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Negative Skin Friction on Pile Foundation

By
Ivy Ugorji
-

Negative skin friction is a downward drag force exerted on a pile by the soil surrounding it. This is the reverse of the normal skin friction or shaft resistance needed to support piles. If the downward drag force is excessive, it can cause the failure of the pile foundation.

Where applicable, negative skin friction must be allowed when considering the factor of safety on the ultimate load-carrying capacity of a pile. The factor of safety (FOS) where negative skin friction is likely to occur is given by;

FOS = Ultimate load carrying capacity of pile/(Working load + Negative skin friction load)

Negative skin friction may occur in pile foundations due to the following circumstances;

  1. If a fill of clay soil is placed over a granular soil or completely consolidated soil into which a pile is driven, the consolidation process of the recently placed fill will exert a downward drag of the pile during the process of consolidation.
  2. If a granular fill material is placed over a layer of compressible clay, it will induce the process of consolidation in the clay layer and exert a downward drag on the pile.
  3. Lowering of the water table will increase the vertical effective stress on the soil at any depth which will induce consolidation settlement of the clay. If a pile is located in the clay layer, it will be subjected to a downward drag force.
negative skin friction

According to section 7.3.2.2 of EN 1997-1:2004 (Eurocode 7), if ultimate limit state design calculations are carried out with the downdrag load as an action, its value shall be the maximum, which could be generated by the downward movement of the ground relative to the pile.

Furthermore, the calculation of maximum downdrag loads should take account of the shear resistance at the interface between the soil and the pile shaft and downward movement of the ground due to self-weight compression and any surface load around the pile. An upper bound to the downdrag load on a group of piles may be calculated from the weight of the surcharge causing the movement and taking into account any changes in groundwater pressure due to ground-water lowering, consolidation or pile driving.

Computation of negative skin friction on a single pile

The magnitude of negative skin friction (Fn) for a single pile in filled up soil may be taken as;

(a) Cohesionless soil
Fn = 0.5K’γf‘Hf2tanδ’

Where;
K’ = coefficient of earth pressure = Ko = 1 – sinφ’
γf‘ = Effective unit weight of material causing down drag
Hf = Depth of compressible layer causing down drag
δ’ = soil-pile angle of friction ≈ 0.5φ’ – 0.6φ’

(b) Cohesive soil
Fn = PHfS

Where;
P = Perimeter of pile
S = Shear strength of soil

Negative skin friction on pile groups
When a group of piles passes through a compressible fill, the negative skin friction may be obtained using any of the following methods;

(a) Fng = nFn
(b) Fng = sHfPg + γf‘HfAg

Where;
n = number of piles in the group
γf‘ = Effective unit weight of material causing down drag
Pg = Perimeter of the pile group
Ag = Cross-sectional area of the pile group within the perimeter Pg
S = Shear strength of the soil along the perimeter of the group


Minimum Area of Reinforcement Required for Reinforced Concrete Beams

By
Ubani Obinna
-

In reinforced concrete beams, the minimum area of longitudinal reinforcement required is given by;

As,min = 0.26 (fctm / fyk) bt d   ≥ 0.0013 bt d —- (1)

where:
fctm is the mean value of axial tensile strength of concrete at 28 days, see Table 3.1 of EN 1992-1-1:2004
fyk is the characteristic yield stress of reinforcement steel
bt is the mean width of the tension zone; for a T-beam with the flange in compression, only the width of the web is taken into account in calculating the value of bt
d is the effective depth of concrete cross-section.

Any section containing less than As,min as given in equation (1) should be treated as an unreinforced section.

The minimum area of reinforcement for different classes of concrete strengths assuming fyk = 500 MPa is given in the Table below;

fck (MPa)fctm (MPa)0.26fctm/fyk
252.60.13%
282.80.14%
302.90.15%
323.00.16%
353.20.17%
403.50.18%
453.80.20%
504.20.21%

For a T-beam with the flange in tension (such as upstand beams or ground beams), the minimum area of reinforcement is given by equation (2);

As,min = 0.26 (fctm / fyk) Ac,t —– (2)

Where Ac,t is the area of the tension zone above the neutral axis of the reinforcement.

BS 8110-1:1997 made express provisions for flanged beams for different stress states of the web or the flange. The requirements are given in the table below;

Flanged beam in flexure (tension reinforcement)Definition of PercentageMinimum Percentage (fy = 500 MPa)
(a) Web in tension
bw/b < 0.4100As/bwh0.18%
bw/b ≥ 0.4 100As/bwh0.13%
(b) Flange in tension
T-beam100As/bwh0.26%
L-beam100As/bwh0.20%

Maximum area of longitudinal reinforcement
The maximum area of tension and compression reinforcement in reinforced concrete beams is;

As,max = 0.04Ac —- (3)

Where;
Ac is the area of the cross-section

Shear Reinforcements (links and stirrups)
The minimum area of shear reinforcement in beams should be calculated from;

Asw/sbw ≥ ρw,min —– (4)

Where;
ρw,min = (0.08√fck)/fyk
Asw = Area of shear reinforcement
s = spacing of shear reinforcement
bw = width of the web

Solved Example
For the concrete section shown below, find the minimum area of longitudinal reinforcement required for;
(a) When the web is in tension
(b) When the flange is in tension
fck = 25 MPa, fyk = 500 MPa

Beam section

(a) When the web is in tension
Using Eurocode;
As,min = 0.26 (fctm / fyk) bt d 
As,min = 0.26 x (2.6/500) x 250 x 695 = 235 mm2 > 0.0013btd (0.0013 x 250 x 695) = 226 mm2

Using Table 3.25, BS 8110-1:1997;
bw/b = 250/1500 = 0.167 < 0.4
As,min = 0.18bwh/100 = (0.18 x 250 x 750)/100 = 338 mm2

(b) When the flange is in tension
Using Eurocode;
As,min = 0.26 (fctm / fyk) Ac,t
Ac,t = (550 x 250) + (1500 x 145) = 355000 mm2 
As,min = 0.26 x (2.6/500) x 355000 = 480 mm2

Using Table 3.25, BS 8110-1:1997;
As,min = 0.26bwh/100 = (0.26 x 250 x 750)/100 = 488 mm2

Construction Commences on the World’s Longest Underwater Tunnel

By
Muyiwa Ajumobi
-

The construction of the world’s longest immersed tunnel has officially begun. The Fehmarnbelt Tunnel that will connect Denmark and Germany, is scheduled to be officially opened by 2029. It’s one of Europe’s largest ongoing infrastructure projects, with a budget of more than US$8 million.

The tunnel will have an 18 kilometers extension and will be built across the Fehmarn Belt, a strait between the German island of Fehmarn and the Danish island of Lolland. It will be an alternative to the current ferry service, which takes 45 minutes. Traveling through the tunnel will take seven minutes by train and ten minutes by car.

1280px Fehmarn bridge.svg

It will be the longest combined road and rail tunnel in the world, with two double-lane motorways, separated by a service passageway, and two electrified rail tracks. Besides the benefits to passenger trains and cars, it will have a positive impact on the flow of freight trucks and trains. 

image

According to Femern website, the contract for the construction of the 18km tunnel was signed on 30 May 2016. The contract which is worth almost 4 billion Euro was signed between Femern A/S  (the Danish state-owned company tasked with designing and planning the link) and international contractors responsible for the establishment of the 18 km Fehmarnbelt tunnel between Rødbyhavn and Puttgarden. The contract with the Dutch consortium, Fehmarn Belt Contractors (FBC) came into force in November 2019. This covers dredging and reclamation.

In May 2020, Femern A/S initiated conditional contracts for the tunnel as well as the portals and ramps, which were signed with the consortium, Femern Link Contractors (FLC). The contracts will be activated with effect from 1 January 2021.

Construction work started in the summer on the Danish side. Work will carry on for a few years in Denmark before moving into German territory. Workers are now building a new harbor in Lolland and in 2021 they will start construction of a factory, both meant to support work on the tunnel. Located behind the port, the factory will have six production lines to assemble the 89 massive concrete sections that will make up the tunnel.

The immersed tunnel and the tunnel factory as well as the portals and ramps

Consortium: Femern Link Contractors (FLC)
Contractors:
VINCI Construction Grands Projets S.A.S. (France)
Per Aarsleff Holding A/S (Denmark)
Wayss & Freytag Ingenieurbau AG (Germany)
Max Bögl Stiftung & Co. KG (Germany)
CFE SA (Belgium)
Solétanche-Bachy International S.A.S. (France)
BAM Infra B.V. (Holland)
BAM International B.V. (Holland)
Sub-contractors:
Dredging International N.V. (Belgium)
Consutants:
COWI A/S (Denmark)

Lateral Load Resistance of High-Rise Buildings

By
Ubani Obinna
-

The lateral load resistance and stability of buildings get increasingly important as the height of the building increases. Gravity loads on buildings can be said to vary linearly with height. In a fairly regular building, the increment in axial load in columns can be said to increase linearly as you move from the roof to the ground floor. On the other hand, lateral loads are quite variable and increase rapidly with height [1].

For example, under a uniform wind load, the overturning moment at the base varies in proportion to the square of the height of the building, while the lateral deflection varies as the fourth power.  However, wind load distribution actually increases with height, and this gives rise to a greater base bending moment [2].

In actual sense, the pressure exerted by wind on a building are not steady, but dynamic and highly fluctuating [3]. This fluctuating pressure could bring severe damage to the building other than the wind force itself, especially due to fatigue [4].

winds
Wind action simulation on a tall building [5]

For structural designs, one of the ways to safely assume wind pressures is through the quasi-steady assumption in which the building is subjected to a steady lateral wind force. This is the approach reported in many codes of practice for the loading of buildings. It is the duty of the structural engineer to select the structural system that will resist the gravity and lateral actions, and at the same time satisfy serviceability requirements of the structure, especially in terms of occupancy comfort due to vibration and sway.

One of the main tasks when designing high-rise buildings is to give the structure the ability to absorb the horizontal forces, and to transmit the resulting moment into the foundation [2, 4]. The loads acting on a tall building can be simply divided into vertical and horizontal actions.

The vertical loads are the weight of the building, imposed load and snow loads (where applicable). The horizontal loads are wind loads, seismic response, unintended inclinations/tilt, impact forces, blasts etc.

The vertical loads are taken up by the bearing walls, columns or towers and are led to the foundations. The loads occurring from the wind are first taken by the façades and are then further distributed to the slabs [6]. The floor slabs act as diaphragms and are often considered to be stiff in their plane and deformations in its plane are usually disregarded. The slabs are connected to the stabilising units, such as shear walls, cores, or stabilising columns.

wind load on a tall building
Wind load distribution to floor slabs [6]

If the façade which takes the wind load is supported by the floor slabs, then the floor slabs will be subjected to a distributed load. However, when the facades have columns attached directly to them, the loads are first transferred to the columns resulting in concentrated loads on the floor slabs. The stress distributions have to be dealt with through careful planning of how the slabs and the facade are connected.

Floor slabs are often considered to be stiff, and the horizontal load distribution through the building is due to the stiffness of the different stabilising components. If the floor slab is not stiff enough, or slip occurs in joints between slab elements in the same plane, then the displacement of the floor slab will not be the same along the loaded side of the floor slab. Stress distribution in floors depends on both loads and supports [6].

The floor of a tall building is supported by the stabilising units through a shear force distributed along the width of the wall. The walls are subjected to both bending and shear deformations but in low robust walls the bending contribution is negligible. If slender units are used for stabilising then bending mainly occurs and shear deformation is negligible.

bending and shear deformation of a floor slab
Bending and shear deformation of a floor slab [6]

Based on holistic considerations, shear walls become more slender in taller structures, even though shear walls are usually considered as low and robust on each floor. It is, therefore, necessary to consider both bending and shear deformations when designing tall buildings. The deformation from bending is curved in the opposite direction to the shear deformation. The deformation from shear is due to the shear forces applied through the floor slabs in each storey. As the loads accumulate and increase through the building the largest singular deformation occurs at the first floor for the shear contribution.

bending and shear deformation of a shear wall

Bending and shear deformation of a shear wall [6]

References

[1] Bungale S. T. (2010): Reinforced Concrete Design of Tall Buildings. CRC Press, Taylor and Francis Group
[2] Hallebrand E., and Jakobsson W. (2016): Structural design of high-rise buildings. M.Sc thesis presented to the Department of Construction Sciences (Division of structural mechanics), Lund University, Sweden
[3] Nizamani J., Thang B.C., Hiader B., and Sharrif M. (2018): Wind load effects on high rise buildings Peninsular Malaysia. IOP Conference Series: Earth and Environmental Science 140(2018)01215
[4] Sandelin C. and Bujadev E. (2013): The stabilization of high-rise buildings: An evaluation of the tubed mega frame concept. M.Sc Dissertation submitted to the Department of Engineering Science, Applied Mechanics, Civil Engineering Uppsala University.
[5] https://www.theb1m.com/video/how-tall-buildings-tame-the-wind Assessed on the 23rd of October, 2020
[6] Gustaffson D., and Hehir J. (2005): Stability of tall buildings. M.Sc thesis submitted to the Department of Civil and Environmental Engineering, Chalmers University of Technology, Sweden

Bentley Systems Announces Winners of ‘Year in Infrastructure 2020’ Awards

By
Ivy Ugorji
-

Bentley Systems Incorporated has announced the winners of the Year in Infrastructure 2020 Awards. The annual awards program honors the extraordinary work of Bentley users advancing design, construction, and operations of infrastructure throughout the world. 

Bentley Systems is an infrastructure engineering software company. They provide innovative software to advance the world’s infrastructure – sustaining both the global economy and environment. The software solutions from Bentley Systems are used by professionals, and organizations of every size, for the design, construction, and operations of roads and bridges, rail and transit, water and wastewater, public works and utilities, buildings and campuses, and industrial facilities. 

The Year in Infrastructure annual awards program honors the extraordinary work of Bentley users advancing design, construction, and operations of infrastructure throughout the world. Sixteen independent jury panels selected the 57 finalists from over 400 nominations submitted by more than 330 organizations from more than 60 countries.  

The Year in Infrastructure 2020 Special Recognition awardees are: 

Category 1: Advancing Project and Asset Longevity 

hdr basnight bridge large


Firm/Organisation/Institution: HDR 
Project: Marc Basnight Bridge 
Location: Dare County, North Carolina, United States  

Category 2: Advancing Bridge Asset Performance Modeling 
Firm/Organisation/Institution: Ulsan National Institute of Science and Technology (UNIST) 
Project: A Smartwatch on the Bridge 
Location: Ulsan, Ulju-gun, South Korea 

Category 3: Advancing Industrial Asset Performance Modeling 
Firm/Organisation/Institution: The Institute of Engineering and Ocean Technology/Oil and Natural Gas Corporation Limited 
Project: Challenges in Addressing Life Extension of Ageing Platforms in Western Offshore of India 
Location: Mumbai, India  

Category 4: Comprehensiveness in Industrial Digital Twins 

Volgogradnefteproekt LLC

Firm/Organisation/Institution: Volgogradnefteproekt, LLC 
Project: Ethane-Containing Gas Processing Complex Construction Support 
Location: Ust-Luga, St. Petersburg, Russia  

Category 5: Comprehensiveness in Transportation Digital Twins 

PT Waskita Manggarai realitymodel2

Firm/Organisation/Institution: PT. WASKITA Karya (Persero) Tbk 
Project: Railway Facility for Manggarai to Jatinegara: Package A – Phase II ( Main Line II) 
Location: South Jakarta, Jakarta, Indonesia  

Category 6: Comprehensiveness in Urban Digital Twins 
Firm/Organisation/Institution: JSTI Group Co., Ltd. 
Project: Hengjiang Avenue Rapid Transformation Project  
Location: Nanjing, Jiangsu, China 

Category 7: Comprehensiveness in a Connected Data Environment 
Firm/Organisation/Institution: Roads & Transport Authority (RTA) 
Project: Collaborative Information System Implementation – Whole Lifecycle Common Data Environment 
Location: Dubai, United Arab Emirates  

Category 8: Advancing Virtualization through Digital Twins 
Firm/Organisation/Institution: Network Rail 
Project: Overcoming Challenges Under COVID-19 Lockdown 
Location: Wales and Western Region, United Kingdom 

Category 9: Advancing Model-based Delivery through Digital Twins 
Firm/Organisation/Institution: NYS Department of Transportation 
Project: Model Based Contracting – NYS RT 28 over the Esopus 
Location: Mount Tremper, New York, United States 

Category 10: Advancing Mixed-Reality Workflows 
Firm/Organisation/Institution: Liaoning Water Conservancy and Hydropower Survey and Design Research Institute Co., Ltd. 
Project: Chaoyang Underground Pumping Station Project of the LXB Water Supply Project Phase II 
Location: Chaoyang, Liaoning, China 

Category 11: Advancing Sustainability Digital Twins 
Firm/Organisation/Institution: Shanghai Institute of Mechanical and Electrical Engineering Co., Ltd. 
Project: Shanghai Electric Environmental Protection Group Technology Renovation and Expansion Project for Nantong Thermoelectric Waste Incineration  
Location: Nantong, Jiangsu, China  

Category 12: Advancing Sustainable Architecture 

swatch headquarters

Firm/Organisation/Institution: Swatch Ltd., Shigeru Ban, Itten+Brechbühl AG 
Project: Swatch Headquarters 
Location: Biel, Bern, Switzerland  

Category 13: Advancing Sustainable Energy  
Firm/Organisation/Institution: Guangdong Hydropower Planning & Design Institute 
Project: Guangdong Yangjiang Pumped Storage Power Station  
Location: Yangjiang, Guangdong, China 

Category 14: Advancing Sustainable Water 
Firm/Organisation/Institution: Jacobs 
Project: San Jose Headworks 
Location: San Jose, California, United States 

The winners of the Year in Infrastructure 2020 Awards for going digital advancements in infrastructure are: 

4D Digital Construction 
Firm/Organisation/Institution: DPR Construction 
Project: 2019 LSM DS Tech Upgrade 
Location: Durham, North Carolina, United States 

Bridges 
Firm/Organisation/Institution:
Chongqing Communications Planning, Survey & Design Institute Co., Ltd., 
Guizhou Communications Construction Group Co., Ltd., 
Guizhou Bridge Construction Group Co., Ltd. 
Project: Digital Design and Construction of Taihong Yangtze River Bridge 
Location: Chongqing, China 

Buildings and Campuses 
Firm/Organisation/Institution: Voyants Solutions Private Limited 
Project: Bangladesh Regional Waterway Transport Project 1 – Shasanghat (New Dhaka) IWT Terminal 
Location: Dhaka-Shasanghat, Narayanganj, Chandpur, and Barisal; Bangladesh 

Digital Cities 
Firm/Organisation/Institution: City of Helsinki 
Project: Digital City of Synergy 
Location: Helsinki, Finland 

Geotechnical Engineering 
Firm/Organisation/Institution: Golder Associates Hong Kong Ltd. 
Project: Tuen Mun-Chek Lap Kok Link Tunnel, Southern Landfall 
Location: Hong Kong 

Land and Site Development 
Firm/Organisation/Institution: AAEngineering Group 
Project: Dzhamgyr Mine – Project Implementation in Extreme Conditions 
Location: Talas Region, Kyrgyzstan 

Manufacturing 
Firm/Organisation/Institution: MCC Capital Engineering & Research Incorporation Ltd. 
Project: BIM Technology-Based Construction of Digital Plant for Iron & Steel Base in Lingang, Laoting of HBIS Group Co., Ltd. 
Location: Tangshan, Hebei, China 

Mining and Offshore Engineering 
Firm/Organisation/Institution: AAEngineering Group 
Project: Digital Twin of AKSU Plant: From Concept to Startup 
Location: Aksu, Akmola Region, Kazakhstan 

Power Generation 
Firm/Organisation/Institution: Shanghai Institute of Mechanical and Electrical Engineering Co., Ltd. 
Project: Shanghai Electric Environmental Protection Group Technology Renovation and Expansion Project for Nantong Thermoelectric Waste Incineration 
Location: Nantong, Jiangsu, China 

Project Delivery 
Firm/Organisation/Institution: Sweco 
Project: Sweco | Digitalisation with BIM 
Location: United Kingdom 

Rail and Transit 
Firm/Organisation/Institution: POWERCHINA Huadong Engineering Corporation Limited 
Project: Innovative Application of Digital Engineering Technology in Shaoxing Rail and Transit Construction 
Location: Shaoxing, Zhejiang, China 

Reality Modeling 
Firm/Organisation/Institution: Khatib & Alami 
Project: Geo-enabling Reality Model Tips and Tricks 
Location: Muscat, Oman 

Road and Rail Asset Performance 
Firm/Organisation/Institution: Roads and Transport Authority (RTA) 
Project: Collaborative Information System Implementation – Whole Lifecycle Common Data Environment 
Location: Dubai, United Arab Emirates 

Roads and Highways 
Firm/Organisation/Institution: Sichuan Road & Bridge (Group) Co., Ltd. 
Project: BIM Technology Application on Chengdu-Yibin Expressway 
Location: Chengdu, Sichuan, China 

Structural Engineering 
Firm/Organisation/Institution: WSP 
Project: WSP Overcomes Complex Challenges with Bentley’s Technology to Deliver Principal Tower 
Location: London, England, United Kingdom 

Utilities and Communications 
Firm/Organisation/Institution: Sterlite Power Transmission Limited 
Project: Sterlite BIM 
Location: Tripura, India 

Utilities and Industrial Asset Performance
Firm/Organisation/Institution: Shell’s QGC business 
Project: Evolution of Engineering Data, Documents and Information Management 
Location: Brisbane, Queensland, Australia 

Water and Wastewater Treatment Plants 
Firm/Organisation/Institution: Hatch 
Project: Ashbridges Bay Treatment Plant Outfall 
Location: Toronto, Ontario, Canada 

Water, Wastewater, and Stormwater Networks 
Firm/Organisation/Institution: DTK Hydronet Solutions 
Project: Digital Water Network Engineering & Asset Management of Dibrugarh Water Supply Project 
Location: Dibrugarh, Assam, India 

Detailed descriptions of all nominated projects are in the print and digital versions of Bentley’s 2020 Infrastructure Yearbook, which will be published in early 2021. 

Column Axial Shortening in Tall Buildings

By
Ubani Obinna
-

When a vertical compressive load is applied to a column, it shortens. Axial shortening takes place in all structures but when reaching great heights its effect has significant importance [1]. As the columns shortening are added together the overall shortening of a high-rise building becomes big enough to have real consequences. Shortening has to be taken into account by the structural engineer when the building is being erected since it will vary as more stories are added [2, 3].

Floor slabs can start to tilt because of differential column shortening which in turn affects the cladding, partitions, mechanical equipment, and more, a possible result is shown in Figure 1. Axial shortening of columns can also affect stress distribution in horizontal structural elements.

Differential column axial shortening
Figure 1: Differential Column Shortening in a Building [4]

Depending on the material used for the construction, the magnitude of column shortening varies. Steel columns have more tendency to be affected by column axial shortening than reinforced concrete [5]. However, reinforced concrete does have length changes from creep and shrinkage making the two materials almost equal in total length change [4]. While shortening in steel columns have been attributed to elastic deformation only, it has been attributed to the summation of elastic strain caused by load application, shrinkage strain caused by drying, and creep strain induced by sustained stress over a long-term period in reinforced concrete.

differential shortening
Figure 2: Shortening in Columns [11]

Concrete columns and walls can potentially shorten at different rates within the same floor resulting in differential shortening [3, 6]. However, according to The Concrete Society (UK), column shortening is not significant in reinforced concrete buildings less than 10 – 15 storeys. At each storey height, a maximum shortening of 4 – 5 mm corresponding to deformation of about 1.4 mm/m is possible. Report suggests that it is difficult to reduce this shortening significantly. A better strategy is to minimise differential shortening by designing all columns to the same criteria.

In a research carried out by Ali [7] on steel buildings using SAP 2000 finite element analysis package, it was observed that the maximum axial shortening occurs in the top storey of the structure and almost zero magnitude at the bottom level. The increasing rate of axial deformation of steel column is quite high from ground level to the mid-floor levels and finally it becomes moderate from the mid floor level to the top floor. Interior columns experience higher axial shortening compared to side and corner columns which is responsible for the development of differential shortening in steel columns. This is because interior columns are more heavily loaded than exterior columns.

By designing the vertical structural members’ connection to deform without stressing the affected elements (cladding, partitions, etc.) column shortening can be contained. The problem of differential shortening between adjacent vertical elements still remains however and must be taken into consideration.

To compensate for columns shortening, a few different methods can be applied;

  • If the stresses are made equal between the vertical elements the length change will be more equal, especially if the material is the same.
  • Designing all columns to the same criteria
  • Keeping long clear spans between different structural types such as cores and columns
  • Steel columns can be compensated by making them longer in fabrication.
  • Concrete columns can be adjusted by the formwork.
  • Counteraction can be made by tilting the floor slabs the opposite way than that because of column shortening during construction
  • Use of shims [2]

Calculation of exact values of axial shortening in reinforced concrete structures is not a straight forward task. It depends on a number of parameters such as the type of concrete, reinforcement ratio, and the rate and sequence of construction [8]. This information may not be available to the structural engineer at the design stage.

Samarakkody et al [9] developed a technique to evaluate the differential axial shortening in a high-rise building with composite steel and concrete columns. This technique incorporated the effects of construction sequence and concrete levelling, stress relaxation of concrete due to the presence of the steel tube, time-dependent material properties such as creep, and effects of belt and outrigger systems.

Correia and Lobo [10] developed a simplified method of assessing axial shortening in tall buildings using the construction sequence approach and obtained favourable results that are comparable with outputs from finite element packages.

References
[1] Cargnino A., Debernardi P.G., Guila M., Taliano M. (2012): Axial shortening compensation strategies in tall buildings. A case study: The new Piedmont Government Office Tower. Structural Engineering International 22(1):121-129
[2] Sandelin C. and Bujadev E. (2013): The stabilization of high-rise buildings: An evaluation of the tubed mega frame concept. M.sc Dissertation submitted to the Department of Engineering Science, Applied Mechanics, Civil Engineering Uppsala University
[3] Matar S.S. and Faschan W.J. (2017): A structural engineer’s approach to differential shortening in tall buildings. International Journal of High-Rise Buildings 6(1):73-82
[4] Fintel M., Ghosh S. and Iyengar H. (1987): Column shortening in tall structures – Prediction and compensation. Portland Cement Association
[5] Patil D. and Bajad M.N. (2016): Predicting axial shortening of vertical elements in high rise buildings by using PCA method. International Journal of Innovative Research in Science, Engineering and Technology 5(7):12512-12519
[6] Fragomeni S., Whaikawa H., Boonlualoah S. and Loo Y.C. (2014): Axial shortening in an 80-storey concrete building, in ST Smith (ed.), 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23), vol. II, Byron Bay, NSW, 9-12 December, Southern Cross University, Lismore, NSW, pp. 1231-1236. ISBN: 9780994152008
[7] Ali S. (2014): Analysis of Effects of Axial Shortening of Steel Columns in Frame Structure. Proceedings of the World Congress on Engineering and Computer Science 2014 Vol II. WCECS 2014, 22-24 October, 2014, San Francisco, USA
[8] Jayasinghe M. T. R., Jayasena W. M. V. P. K. (2004): Effects of Axial Shortening of Columns on Design and Construction of Tall Reinforced Concrete Buildings. ASCE Practice Periodical on Structural Design and Construction 9(2) https://doi.org/10.1061/(ASCE)1084-0680(2004)9:2(70)
[9] Samakkody D.A., Thambiratnman D.P., Chan T.H.T. and Moragaspitiya P.H.N. (2017): Differential axial shortening and its effects on high rise buildings with composite concrete filled tube columns. Elsevier – Construction and Building Materials (143):659-672 https://doi.org/10.1016/j.conbuildmat.2016.11.091
[10] Correia R. and Lobo P.S. (2017): Simplified assessment of the effects of column shortening on the response of tall concrete buildings. Procedia Structural Integrity 5(2017):179-186
[11] Lee Y., Kim J., Seol H., Yang J., Kim K. (2017):3D numerical analysis of column shortening and shore safety under construction of high-rise building. Elsevier – Engineering Structures (150): 242-255. https://doi.org/10.1016/j.engstruct.2017.07.049




Autodesk Launches BuildingConnected in Australia and NewZealand

By
Ubani Obinna
-

BuildingConnected, a construction tender program owned by Autodesk had been launched in Australia and NewZealand. With this expansion, the BuildingConnected solution is now available in Australia, New Zealand, the United Kingdom and Ireland. More than one million construction professionals use the platform in North America alone, with more than 2000 general contractors and owners actively bidding out projects, totalling USD56 billion in project values each month.

According to Autodesk’s official website, BuildingConnected is a preconstruction solution that combines the largest real-time, construction network with an easy-to-use platform that streamlines the bid and risk management process. BuildingConnected can enhance preconstruction operations by helping contractors find the best subcontractor for the right project, qualify subcontractors and manage project risks, centralise bids and manage them in real time.

buildingconnected

BuildingConnected allows preconstruction teams to:

  • Quickly solicit bids with customisable templates and accurately compare those bids in a side-by-side “apples-to-apples” fashion
  • Track against internal budgets with real-time cost updates
  • Easily collaborate with other estimators on the team, and follow communications and bid versions
  • Export bids and summary sheets for transparent collaboration with owners
  • Gain valuable insight into historical bid data and reports to optimise for future projects

According to Autodesk Construction Solutions Managing Director, Asia Pacific Operations Tomy Praveen, “BuildingConnected will enable construction businesses in Australia and New Zealand operating in single or multiple states and locations to discover and engage with leading subcontractors. With this connected network, they will be able to benchmark pricing and better deliver projects on time and to budget,”.

Join webinars to learn more about preconstruction and BuildingConnected

To learn more about connected construction and tackling procurement challenges in preconstruction, Autodesk will host a webinar that explores recently conducted research.

It will host a panel of building procurement experts who will focus their experiences on procurement technology, plus share findings from our recent research examining how owners, head contractors and subcontractors in Australia and New Zealand approach procurement today.

You can find the registration link to attend here.

BuildingConnected is now available for customers in Australia, New Zealand, United Kingdom and Ireland. For more details and key features, you can visit here.

Structural Analysis and Design of Portal Frames

By
Ubani Obinna
-

Buildings with large spans can be economically constructed using steel portal frames. Such structures are very common in business parks, industrial complexes, warehouses, places of worship, leisure houses, and sports complexes. The functionality of these types of buildings is usually influenced by factors such as the usage of the building, property development laws, availability of space, development plan, and the desired quality of the finished building.

Portal frames are characterized by their unique structural configuration and rigid connections. They comprise two vertical or inclined columns, rigidly connected at their eaves by a horizontal or inclined beam called the rafters. This arrangement forms a rigid frame that is either rectangular or trapezoidal in profile, with inherent in-plane stability.

The design of portal frames involves the selection of adequate column and rafter steel sections that will satisfy critical performance criteria such as bending, shear, axial compression, flexural buckling, lateral torsional buckling, and deflection, under the anticipated loading conditions.

In portal frames, the beam or rafters act as a portal, effectively transferring gravity and wind loads to the columns. Typically, a rigid bolted end-plate connection facilitates this load transfer and plays a critical role in the overall stability of the frame. The columns can be supported as fixed or pinned members connected to the base plates. When the columns are supported as fixed connections, moment-resisting base plate design will need to be carried out.

portal frame model

Steel as a construction material offers numerous possibilities to achieve both pleasant and flexible functional use economically.

portal frame design

Portal frames with hinged column bases are the most common type of industrial building and are used all over Nigeria and the rest of the world. Portal frames possess adequate stability in-plane and majorly require bracing for out-of-plane stability. Other structural forms that can also be used in industrial frames are lattice trusses, cable-stayed structures, etc.

industrial portal frame with mezzanine
Industrial portal frame with mezzanine

As a structure designed to accommodate complex human activities, it is very important to pay adequate attention to the details that will make the building functional, aesthetically pleasing, safe, and efficient. Expertise in the area of human factor engineering should be employed to make every aspect of the building efficient. The general aspects that should be considered before the detailed design of an industrial building are;

  • Space optimization.
  • Speed of construction.
  • Access and security.
  • Flexibility of use.
  • Environmental performance.
  • Standardization of components.
  • Infrastructure of supply.
  • Service integration.
  • Landscaping.
  • Aesthetics and visual impact.
  • Thermal performance and air-tightness.
  • Acoustic insulation.
  • Weather-tightness.
  • Fire safety.
  • Design life.
  • Sustainability considerations.
  • End of life and re-use

Loadings on Portal Frames

Steel portal frames designed and constructed in Europe typically follow the guidance set out in the Eurocodes (EC). These are a series of harmonized standards that define the requirements for the safety and serviceability of structures. Here’s a breakdown of the anticipated loadings considered according to the Eurocodes for steel portal frames:

Permanent Actions

  • Dead loads: This includes the self-weight of the steel frame itself, cladding materials (roofing, wall panels), any permanent fixtures or suspended ceilings.
  • Superimposed dead loads: These are permanent loads that are not part of the structure itself but add weight. Examples include partitions, fixed building services (HVAC units, piping), and permanent building equipment.

Variable Actions

  • Snow loads: The weight of snow accumulation on the roof, determined based on the geographical location and specific snow load zone.
  • Wind loads: Wind pressure acting on the entire structure, considering the building’s shape, size, and location. Both positive (wind suction) and negative (wind pressure) wind loads are evaluated.
  • Imposed loads: These are live loads acting on the structure due to occupancy or use. The specific value depends on the building function (residential, office, storage) and any specific use cases.
  • Seismic loads: In seismic zones, earthquake loads are considered to ensure the structure can withstand potential earthquake forces. The specific design approach depends on the seismic hazard level of the location.

Combination of Actions

The Eurocodes also require considering various combinations of these actions for design. This accounts for the possibility of multiple loads acting simultaneously. Specific factors are applied to each action type depending on whether it’s a permanent, variable, or accidental load. This ensures the design considers realistic scenarios the structure might encounter.

Methods of Portal Frame Analysis

Two primary approaches can be adopted for analyzing portal frame structures. They are:

  • elastic analysis, and
  • plastic analysis.

Elastic Analysis

The elastic analysis approach is based on the assumption that the frame exhibits purely elastic behaviour, meaning it does not experience any permanent deformations beyond its elastic limit when subjected to loading.

When a portal frame undergoes gravity loading, the bending moment reaches its peak values at the eaves (the horizontal line where the roof meets the wall) and at the apex (the highest point of the frame). At the eaves, the bending moment exhibits a hogging behaviour (convexity upwards), while at the apex, it displays a sagging behaviour (convexity downwards).

Elastic analysis tends to yield higher maximum bending moments at both the eaves and apex compared to plastic analysis. Consequently, design based solely on elastic analysis often results in less economical frames, as it necessitates the use of larger and potentially more expensive members to accommodate these higher moments.

Plastic Analysis

In contrast to elastic analysis, plastic analysis of portal frames acknowledges the potential for inelastic deformations within the structure. This allows for a more significant redistribution of bending moments throughout the structure. This redistribution is facilitated by the formation of plastic hinges at specific locations within the frame.

These plastic hinges typically develop at sections where the bending moment reaches the material’s plastic moment resistance, signifying the point at which the material yields. As a result of this redistribution, plastic analysis often leads to the design of lighter and more economical portal frames compared to those solely based on elastic analysis.

Second Order Effects in Portal Frames

While plastic analysis offers a more realistic representation of portal frame behaviour compared to elastic analysis, it is essential to acknowledge that neither method explicitly incorporates the influence of the frame’s stability under load, also known as the second-order effect. This is particularly relevant for portal frames, which are typically slender and lightweight structures susceptible to experiencing significant deformations under load. As a consequence, they are inherently more prone to these second-order effects.

To address this critical aspect, BS EN 1993-1-1 Clause 5.2.2 provides a comprehensive framework encompassing various methodologies for accounting for second-order effects during the design and analysis of steel structures. Furthermore, Clause 5.2 of the same standard establishes well-defined criteria to assist engineers in determining the significance of second-order effects in specific steel structures.

In the context of portal frame structures, a critical parameter known as αcr is employed to assess the frame’s sensitivity to second-order effects. This factor represents the ratio between the structure’s elastic critical buckling load (Fcr) for global instability and the applied design load (FEd). When αcr meets or exceeds a value of 10 for elastic analysis, second-order effects are generally considered negligible.

αcr = Fcr/FEd ≥ 10

However. BS EN 1993-1-1 has a simple approximate method to evaluate αcr when the roof slope is less than 26° and the axial force in the rafter is not significant. The axial force in the rafter is significant if NEd ≥ 0.09Ncr

The buckling amplification factor αcr can be calculated using the relationship below;
αcr = h/(200 × δNHF)

Where;
h is the height to the eaves
δNHF is the horizontal deflection at the eaves under a notional horizontal force applied at each eaves node, equal to 1/200 of the factored vertical base reaction.

Member Stability Analysis

Portal frames are typically constructed from open steel sections and are susceptible to a specific buckling mode known as lateral-torsional buckling. This phenomenon occurs when the member experiences combined bending and twisting deformations. To mitigate this risk, it is often necessary to incorporate various restraint mechanisms within the frame.

Both the rafters (horizontal beams) and columns (vertical supports) in a portal frame require careful evaluation to ensure they possess adequate stability against buckling. To address this challenge, three primary categories of restraints can be employed in portal frames:

  • Lateral Restraint: As the name suggests, lateral restraints primarily focus on preventing lateral movement of the compression flange. This is often achieved through:
    • Purlins or Side Rails: These horizontal or slightly inclined members, when securely connected to the top flange of the rafter, act as a barrier against lateral movement, particularly when the compression flange is on the top side of the rafter.
  • Torsional Restraint: This type of restraint aims to prevent the entire member (rafter or column) from twisting about its longitudinal axis. It typically involves a combination of:
    • Purlins or Side Rails: Similar to lateral restraints, purlins or side rails can contribute to torsional restraint when used in conjunction with:
      • Rafter or Column Stays: These are additional steel members that connect the flange experiencing compression (typically the top flange of a rafter) to a more stable structural element, like a column or another rafter. The stay effectively restricts the twisting motion.
members of portal frame structure

Intermediate Restraints

An additional concept to consider is the use of intermediate restraints. These can also be purlins or side rails, but strategically placed between the primary torsional restraints. Their primary function is to provide lateral support to the tension flange (typically the bottom flange of a rafter) when it’s experiencing tension. This allows for increased spacing between the more robust torsional restraint systems, potentially offering a more economical design.

The choice of the most suitable restraint system depends on several factors, including:

  • Span length: Longer spans generally necessitate more robust restraints, like closely spaced purlins or combined torsional restraint systems.
  • Loading conditions: Heavier loads require stronger restraints to maintain stability.
  • Cost and complexity: Bracing systems with frequent purlins or closely spaced torsional restraints might be more expensive and labour-intensive compared to using intermediate restraints with wider spacing between the primary torsional restraints.
  • Architectural considerations: The visual impact of different restraint systems should be considered. While closely spaced purlins might be visually busy, strategically placed intermediate restraints can offer a more streamlined appearance.

New Textbook Publication on Portal Frame Design

In our commitment to spreading civil engineering knowledge, a simple textbook has been written (part of Structville webinar proceedings) to present a brief but important aspect of portal frame design. The publication contains explanations on different types of portal frames, considerations in the design of portal frames, functional components of portal frames, actions on portal frames, and a design example of portal frames using elastic analysis. The design code adopted in the publication is BS EN 1993-1-1:2005 (Eurocode 3).

30m span portal frame
portal 1
portal 2

In addition, anyone who purchases the publication (price is ₦2,050) receives a video tutorial on modelling of portal frames in Staad Pro for free.

portal frame design

The cost of the publication is ₦2,050 only. To purchase, click HERE

To purchase the full webinar materials (including videos of discussions and models) for ₦4,100 only, click HERE

Which Engineering Judgement Informed this Decision?

By
Ivy Ugorji
-

Looking at the image above, it can be seen that the contractor lapped and cranked the column reinforcement to an entirely new position. In your own opinion, which engineering judgement or principle informed this decision taken by the contractor, or is it plain lack of structural engineering knowledge?

badly cranked column

Comment your answer below with reasons and stand a chance of winning our new publication on ‘Structural Analysis and Design of Industrial Portal Frames‘. Winners will be chosen randomly. Kindly show appreciation by notifying us in the comment section when you receive yours.

portal frame design

To purchase this publication for ₦2,050 only, click HERE.

Application of Wind Load to Shear Walls – A Manual Approach

By
Ubani Obinna
-

The higher a building goes, the higher the effects of lateral forces such as the wind on the structure become. Shear walls are often used for providing lateral stability against destabilising actions of wind in reinforced concrete structures. Shear walls can be independently relied on to provide lateral stability in a building, or sometimes other elements such as beams, columns, and staircases can interact with shear walls to provide the required lateral stiffness to a building.

Shear-wall frame interaction for resisting lateral forces is usually complex and may involve tedious manual calculations. However, this can be made simple by using finite element analysis software. It is however easy to transfer wind load manually to shear walls without recourse to the columns. This is the concept that this article explores.

Read Also…
Analysis of Coupled Shear Wall Under the Effect of Wind Load
Calculation of the natural frequency of multistorey frames

To determine the wind load transferred to shear walls in a building, the load is divided among the stabilising elements based on their stiffness (second moment of area). The stiffer elements attract a greater portion of the load. If the shear walls and other stabilising elements are symmetrically arranged (which is recommended), the centre of gravity and the shear centre (centre of stiffness) coincides and eliminate any potential torsion (twisting) in the building due to the lateral load. However, if the arrangement is unsymmetrical, there will be a twisting moment in the structure that must be properly accounted for in the design.

Let us use an example to show how this is done. Consider the general arrangement of a 7-storey building shown below;

SHEAR WALL ARRANGEMENT

The characteristic wind pressure coming from the y-direction to the building is 1.25 kN/m2. The thickness of the shear walls and the lift core is 225 mm. Let us distribute the wind load to the stabilising elements of the building (shear walls and lift core).

The relative stiffness of the elements are as follows;

Moment of inertia
Wall 1 = Wall 2 = Iw1,x = Iw2,x =bh3/12 = (0.225 x 53)/12 = 2.34 m4
Lift core = IL,x = (2 x 23)/12 – (1.55 x 1.553)/12 – (1.0 x 0.2253)/12 = 0.851 m4

∑Ix = 2Iw,x + IL,x = 2(2.34) + 0.851 = 5.531 m4

The ratio of wind force transferred to each wall (wall 1 and wall 2) = Iwi,x/∑Ix = 2.34/5.531 = 0.423

Therefore each wall will carry 42.3% of the wind force. This implies that about 85% of the wind force is resisted by the shear wall, while the remaining 15% is resisted by the lift core.

To calculate the shear centre of the structure, the following procedure can be followed;

We will need to determine the centre of gravity of the lift core. We can easily form a table for that;

lift core 1
Area A (m2)Lever arm (x) mAx (m3)
2.0 x 2.041.04
-1.55 x 1.55-2.41.0-2.4
-1.0 x 0.225-0.2251.0-0.225
∑A = 1.375 m2∑Ax = 1.375 m2


Therefore the centre of gravity in the x-direction is ∑Ax/∑A = 1.0 m

Taking moment about the centreline of wall 2;

(Iw1,x × 24) + (IL,x × 12) = dx∑Ix
(2.34 × 24) + (0.851 × 12) = (dx × 5.531)

Therefore, dx = 12 m = L/2 = 12 m

This shows that in the direction considered, the shear centre coincides with the centroid of the building, hence no torsion. Had there been significant torsion this would have been resolved into +/– forces in a couple based on the shear walls.

The force on each shear wall is therefore as follows;

wk1 = wk2 = 0.423 x 1.25 x 24 = 12.69 kN/m

loading on shear wall
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