid seismic requirements for moment frames
resisting earthquake forces. Although we have requested preliminary
foundation reactions from local pre-engineered metal building suppliers,
they will not provide this information for "free". Although we would be
willing to pay for a preliminary design, legal counsel has said that if we
pay a supplier for a design, that supplier should be excluded from bidding
as they would have a competitive advantage over other bidders (for a public
works project). Thus we plan to develop our own preliminary design to get
foundation reactions. 

Developing the required design load cases has been a lesson in building code
complexity. The governing building code is the 2000 IBC. The building is an
L-shaped building and will have non-symmetrical bracing and irregular bay
sizes, due to equipment layout. I have currently come up with a "minimum" of
83 load cases to be evaluated! (I guess I can understand why the building
manufacturers no longer want to design for free.) 

FYI, following is a summary of the code requirements. 

Seismic Design: 

Basic Seismic Design Parameters per IBC 2000: 
	- Seismic Use Group III, Importance Factor = 1.50 (per IBC Table
1604.5, required for fire suppression) 
	- Site Class D (based on Soil Profile, per Geotech Report and IBC
Table 1615.1.1) 
	- Sds = 0.401; Sd1 = 0.163 (based on zip code) 
	- Seismic Design Category D (per IBC Tables 1616.3(1)&(2))

Per IBC Section 2212.1.2, the steel structure must conform with AISC Seismic
Provisions, Part I (LRFD) or Part III (ASD). 

It is proposed to use AISC Seismic Provisions Part I, Section 14.5 for Low
Buildings (recommended by a building manufacturer). Members and connections
are to be designed for Load Combinations 4-1 and 4-2 (with overstrength
factors), as given in Section 4.1 of Part I. It should also be noted that
Part III for Allowable Stress Design uses the same load combinations, so
these may be used for LRFD or ASD. In these equations, the required load
factor for seismic (omega-zero) is 2.0 (i.e., 2.0E is used). Orthogonal
effects need not be considered with these load combinations. However,
standard load combinations using 1.0E must also be analyzed, including
orthogonal effects. Thus, there are 2 standard load combinations and 2
special load combinations for seismic effects. 

Note: Where Allowable Stress Design is used in accordance with Part III of
the AISC Seismic Provisions, allowable stresses must be factored up by a
factor of 1.7 and then multiplied by a resistance factor which varies by
type of stress and component (e.g., flexure vs shear, tension vs
compression, welds vs bolts, etc). ASD load combinations include the same
load factors as for LRFD. Thus in effect, the same analysis is performed for
LRFD and ASD but different formulas are used for effective strengths. One
effect of this is that standard allowable stress design checks built into
software for ASD design, cannot be used directly since the varying
resistance factors have not been incorporated. Thus member design must be
done manually from the analysis results for forces and stresses, when ASD is
used. However, if the software includes LRFD code requirements, that may be
used for member design. (I guess AISC will succeed in forcing LRFD upon us.)

Due to the L-shaped building, plan irregularities must be accounted for in
accordance with IBC Table 1616.5.1. Type 1a or 1b irregularity must be
evaluated due to differential story drift at the long end of the building vs
the short end of the building. Type 2 irregularity must be evaluated due to
the re-entrant corner. (Type 5 irregularity may also apply due to bracing
not being symmetrical about the major axes.) 

IBC Table 1616.6.3 permits the equivalent lateral force method to be used
"with dynamic characteristics included in the analytical model". Equation
16-35 will be used to determine the seismic response coefficient (i.e., the
peak seismic response; Equation 16-36 does not govern based on the estimated
period per 1617.4.2.1). Redundancy must be evaluated in accordance with IBC
Section 1617.2.2. Torsion must be evaluated in accordance with IBC Section
1617.4.4 (including torsion due to mass and stiffness distribution,
accidental torsion, and dynamic amplification of torsion). Where orthogonal
effects must be evaluated, design must conform to IBC Section 1620.2.2. 

Seismic effects should be evaluated in two directions for each orthogonal
axis, to get the maximum tension and compression reactions at braces and at
foundations. Furthermore, positive and negative accidental torsion should be
evaluated for each of these cases. This results in 4 basic load cases * 2
axes * 2 directions * 2 torsion eccentricities = 32 load cases. Additional
combined load cases must also be evaluated on a case-by-case basis to
account for orthogonal effects (such as where orthogonal braces attach to a
single column base). 

Story drift must be evaluated in accordance with IBC Table 1617.3 (max
allowable drift = 0.010*h) and Section 1617.4.6; but for drift the
redundancy coefficient should be equal to 1.0. If the redundancy coefficient
calculated from IBC Section 1617.2.2 is greater than 1.0, additional load
cases should be evaluated. It is proposed to select the most critical cases
for building deformation from the original strength design load cases by
judgment and evaluate drift as needed.  

Wind Design Parameters per IBC 2000: 

Wind Load: 
	- Design per ASCE 7-98, Chapter 6, Method 2 - Analytical Procedure 
	- Basic Wind Speed = 100 mph, Importance Factor = = 1.15 
	- Exposure Category B (wooded area) 
	- Topographic Factor, Kzt = 1.0 

Note that this building is not a "simple diaphragm" building or "regular
shaped building" due to lack of symmetry. There are 3 basic load
combinations in the 2000 IBC including wind. However in accordance with ASCE
7-98, cases with positive and negative internal pressures must be considered
and, as shown in Figure 6-4 of ASCE 7-98, two cases must be evaluated for
each of the four building faces (similar to "accidental torsion" for seismic
design). This results in 3 basic load cases * 2 internal pressures * 4 faces
* 2 cases = 48 load cases. 

Additional Basic Load Cases: 

There are 3 basic load combinations to account for dead load, floor live
load, and roof snow load, without lateral wind or seismic loads. Although
there are no "floor live loads" acting on our building frame, one bay will
be affected by monorail loads. Also, collateral loads should be included to
account for lighting and HVAC hanging from the ceiling. 

Snow Design Parameters per IBC 2000: 
	- Ground Snow Load = 50 psf, Importance Factor = 1.20 
	- Terrain Category B, Partially Exposed, Thermal Factor = 1.1 at
process area and 1.0 at admin area 
	- Roof slope varies 4:1 or 8:1 (14-deg to 7-deg) 

For gable roofs, balanced and unbalanced snow loads must be evaluated and
wind from all directions must be accounted for to determine unbalanced
loads. Thus 4 wind directions must be evaluated for unbalanced snow
conditions plus one full balanced snow load case, for 5 snow load cases.
However, the 4 unbalanced snow load conditions should be able to be included
in wind load combinations already accounted for. 

It appears that this single story steel building design will require 32
seismic load cases, 48 wind load cases, and 3 basic load combinations, as a

(For reference, for a rectangular building with symmetrical bracing, the
load cases could be reduced to 23: the seismic and wind load cases could
each be reduced by a factor of 4, including a factor of 2 by avoiding
torsional eccentricities in two directions for each case and a factor of 2
by avoiding loads in each direction for north-south or east-west load cases.
Results for the most critical stresses on one side of the building would be
used for the opposite side of the building.) 

William C. Sherman, PE
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

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