NewbieInSE
Structural
- Dec 19, 2019
- 234
This is a document prepared for a local compliance authority of Bangladesh.
I would like to know your thoughts and suggestions on it.
DISCUSSION AND SUGGESTIONS FOR CLASSIFYING SITE CLASS
Abbreviations:
BNBC: Bangladesh National Building Code-2020
ASCE: American Society of Civil Engineers
FEMA: Federal Emergency Management Agency
NEHRP: National Earthquake Hazards Reduction Program
FOREWORDS: The site classes in the BNBC-2020 (which is based on ASCE 7-05) directly impact the seismic design force for all the buildings, whether a low-rise or high-rise building. In regions of moderate seismicity, a difference in site class may change the seismic design category (SDC), resulting in a difference in design and detailing requirements.
BNBC-2020 provides straightforward guidelines in determining the site classification. As the determination of site class plays a very important role in determining seismic design category and cost of construction of the structure, we feel it important to discuss on the matter based on the international codes and guidelines.
It is to emphasize that, there is always room to discuss code issues as the codes give plain guidelines but the practical situations may warrant engineering judgement in conjunction with the application of code guideline.
BASIS FOR SITE CLASSIFICATION: The source document for the site classifications defined in the ASCE 7-05 is NEHRP Recommended “Provisions for Seismic Regulations for New Buildings and Other Structures (FEMA 450)”. The proof of it can be found from ASCE 7-16’s reference list of Chapter 20’s commentary.
Figure: A portion from ASCE 7-05 page 710
Information regarding the basis for the site classifications is provided in FEMA 450’s commentary. The commentary describes how soil deposits amplify the level of ground shaking relative to the level of shaking of bedrock.
FEMA 450’s recommendation in classifying site class:
FEMA 450 commentary states: Statement Number 1
“A step- by- step procedure for classifying a site is given in the Provisions. Although the procedure and criteria in Sec. 3.5.1 and 3.5.2 are straightforward, there are aspects of these assessments that may require additional judgment and interpretation. Highly variable subsurface conditions beneath a building footprint could result in overly conservative or unconservative site classification. Isolated soft soil layers within an otherwise firm soil site may not affect the overall site response if the predominant soil conditions do not include such strata.” Reference: FEMA 450 Commentary page 30.
Here several points are important to note:
1. There is space to apply Engineering Judgement in determining site class.
2. If the soil in the 100 ft. is abrupt, then the engineer may overestimate or underestimate the site classification. Therefore, special attention should be applied and the soil strata which are causing the underestimation or overestimation of site class need to be identified.
3. Isolated soft soil at several layers in the 100 feet may not affect the overall site response if the predominant soil is firm (better than those soft soils). Therefore, it is conservative to classify a site as site class D just because it has one or two soft soils at the top.
The document again states: (Statement number 2)
“Conversely, site response studies have shown that continuous, thin, soft clay strata may increase the site amplification.” Reference: FEMA 450 Commentary page 30. It indicates that, if there is a thin continuous clay stratum, then it may increase the site amplification. Here it is to note that, the thin clay strata need to be continuous which can be verified by several bore logs. If there are several bore logs and only one of them shows a thin clay layer, then it is not reasonable to penalize the structure for that single localized thin clay strata. This is clarified further in FEMA 450 commentary.
Statement number 3:
“The site class should reflect the soil conditions that will affect the ground motion input to the structure or a significant portion of the structure.” Reference: FEMA 450 Commentary page 30. Based on this, it can be said that, the site class should be determined depending on the predominant soil strata which are likely to impart most of the seismic excitation to the structure.
Level from which soil layers need to be considered:
There are several interpretations regarding the starting level of considering SPTs and Shear wave Velocities to determine site class. For example, the FEMA 450 Commentary mentions the following.
Statement number 4:
“For structures receiving substantial ground motion input from shallow soils (e.g., structures with shallow spread footings, laterally flexible piles, or structures with basements where it is judged that substantial ground motion input to the structure may come through the side walls), it is reasonable to classify the site on the basis of the top 100 ft (30 m) of soils below the ground surface.”
Here emphasis should be given on the phrase “structures receiving substantial ground motion input from shallow soils”. In case of a shallow footing, the seismic excitation propagates to the structure by the soil beneath it. Therefore, the soil below the shallow footings needs to be included in determining site class because they transfer substantial ground motion to the structure. However, the soil above shallow footing has little or no effect in transferring seismic excitation to the structure. So, if the soil above shallow footing is weak, then engineering judgement might be applied to exclude them from the determination of site class as mentioned in Statement number 1. However, in case of structures with basement wall which are interconnected with the building frame members, the seismic excitation may transmit through the soil around the basement wall and reach the structural members. Due to this if such is the case, FEMA indicates to include the soils above mat up to basement wall top.
The permissibility of avoiding the soil layers above footing (if applies e.g., basement wall connectivity etc.) can be understood from this statement clearly.
Statement number 5:
“Conversely, for structures with basements supported on firm soils or rock below soft soils, it is reasonable to classify the site on the basis of the soils or rock below the mat, if it can be justified that the soft soils contribute very little to the response of the structure.” Reference: FEMA 450 Commentary page 30.
The above statement again indicates the applicability of engineering judgement. This statement clearly indicates that if seismic excitation cannot be transmitted through the basement walls, then the soil beneath the mat level can be used in determining site class. This same consideration (based on seismic excitation transmission) can be applied to shallow spread footings.
Dominic Kelly, a member of ASCE 7 Seismic Task Committee for the 2002 and 2005 versions of ASCE-7 explains, “The ASCE 7-05 defines six classes, site class A to site class F, based on the upper 100 feet of soil from the base of a building. Base is defined as the level at which the horizontal seismic ground motions are considered to be imparted to the structure.” Reference: Link to Dominic Kelly's Discussion on Site Classification.
Summary:
From the above discussion it can be summarized that,
1. There is space to apply engineering judgement.
2. It is overly conservative to classify a site class SD, based on upper one or two layers (5 or 10 feet) which is not necessary.
3. For shallow footings, SPTs below footing bottom level up to 100 ft. below should be used to determine site class.
4. For deep foundations, SPTs from the pile beginning level (because seismic excitation is transferred from soil to the piles) up to 100 feet below should be used to determine site class. However, if soft soil of insignificant depth (5 to 10 feet) makes the site class D, then it may be reasonable to ignore those few soft soil layers if rest of the predominant soil depth doesn’t include such soil.
5. For mat with basement, it may be left to engineer’s judgement whether to consider soil above mat level in calculating site class or using soil below mat level to determine site class, depending on the basement wall to structural frame’s connectivity conditions.
I would like to know your thoughts and suggestions on it.
DISCUSSION AND SUGGESTIONS FOR CLASSIFYING SITE CLASS
Abbreviations:
BNBC: Bangladesh National Building Code-2020
ASCE: American Society of Civil Engineers
FEMA: Federal Emergency Management Agency
NEHRP: National Earthquake Hazards Reduction Program
FOREWORDS: The site classes in the BNBC-2020 (which is based on ASCE 7-05) directly impact the seismic design force for all the buildings, whether a low-rise or high-rise building. In regions of moderate seismicity, a difference in site class may change the seismic design category (SDC), resulting in a difference in design and detailing requirements.
BNBC-2020 provides straightforward guidelines in determining the site classification. As the determination of site class plays a very important role in determining seismic design category and cost of construction of the structure, we feel it important to discuss on the matter based on the international codes and guidelines.
It is to emphasize that, there is always room to discuss code issues as the codes give plain guidelines but the practical situations may warrant engineering judgement in conjunction with the application of code guideline.
BASIS FOR SITE CLASSIFICATION: The source document for the site classifications defined in the ASCE 7-05 is NEHRP Recommended “Provisions for Seismic Regulations for New Buildings and Other Structures (FEMA 450)”. The proof of it can be found from ASCE 7-16’s reference list of Chapter 20’s commentary.
Figure: A portion from ASCE 7-05 page 710
Information regarding the basis for the site classifications is provided in FEMA 450’s commentary. The commentary describes how soil deposits amplify the level of ground shaking relative to the level of shaking of bedrock.
FEMA 450’s recommendation in classifying site class:
FEMA 450 commentary states: Statement Number 1
“A step- by- step procedure for classifying a site is given in the Provisions. Although the procedure and criteria in Sec. 3.5.1 and 3.5.2 are straightforward, there are aspects of these assessments that may require additional judgment and interpretation. Highly variable subsurface conditions beneath a building footprint could result in overly conservative or unconservative site classification. Isolated soft soil layers within an otherwise firm soil site may not affect the overall site response if the predominant soil conditions do not include such strata.” Reference: FEMA 450 Commentary page 30.
Here several points are important to note:
1. There is space to apply Engineering Judgement in determining site class.
2. If the soil in the 100 ft. is abrupt, then the engineer may overestimate or underestimate the site classification. Therefore, special attention should be applied and the soil strata which are causing the underestimation or overestimation of site class need to be identified.
3. Isolated soft soil at several layers in the 100 feet may not affect the overall site response if the predominant soil is firm (better than those soft soils). Therefore, it is conservative to classify a site as site class D just because it has one or two soft soils at the top.
The document again states: (Statement number 2)
“Conversely, site response studies have shown that continuous, thin, soft clay strata may increase the site amplification.” Reference: FEMA 450 Commentary page 30. It indicates that, if there is a thin continuous clay stratum, then it may increase the site amplification. Here it is to note that, the thin clay strata need to be continuous which can be verified by several bore logs. If there are several bore logs and only one of them shows a thin clay layer, then it is not reasonable to penalize the structure for that single localized thin clay strata. This is clarified further in FEMA 450 commentary.
Statement number 3:
“The site class should reflect the soil conditions that will affect the ground motion input to the structure or a significant portion of the structure.” Reference: FEMA 450 Commentary page 30. Based on this, it can be said that, the site class should be determined depending on the predominant soil strata which are likely to impart most of the seismic excitation to the structure.
Level from which soil layers need to be considered:
There are several interpretations regarding the starting level of considering SPTs and Shear wave Velocities to determine site class. For example, the FEMA 450 Commentary mentions the following.
Statement number 4:
“For structures receiving substantial ground motion input from shallow soils (e.g., structures with shallow spread footings, laterally flexible piles, or structures with basements where it is judged that substantial ground motion input to the structure may come through the side walls), it is reasonable to classify the site on the basis of the top 100 ft (30 m) of soils below the ground surface.”
Here emphasis should be given on the phrase “structures receiving substantial ground motion input from shallow soils”. In case of a shallow footing, the seismic excitation propagates to the structure by the soil beneath it. Therefore, the soil below the shallow footings needs to be included in determining site class because they transfer substantial ground motion to the structure. However, the soil above shallow footing has little or no effect in transferring seismic excitation to the structure. So, if the soil above shallow footing is weak, then engineering judgement might be applied to exclude them from the determination of site class as mentioned in Statement number 1. However, in case of structures with basement wall which are interconnected with the building frame members, the seismic excitation may transmit through the soil around the basement wall and reach the structural members. Due to this if such is the case, FEMA indicates to include the soils above mat up to basement wall top.
The permissibility of avoiding the soil layers above footing (if applies e.g., basement wall connectivity etc.) can be understood from this statement clearly.
Statement number 5:
“Conversely, for structures with basements supported on firm soils or rock below soft soils, it is reasonable to classify the site on the basis of the soils or rock below the mat, if it can be justified that the soft soils contribute very little to the response of the structure.” Reference: FEMA 450 Commentary page 30.
The above statement again indicates the applicability of engineering judgement. This statement clearly indicates that if seismic excitation cannot be transmitted through the basement walls, then the soil beneath the mat level can be used in determining site class. This same consideration (based on seismic excitation transmission) can be applied to shallow spread footings.
Dominic Kelly, a member of ASCE 7 Seismic Task Committee for the 2002 and 2005 versions of ASCE-7 explains, “The ASCE 7-05 defines six classes, site class A to site class F, based on the upper 100 feet of soil from the base of a building. Base is defined as the level at which the horizontal seismic ground motions are considered to be imparted to the structure.” Reference: Link to Dominic Kelly's Discussion on Site Classification.
Summary:
From the above discussion it can be summarized that,
1. There is space to apply engineering judgement.
2. It is overly conservative to classify a site class SD, based on upper one or two layers (5 or 10 feet) which is not necessary.
3. For shallow footings, SPTs below footing bottom level up to 100 ft. below should be used to determine site class.
4. For deep foundations, SPTs from the pile beginning level (because seismic excitation is transferred from soil to the piles) up to 100 feet below should be used to determine site class. However, if soft soil of insignificant depth (5 to 10 feet) makes the site class D, then it may be reasonable to ignore those few soft soil layers if rest of the predominant soil depth doesn’t include such soil.
5. For mat with basement, it may be left to engineer’s judgement whether to consider soil above mat level in calculating site class or using soil below mat level to determine site class, depending on the basement wall to structural frame’s connectivity conditions.
