Helpful Links for Determining Minimum Design Loads

Hopefully these links can save you some time and help get you more accurate design loads. A quick heads-up – you will usually need to search the town/county/state to see if the Authority Having Jurisdiction (AHJ) has a specific requirement.

Wind Load

A favorite for Wind Loads in accordance w/ ASCE 7

http://www.atcouncil.org/windspeed/index.php

Seismic

A favorite for determining your ‘base acceleration’ coefficients:

http://earthquake.usgs.gov/hazards/designmaps/

Snow

This site is no longer free but when I used it, it was useful:

http://www.groundsnowbyzip.com/

This is a little dated and really not that useful but I’ll mention it anyway:

http://www.fs.fed.us/t-d/snow_load/states.htm

Others

This is a ‘pay-for’ site but some may use it:

http://www.groundsnowbyzip.com/

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ASCE 7 Seismic Design – Part 1

We are going to break down and review seismic design in regards to ASCE7-05. We are going to cover the basics and some commentary. Hopefully I will be able to elaborate sometime in the future and include some discussion.

Based ASCE7-05

1)        Exceptions

• a)        Detached 1 and 2 family dwellings with a Ss<0.4 and SDC (Seismic Design Category) = A, B or C.
• b)       Detached 1 and 2 family dwellings (not included above), wood framed, less than 2 stories, and designed in accordance with the IRC.
• c)        Agricultural buildings
• d)       Structures not included in ASCE7 such as bridges, transmission towers, nuclear, and buried structures.

2)       Existing Structures – Alterations and additions

• a)        See appendix 11B .
• b)       To summarize – You have three options
  • i)        Design the addition/alteration separately
  • ii)       If the alteration/addition does NOT increase the seismic force or reduce the strength of the existing structural member(s) by more than 10% than you do NOT need to upgrade the existing member(s) to meet the current standard.
  • Upgrade the existing to meet the current standard (code)

Seismic Loads

First determine how the earth movement will influence the building and what acceleration should be used to model this movement.

1. Determine Ss and S1 from the 0.2 and 1.0s spectral response accelerations shown on Figs. 22-1 through 22-14.
2. Determine the Site Class based on soil properties. (See chapter 20 for more info on determining site classes). The site class shall be A,B,C,D,E,F based on the geotechnical report or local AHJ. If there has not been a site class provided site class D may be assumed (unless otherwise specified by AHJ).
3. Determine the appropriate site coefficient, Fa based on the short period, Ss, and Fv based on the 1sec period, S1, and the site class (Table 11.4-1 and 11.4-2).
   • Evaluate the Maximum Considered Earthquake (MCE) spectral response acceleration for short periods and 1 sec periods.
     • Sms=Fa*Ss
     • Sm1=FvS1
4. Design Spectral Acceleration Parameters (these are the values used in design).
   • Sds=2/3*Sms (short period)
   • Sd1=2/3*Sm1 (1 s period)
5. Determine Occupancy Category from Table 1-1
6. Determine Importance Factor from Table 11.5-1
7. Determine Seismic Design Category (SDC) A,B,C ,D, E or F. (E is reserved for S1>0.75 and F is reserved for Occupancy Category IV w/ S1>0.75) based occupancy category and period response acceleration parameter.
   • Table 11.6-1 SDC based on Sds
   • Table 11.6-2 SDC based Sd1
   • Use the most severe case. It is permitted to use Table 11.6-1 if S1 < 0.75 and all of the following apply:
   1. Unless Ta<0.8*Ts ,
   2. In each orthogonal direction the fundamental period of the structure used to calculate story drift is less than Ts
   3. Eq 12.8-2 is used to calc Cs (Cs=Sds/(R*I)) seismic response coefficient.
   4. Diaphragms are rigid or flexible diaphragms with vertical elements of the LFRS do not exceed 40′ spacing.
8. Some quick notes for designing building in SDC A:
   1. For SDC A the force at each level may be determined by Fx=0.01*Wx
      • Essentially 1% of the weight is used as horizontal force.
   2. Load path connections for a smaller portion of the structure connected to the ‘main’ structure shall be designed using 0.05*(Rdl+Rll) (Dead load and live load reaction) of the smaller portions weight. Connections shall also be designed for 0.05*W. Unless the lateral system cannot provide for this force than the maximum force the the lateral system can provide should be used.
   3. Anchorage to concrete or masonry walls. The minimum connection to the diaphragm shall be 280 plf (strength level). This should be substituted for E in the load combinations.

Second – How will the building respond and what forces should be used based on the ground acceleration (found previously) and how the building responds.

1. Select a lateral force resisting system. For each system Table 12.2-1 lists the following: Response Modification Factor (R), System Overstrength factor (Ω Omega) and Deflection Amplification Factor (Cd).
   • For a more indepth explanation see here:http://www.structuremag.org/article.aspx?articleID=756
   • R: The response modification factor reduces the seismic load to strength level design forces. Essentially this accounts for the ductility of the structure. Systems that can dissipate energy through a ductile response have higher R factor. These systems may require a higher level of design and detailing for certain SDC’s.
   • Ω Omega: The Overstrength factor increases the required seismic forces and is applied in specific cases or in the design of certain parts of the structure. Ω0 is intended to reflect the upper bound lateral strength of the structure and estimates the maximum forces in elements that are to remain non-yielding during the design basis ground motion. In summary, R reduces the required seismic forces realizing the some yielding of the structure will help dissipate energy. To force a more ductile response some ‘brittle’ members are designed to resist higher forces so that they stay in the elastic range during the seismic event.
   • (Cd) Deflection Amplification Factor: Realizing that the structure is intended to yield (ductile response) deflection will be greater than that found from an elastic analysis. Cd amplifies the deflection of the structure based on an elastic analysis.

   • 

     Response Factor, Deflection Amplification Factor and Overstrength Factor

2. Different systems may be used in the same structure. If the systems are in orthogonal directions the R, Omega, Cd shall be applied to each system. Systems used in combination to resist lateral forces in the same direction are referred to as dual systems. Some dual systems are listed in Table 12.2-1. For other systems the more stringent system limitation shall apply.
   • If R, C and Ω vary over the height of the structure; the story below shall meet the most stringent of the stories above (avoid weak story) for systems in the same direction.
     • There are multiple exceptions see (12.2.3.1)
   • If  R, C and Ω vary within the same story (Horizontal Combinations). R shall be the lowest of the different systems for that story. R may vary for different lines of LFRS if the building category is 1 or 2, two stories or less and the use of flexible diagrams. However the diaphragm shall use the lowest R value. Cd and Omega  in the direction under consideration under consideration at any story shall not be less than the largest value of this factor for the R factor used in the same direction being considered.
   • Further restrictions and direction is given in 12.2.5 for specific system requirements.
3. Irregularities – Irregularities are covered in chapter 12.3. They are specific to certain geometries and mass distributions.
   • Vertical Irregularities – Differences from story to story including – Variable stiffness, variable weight distribution, offset of vertical elements.
   • Horizontal Irregularities – Reentrant corners, torsional, discontinuous diaphragms, non parallel systems.
4. Redundancy Factor, ρ – equal to 1 for the following:
   • Structures assigned to SDC B or C
   • Calculating drift and P-Delta effects.
   • Design of nonbuilding structures
   • Design of collector elements, splices and connections when using the overstrength factor.
   • Diaphragm loads using Eq 12.10-1
   • Structures with damping  systems (Section 18).
5. Redundancy Factor, ρ – equal to 1.3 for SDC D,E and F. Unless the exceptions of 12.3.4.2 are met and comply with table 12.3-3.
6. Diaphragm Flexibility – Rigid, Flexible and Semi-Rigid. All diaphragms are semi-rigid, meaning that load is distributed to from the diaphragm to the vertical elements depends on the stiffness of the diaphragm and stiffness of the vertical elements.
   1. Rigid – When concrete is used, span-to-width is <3 and no horizontal irregularities.
   2. Flexible – selective combinations of materials used for vertical elements and diaphragms. In general if the maximum diaphragm deflection is  less than 2 x the average drift of the vertical element.
7. Continuous Load Path – Any smaller portion of the structure shall be connected to the remainder of the structure and designed to transmit 0.133*Sds*W or 0.05*W of the smaller portion.
8. Connection to supports  – a minimum of 0.05*(Wdl+WLL) of the beam/girder/truss reaction.

Next we will consider how to apply the loads including load combinations, magnitude, direction and modeling.

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In comparing the 2010 edition to the 2005 edition of the ASCE 7 we see that there are significant changes to the layout, format, load factors used for wind and basic wind speed maps. These changes affect how you determine wind design wind pressures.

References

ASCE 7-10 Minimum Design Loads for Buildings and Other Structures. Found here

ASCE 7-05 Minimum Design Loads for Buildings and Other Structures. Found here

The Basics

ASCE 7-05 uses a single basic wind speed map. For each building risk category an importance factor is applied. Note that these importance factors only depend on the type of building, not where the building is located. The wind-load factor is then applied to determine the design wind pressure. For this edition (05), the ASD wind-load factor is 1.0 and the strength design wind-load factor is 1.6.

ASCE 7-10 uses three different basic wind speed maps for different categories of building occupancies. These maps provide basic wind speeds that are directly applicable for determining pressures for strength design. Consequently, the strength design wind-load factor was changed to 1.0 in this version. Simply put, ASCE 7-10 uses three maps based on strength design in conjunction with a wind-load factor of 1.0 for strength design (LRFD) and 0.6 for service level loads (ASD), while ASCE 7-05 uses a single map with an importance factor and wind-load factor of 1.6 for strength design (LRFD) and 1.0 for service level loads (ASD).

Why the Change?

The commentary in ASCE 7-10 (section states 26.5.1) a few reasons for basic wind speed changes:

1. A strength design wind speed map brings the design approach used for wind ‘in-line’ with that used for seismic loads.
2. Multiple maps remove the inconsistencies inherent the importance factor approach. With multiple maps a distinction may be made based on location (i.e. hurricane prone vs non-hurricane prone which also changes the recurrence interval).
3. New maps establish a more uniform return period for the design-basis winds.
4. The maps more clearly inform owners and their consultants (that’s you) about the storm intensities for which designs are preformed.
5. We have justify our pay check somehow

Summary:

ASCE 7-10: 3 wind speed map based on 3-sec gust at 33ft above ground. The different maps are ‘calibrated’ to strength level design (LRFD LF=1.0) and also include building classification and location.

ASCE 7-05: (1) wind speed map based on 3-sec gust at 33ft above ground. Importance factors and Load Factors are used to increase design pressures.

(Bonus Info)

EIA-TIA-222 Rev G: Wind speeds are similar to 7-05 with different definitions of classification of structures and gust effect factors.

EIA-TIA-222 Rev F: Wind speed maps based on fastest mile. These are not directly comparable to ASCE 7-05 or 10, as the ASCE 7 uses 3-sec gust. The 3-sec gust represents the peak gust wind speed where as the fastest-mile wind speed represents the average wind speed over the time required for one mile of wind to pass the site.  The design pressures are derived using different adjustments for height/exposure and gust effects than that of Rev G and/or the ASCE 7 standard

How ASCE 7-10 Wind speed were developed – return periods:

Risk Cat I which is based on 25-yr return period equates to 300yr return period

Risk Cat II: 700yrs or 0.0014 annual exceedance probability

Risk Cat III and IV which are based on a 100-yr return period (thus there importance factor was greater in -05): 1,700yrs or 0.000588 annual exceedance probability

Note

Interestingly enough new research gathered since 2005 indicated that design wind speeds should be reduced (they also note that the overall rate of ‘intense’ storms increased). Therefore it is likely that you will noticed reduced wind pressures along coastal regions.

For most of the US of A the wind load remains basically unchanged. A quick look at the basics –

ASCE 7-10 (eqn 27.3-1) or ASCE 7-05 (eqn 6-15) wind pressure:

Assuming that and V = 90 mph then we have
ASCE 7-05 => (ASD)
ASCE 7-10 => (ASD)

A nice paper by AWC (American Wood Council)

http://www.awc.org/pdf/ASCE7-10WindChanges.pdf

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ASD

Allowable Stress Design (1989 9th Edition AISC Manual) or Allowable Strength Design (2005 13th Edition AISC Manual). Both use service level loads and a safety factor to member strength.

WSD

Working Stress Design (not used in design anymore). Uses services level loads and a safety factor to member strength.

LRFD

Load and Resistance Factor Design. Uses factored loads and applies a reduction factor to member strength.

LSD

Limit States Design. A design methodology where different failure mechanisms or states are checked and allowable strengths for each failure mechanism or state are determined. The controlling limit state is normally the one that results in the least available strength. This is more of a general term and includes ASD ’89, ASD 2005 and LRFD.

Strength Design = Generally refers to LRFD however the most new manuals which include ASD could be considered strength design methods as well. Meaning stresses are typically not calculated anymore…well they are but the end result is usually in terms of a members strength. In concrete you may also hear the term Ultimate Strength Design (where the old ’63 code used Working Stress Design) which is referring to LRFD.

Ultimate or Strength Level = Generally strength or ultimate level loads refers to Factored Loads in LRFD design. Ultimate capacity is generally the Factored Resistance or Capacity of the member being designed with LRFD.

Service Level = Generally service level loads are used with ASD methods. They are also used when checking deflection for serviceability.

Nominal Strength = This is the strength of the member for a given limit state before any safety factor or reduction factor is applied to the member. This is used with ASD or LRFD and is normal given in manuals that present a “Unified Approach” aka they give you a nominal capacity then  the user applies a safety factor or resistance factor.

Available Strength = This is the strength of the member based on the nominal strength reduced by the applicable safety factor or reduction factor. In LRFD it is common to refer to this as the Ultimate Strength. In ASD it is commonly referred to as the Allowable Strength.

Required Strength = This is the strength required based on the applicable ASD or LRFD combination. The required strength should always be less than the available strength.

Resistance Factor = The reduction factor applied to the nominal strength as used in LRFD.

Safety Factor = This is the factor which reduces the nominal strength as used in ASD.

These terms can be confusing when your fresh out of school. Most likely in school you predominantly used LRFD design. However when you show up to work you may find some who still use a lot of ASD. Or you may see alot of old ASD example problems or even need or want to use it in your design. I will try to clear some of this up for you.

ASD can mean either Allowable Stress Design or Allowable Strength Design. The Allowable Stress Design is the older or original designation which was used in the 9th Edition of the AISC Steel Construction Manual (1989 AISC) and the old ACI Concrete code (called Working Stress Design. Side note: working stress design can be helpful in reducing cracks and crack size. Therefore the method is sometimes still used in water applications). In these codes service level loads where applied to members. The stresses in the members where found and then checked against an allowable stress value which had a safety factor incorporated into it. Many ‘old timers’ will say that this used to give you more of a feel for the design as you better understood how the material and members where stressed. Allowable Strength Design (2005 AISC) – was mostly developed so that engineers who did not want to use LRFD could still use ASD and service level loads therefore both the ’89 ASD and ’05 ASD both use the same load combinations. It differs from the allowable stress design in that it is a ‘Strength Design’ methodology. The ’05 ASD uses safety factors on the nominal strength of the member based the particular limit state. The 05′ ASD allowable strength values maybe transformed into 89′ ASD stress values by factoring out the appropriate section property. Both ASD methods utilize Limit States Design however they are ‘hidden’ in the ’89 ASD code. Meaning that in the ’05 ASD each limit state is checked (i.e. yeilding, local buckling, lateral-torsional buckling, etc.). In the ’89 ASD code the allowable stress is reduced to the lowest applicable limit state. They also both take advantage of inelastic behavior in some limit states.

LRFD refers to Load and Resistance Factor Design which is also a Limit States Design methodology. This method uses a load factor to ‘factor up or down’ service level loads and also reduce member strength based on reliability and statistical data. When using LRFD you must design the strength based on the LRFD load combinations and factors however deflection should be based on service level loads, so you must keep track of your loads!

In the 2005 AISC both the ASD and LRFD methods for determining nominal strengths are presented side by side. The nominal strength will be the same for both methods and only the allowable strength will differ due to the fact that the safety factor applied for ASD and the reduction factor applied for LRFD will be different.

So why the switch, whats behind it? LRFD is a more reliable and statistical based method for predicting loads and material strengths. Whereas the allowable stress saftey factors where based on engineering judgement and past experiences. It is debated which will give you a more efficient design however it seems in most situations LRFD will produce a smaller sized beam based on strength but not always. Also serviceability and deflection control many designs, in which case both methods will yield the same result as the design is not base on strength at that point.

Check the code you are using for ASD safety factors/combinations and LRFD factors/combinations i.e. IBC, ASCE, ACI, etc.

Also see Chapter 2 of the 2005 AISC manual for further discussion.

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