TMT Reinforcements Can Change Design Specifications in Nigeria

By

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April 30, 2020

Nigeria’s reinforcement market is usually categorised into ‘local’ and ‘foreign’ reinforcements which are priced differently in the market. Local reinforcements are manufactured in the country, while foreign reinforcements are usually imported into the country from Germany, Ukraine, and Russia. Different researchers and quality control agencies have often reported low quality of reinforcements manufactured in the country, which usually fail to meet the standards required for reinforcing steel. Due to the problems of quality assurance of reinforcements in Nigeria, most structural engineers in Nigeria use yield strength of 410 MPa in reinforced concrete design (using BS 8110-1:1997).

However, in the middle of local and foreign reinforcements in Nigeria are the Thermo Mechanically Treated (TMT) reinforcements. TMT reinforcements are currently produced worldwide on a large scale for high strength steel. They are usually manufactured using recycled metal scraps.

TMT reinforcements are characterised by their softer inner core, and hardened outer core, and are manufactured by a process called Thermo Mechanical Treatment. This process combines plastic deformation processes such as forging, rolling, etc with thermal processes like heat treatment, water quenching, heating, and cooling at various rates into a single process. During the cooling process of TMT reinforcements, the inner core remains red hot, while the cooled outer surface gets auto tempered due to heat flow from the core to the surface, and turns the outer surface into a hardened martensitic layer.

A recent research carried out in Nigeria have shown that TMT reinforcements can change the face of design in the country, given the gradual switch of design code from BS 8110-1 to Eurocode 2. Researchers from Nnamdi Azikiwe University, Awka, tested the mechanical properties of 70 samples of TMT reinforcements produced by four different companies in the country. The reinforcements tested were;

Tiger TMT – African Foundries Limited
TMT Shield – Landcraft Industries
Rock TMT, and
Real TMT – Quantum steel Nigeria

The diameter of reinforcement tested ranged from 10 mm diameter to 25 mm diameter. The research was published in the Journal of Science and Technology Research.

Out of the 70 samples tested, 91.5% met the required characteristic strength of 500 MPa (Eurocode 2, UK), and the percentage elongation at fracture satisfied all the requirements of BS 4449:2005. The mean yield strength of the samples was found to be 532.8 MPa with a standard deviation of 24.926 MPa, and coefficient of variation of 4.678%. The probability of the samples tested falling below the yield strength of 500 MPa was found to be 9.4% with a reliability index of 1.316. However, the ultimate tensile strength to yield strength ratio (Rm/Re) of the samples were found to be averagely high (with a mean of 1.356 and a standard deviation of 0.095).

According to the authors,

Design engineers are free to decide on the characteristic value of yield strength to use for design, since Eurocodes permits the use of yield strength ranging from 400 – 600 MPa. Manufacturers should however follow the recommendations in clause 8.2.2 of BS 4449 for assessment of long-term quality level of their characteristic strength… Future work should involve extensive testing of the chemical properties of TMT reinforcements produced in Nigeria, to see how they impart on the mechanical properties. Subsequently, reinforced concrete designers in Nigeria can confidently use f_yk = 500 MPa, and a material factor of safety of 1.15 at ultimate limit state (design strength = 0.87f_yk = 435 MPa) provided TMT reinforcements have been specified.

The mechanical properties of TMT reinforcements as reported in the study will therefore likely change the way designers specify reinforcements during designs. It is obvious that any design done using f_yk = 410 N/mm² will be more expensive than design done using reinforcement of yield strength, 500 N/mm². Reinforcement dealers in Lagos are complaining that the demand for foreign reinforcement has reduced as more attention is being paid to TMT reinforcement by top construction companies. It will be interesting to see how TMT reinforcements will influence the construction market and design standards in Nigeria in the nearest future.

Reference:
Ubani O.U., Okonkwo V.O., Osayanmon O. (2020): Variability of Mechanical Properties and Reliability of Thermo Mechanically Treated Reinforcements in Nigeria. Journal of Science and Technology Research 2(1):1-12

To download the full research publication and findings, click HERE

How to Offset Beams in Staad Pro (with video tutorials)

By

Ubani Obinna

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April 28, 2020

In some cases, structural members are not perfectly connected to each other along their centroidal axis. This can be as a result of structural arrangement, construction specifications, or feasibility of execution. There are provisions on Staad Pro software to offset beam members in order to reflect as closely as possible the real structural arrangement. In this article, we are going to show how you can offset beams on Staad Pro.

Step 1: Do a little calculation
You should know the value through which you wish to offset on structural member from the other. A little but simple calculation is required in order to know the value to input into staad Pro. Let us assume that a primary beam of 450 mm depth is supporting a secondary beam of 300 mm depth. By default, Staad will join the beams along their centrelines as shown in Figure 1.

Connection of two beams — Fig 1: Schematic representation of default connection of two beams on staad Pro

Let us assume that you want the top fibre of the primary and secondary beams to flush, you will notice that the secondary beam will need to move up by (450/2) – (300/2) = +75 mm. Note that the same effect will be achieved if the primary beam comes down by -75mm.

Also, if we want to the secondary beam to rest on top of the primary beam, then the secondary beam will need to move up by (75 mm + 300 mm = 375 mm). Note that additional internal stresses will be induced in the members due to the eccentricities in the connection.

Step 2: Input your values

After modelling your structure, go to GENERAL → SPEC → BEAM → OFFSET

Depending on the arrangement of the structure, you can offset in any direction you wish at the start and end of each member. Assign the the command to the beam in question.

Step 3: Load the structure and analyse as usual

You can now apply the loading on the structure, and analyse as appropriate.

Watch a sample video tutorial below;

Cracking Due to Edge Restraint and Early Thermal Effects in Concrete

By

Peace Ikpotokin

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April 28, 2020

Crack width is calculated by multiplying the crack inducing strain by the crack spacing (i.e. the movement over a length equal to the crack spacing). Crack inducing strain ε_cr is calculated based on whether the element is subjected to edge restraint (which can be early or long term thermal effects), end restraint, and flexure/direct tension. In this article, we are going to shown how to calculate the crack width of a structural element subjected to edge restraint and early thermal cracking.

Edge restraint occurs where the young concrete section (say a wall) is cast on a hardened concrete base. This means that restriction is only in one direction, and there is interaction between the old and new concrete in terms of distribution of cracks. Edge restraint is different from end restraint because the crack width is a function of restrained strain rather than the tensile capacity of the concrete.

At the early age of freshly poured concrete (within 3 days), the crack inducing strain due to edge restraint in concrete element is given in equation (3.6) of CIRIA C660 which is shown below.

ε_cr= K[α_cT₁ + ε_ca] R₁ – 0.5 ε_ctu

where;
K = allowance for creep
= 0.65 when R is calculated using CIRIA C660
= 1.0 when R is calculated using BS EN 1992-3.

α_c = coefficient of thermal expansion
T₁= difference between the peak temperature of concrete during hydration and ambient temperature °C (See Table 1).
ε_ca = autogenous shrinkage strain – value for early age (3 days, see Table 2)
R₁ = restraint factor from Figure L₁ of BS EN 1992-3 for the short-term situation
ε_ctu = tensile strain capacity of the concrete (see Table 3)

Table 1: Typical values of difference between the peak temperature of concrete during hydration and ambient temperature (Narayanan and Goodchild, 2012)

Table 2: Typical Values of Autogenous Shrinkage Strain

Table 3: Values of Tensile Strain Capacity of Concrete (CIRIA C660)

Calculation Example

Calculate the early thermal crack width of concrete wall cast in Nigeria with the following data;

Thickness of wall = 400 mm
Reinforcement provided = H12@150mm c/c on both faces
Concrete cover = 50 mm
Type of restriant = Edge restraint
Concrete grade = C30/37
Type of coarse aggregate = Granite

Solution

Crack width w_k = s_r,maxε_cr

The maximum crack spacing s_r,max= 3.4c + 0.425 (k₁ k₂ϕ/ρ_p,eff)

To see the definition of these terms, see the post below;

Crack width and crack spacing calculation in concrete

c = 50 mm to outer face
k₁ = 0.8
k₂ = 1.0
ϕ = 12 mm
ρ_p,eff= 754/{1000 × min[400/2; 2.5 × (50 + 12/2)]}
ρ_p,eff= 754/(1000 × 140) = 0.00538

s_r,max= 3.4 × 50 + 0.425 (0.8 × 1.0 × 12 / 0.00538) = 170 + 758.36 = 928.36 mm

Early age crack-inducing strain, ε_cr = K[α_cT₁ + ε_ca] R – 0.5 ε_ctu

Using CIRIA C660 the following parameters can be determined;

K = 0.65
α_c = 10 × 10^–6 (granite coarse aggregate, see coefficient of thermal expansion of concrete)
T₁ = Using 35 °C (for a 400 mm thick wall cast in Nigeria, assuming Class N cement, 18 mm marine plywood, concrete grade C30/37 and cement content not less than 360 kg/m³, see Table 1)
ε_ca (autogenous shrinkage strain for grade 30/37 concrete at 3 days) = 15 × 10^–6 (see Table 2)
R = Restraint factor R_j = 1/(1 + E_nA_n/E_oA_o)

Where;

E_n and E_o are the elastic modulus of new and old concrete respectively
Assume E_n/E_o = 0.80 (CIRIA C660)
A_n and A_o are the areas of new and old concrete respectively (use A_n/A_o = h_n/2h_o assuming that the wall is cast remote from the edge of slab) = 0.4/2(0.4) = 0.5

R = 1/[1 + (0.8 × 0.5)] = 0.714

ε_ctu = 76 × 10^–6 (for early age thermal cracking, see Table 3)

ε_cr = K[α_cT₁ + ε_ca] R – 0.5 ε_ctu = 0.65 [(10 × 10^–6 × 35) + 15 × 10^–6] 0.714 – 0.5(76 × 10^–6) = 1.3139 × 10^–4

Therefore the early age crack width = w_k = s_r,maxε_cr = 928.36 x 1.3139 × 10^–4 = 0.121 mm

Since this crack width is less than 0.2mm, the early crack width can be considered acceptable for water retaining structures.

Coefficient of Thermal Expansion of Concrete

By

Ubani Obinna

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April 28, 2020

Coefficient of thermal expansion is defined as the change in unit length per degree of temperature change. In a concrete element, it is therefore a measure of the free strain produced in concrete subject to a unit change in temperature and is usually expressed in microstrain per degree centigrade (με/°C). It is a very important property of concrete which influences its behaviour under thermal actions. Thermal actions in concrete can come from the environment, stored materials, or during hydration reaction.

Concrete as a material will expand and contract when exposed to temperature change, and when this is not catered for in design, there will be cracks in the concrete element. The coefficient of thermal expansion of concrete largely depends on the aggregate, but a conservative value of 12 × 10^–6/°C can be used in the absence of data in the UK. Eurocode states that a value of 10 × 10^–6/°C but this value is deemed not to be conservative. The range of coefficient of thermal expansion of concrete ranges from 7 to 13 × 10^–6/°C.

Factors such as cementitious material content, water-cement ratio, temperature range, concrete age, and ambient relative humidity can also influence the thermal properties of concrete. However, the nature of aggregates is the principal factor in determining the coefficient of thermal expansion, and the resistance of the concrete to fire since they make up about 70% of concrete. In design α_c is assumed to be constant for a particular concrete, in fact it varies with both age and moisture content. Semi-dry concrete has a slightly higher coefficient of thermal expansion than saturated concrete.

Where the type of rock group of the coarse aggregate is known and can be guaranteed to be used, the appropriate value of the coefficient of thermal expansion from the table below may be used e.g. 10 × 10^–6/°C for granites and 9 × 10^–6/°C for limestones.

Table 1: Design values for coefficient of thermal expansion

Coefficient of thermal expansion of different types of concrete aggregates

There is no standard method for measuring the coefficient of thermal expansion for concrete in CEN, ISO or ASTM although a method for repair materials is provided in BS EN 1770. However, in-house methods can be used for laboratory mesurement. Typically, measuring points would be fixed to a concrete specimen that is placed on roller bearings in a water tank. The specimen is left in the water until there is equilibrium of temperature, and a set of length readings taken. The specimen is then heated to, say, 80°C and kept constant until this temperature is achieved throughout the specimen depth. A second set of readings is taken and the coefficient of thermal expansion calculated.

Thank you very much for reading, and God bless you.

Chained lintels in a building – What are the implications?

By

Ubani Obinna

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April 27, 2020

A lintel is a horizontal structural member which spans across the supports of an opening such as a door or window in a building. Depending on the span of the opening, a lintel may be structurally significant or insignificant – considering whether it requires serious structural consideration or not. Lintels can be made of materials such as timber, concrete, or steel. Concrete lintels are usually provided with nominal reinforcement, due to the inherent low tensile strength of concrete. When a lintel is continuous in a building and connects all the structural members together, it is called a chained lintel.

In Nigeria, it is common to see residential buildings chained at the lintel level. What is meant by chaining is that all the structural members are joined monolithically at the lintel level with reinforced concrete. Chained lintels can also be called continuous lintels. This process is usually done when the blockwork is built alongside the columns of the building. It is rarely done in a purely framed building where the columns, beams, and slab are done before the blockwork panels are built.

Building with chained lintel — Building under construction with chained lintel

Eye brows can be raised over this ‘wasteful approach’ of using excessive concrete and reinforcement on elements that are not ‘structurally significant’. This can lead to questions such as;

(1) Is it possible that the amount of money spent on constructing blockwork simultaneously with the frame and adopting a chained lintel offsets the cost of constructing a purely framed building?
(2) Can chained lintels be entirely avoided irrespective of the approach used?
(3) Are there special advantages of doing chained lintel in a building?

This issue is pertinent because the structural frame of a building is usually designed to stand alone without considering the effect of such lintels. Also, such arrangement is not usually captured in the structural drawings of a building, but it is done on site for low scale residential buildings. Why the extra cost? The lintels are usually 230 x 230 mm in dimension.

Advantages of chained lintels

The perceived advantages of chained lintels are as follows;

(1) It can improves the rigidity of the building. This is a no-brainer since more redundants are being added to the frame of the building. As a result, the building can behave more as a unit due to the increased linking members.
(2) It improves the robustness of the building.
(3) It can improve the lateral stability of the building, even though it is rarely critical in simple residential buildings such as duplexes in a region of no seismic event.
(4) It can cover up the inherent dimensional inaccuracies associated with building the blockworks and the columns at the same time, even though strict quality control can improve that.
(5) Introducing chained lintels reduces the buckling length of columns.

Disadvantages of chained lintels
(1) All the advantages highlighted above are usually catered for when designing the building as a pure frame. What is then the need of the extra advantages in terms of cost?
(2) The reinforcements provided in the building are the same as when no consideration for chained lintels is made.
(3) The construction speed is reduced.

A Structural Perspective

To look at this issue from a structural perspective, let us investigate the effect of chained lintels on the structural response of a simple two storey building under the effect of normal actions. In order to achieve this, let us model a simple two storey residential building and investigate its structural behaviour with and without chained lintels. The building data is shown below;

General arrangement of the building — General layout of the building

Building Data
Dimension of all columns = 230 x 230 mm
Dimension of floor beams = 450 x 230 mm
Dimension of roof beams = 300 x 230 mm
Thickness of floor slab = 150 mm
Dimension of lintels (model 2) = 230 x 230 mm
ULS action on floor slabs = 12 kN/m²
ULS action on floor beams (blockwork + rendering + selfweight) = 15 kN/m
ULS action on roof beams = 5 kN/m
ULS action on lintels = 3.5 kN/m

(N/B): All loads were assumed.

MODEL 1 — Model 1 of the building (No lintel)

BENDING MOMENT ON EXTERNAL BEAM — Bending moment on external floor beam of model 1

BENDING MOMENT ON INTERNAL BEAM — *Bending moment on internal floor beam of model 1*

From the figures above, the maximum hogging moment on the internal and external beams are 45.5 kNm and 68.4 kNm respectively, while the sagging moments are 28 kNm and 41.3 kNm respectively.

Internal forces on corner column of building model 1

external intermediate column 1 — *Internal forces on middle external columns of building model 1*

The results below show the building model with lintels and the internal forces acting on the building.

model 2 — Model 2 of the building (with lintels)

BM EXTERNAL MODEL 2 — *Bending moment on external floor beam of model 2*

bm internal model 2 — *Bending moment on internal floor beam of model 2*

corner column model 2 — *Internal forces on corner column of building Model 2*

external intermediate column model 2 — *Internal forces on middle external columns of building model 1*

Comparison of results

Under the effect of gravity actions, the following results were observed in the structural members;

(1) Beams
In the building with chained lintel, the bending moment on the intermediate support of the external beam reduced by about 6.8%. However, the end support moment increased by about 43.8% (from 7.47 kNm to 13.3 kNm). A reduction in span moment was also observed for floor beams in buildings with chained lintel. This same behaviour was observed in the internals beams.

(2) Columns
The bending moment in the corner columns increased from 5.69 kNm to 12.338 kNm when chained lintels were introduced. This is due to the fact that lintels are subjected to the load from the blockwork courses that get to the sofit of the beams. Axial load in the columns also increased from 101 kN to 143 kN. This same increase in internal forces was observed for the intermediate columns.

Therefore, introducing chained lintels appears to worsen action effects on the columns, but favours the floor beams, except for end support moments.

Limitation of the study: The loadings on the lintels appeared to be exaggerated, and some considerations were not made to reduce the wall load on the floor beams since the chained lintel will carry some of the loads. The author therefore welcomes discussions and further investigations in the study.

Thank you for reading.

Download Top Civil Engineering Books for Free on Springer – COVID-19

By

Ubani Obinna

-

April 26, 2020

Springer is a leading global scientific, technical and medical portfolio, providing researchers in academia, scientific institutions and corporate R&D departments with quality content through innovative information, products and services. With more than 2,900 journals and 300,000 books, Springer offers many opportunities for authors, customers and partners. Here is a list of top civil engineering related textbooks that you can download for free on Springer during this COVID-19 period.

(1) Fatigue of Structures and Materials (2nd Edition, 2009)
Author: J. Schijve

To download this textbook, click HERE

(2) Composite Materials (3rd Edition, 2012)
Author: Krishan K. Chawla

To download the text book, click HERE

(3) Introduction to Partial Differential Equations (1st Edition, 2016)
Author: David Borthwick

To download, click HERE

(4) Structural Analysis (2009)
Authors: O. A. Bauchau, J.I. Craig

Click HERE to download

(5) Irrigation and Drainage Engineering (1st Edition, 2016)
Authors: Peter Waller, Muluneh Yitayew

Click HERE to download

(6) Design and Analysis of Experiments (2nd Edition, 2017)
Authors: Angela Dean, Daniel Voss, Danel Draguljić

To download the textbook, click HERE

(7) Engineering Mechanics 1 (2nd Edition, 2013)
Authors: Schröder, Wolfgang A. Wall, Nimal Rajapakse

To download the textbook, click HERE

(8) Fluid Dynamics (2015)
Author: Michel Rieutord

To download this textbook, click HERE

(9) Multivariate Calculus and Geometry (3rd Edition, 2014)
Author: Seán Dineen

Click HERE to download

(10) Fundamentals of Structural Engineering (2nd Edition, 2016)
Authors: Jerome J. Connor, Susan Faraji

Click HERE to download

(11) The Finite Element Method and Applications in Engineering Using ANSYS (2nd Edition, 2015)
Authors: Erdogan Madenci, Ibrahim Guven

Click HERE to download

(12) An Introduction to Soil Mechanics (1st Edition, 2020)
Author: Arnold Verruijt

Click HERE to download

(13) The Finite Volume Method in Computational Fluid Dynamics (1st Edition, 2016)
Authors: F. Moukalled, L. Mangani, M. Darwish

Click HERE to download

(14) Statics and Mechanics of Structures (2013)
Authors: Steen Krenk, Jan Høgsberg

Click HERE to download

(15) Engineering Mechanics 2 (2nd Edition, 2018)
Authors: Schröder, Wolfgang A. Wall, Javier Bonet

Click HERE to download

(16) Structural Dynamics (6th Edition, 2019)
Authors: Mario Paz, Young Hoon Kim

Click HERE to download

(17) Excel Data Analysis (2nd Edition, 2019)
Author: Hector Guerrero

Click HERE to download

(18) Statistical Mechanics for Engineers (1st Edition, 2015)
Author: Isamu Kusaka

Click HERE to download

(19) An Introduction to Machine Learning (2nd Edition, 2017)
Author: Miroslav Kubat

Click HERE to download

(20) Elementary Mechanics Using Matlab (2015)
Author: Anders Malthe-Sørenssen

Click HERE to download

(21) Introduction to Artificial Intelligence (2nd Edition, 2017)
Author: Wolfgang Ertel

Click HERE to download

(22) Computational Geometry (3rd Edition, 2008)
Authors: Mark de Berg, Otfried Cheong, Marc van Kreveld, Mark Overmars

Click HERE to download

(23) Leadership Today (1st Edition, 2017)
Authors: Joan Marques, Satinder Dhiman

Click HERE to download

(24) Handbook of Marriage and the family (3rd Edition, 2013)
Authors: Gary W. PetersonKevin R. Bush

Click HERE to download

(25) Neural Networks and Deep Learning (1st Edition, 2018)
Authors: Charu C. Aggarwal

Click HERE to download

IStructE to hold Webinar on Structural Uses of Stone

By

Peace Ikpotokin

-

April 25, 2020

The Institution of Structural Engineers (IStructE) will be organising a webinar on the structural uses of stone. Stones are naturally occurring rocks of igneous, sedimentary or metamorphic origin, and has been used for thousands of years in construction.

Most rocks are sufficiently consolidated to enable them to be cut or made into various shapes and blocks to be used as walling, paving or roofing materials. Stones are categorised into building stones, ornamental stones and dimension stones.

As admitted in IStructE’s website,

“Dimension stone has been a structural material for thousands of years, yet its use has steadily declined. The perception is that it is too expensive to quarry, cut and transport. Many now see this as suitable only for cladding or flooring. Yet, with a modern approach to structural usage, stone can realise its potential as a material fit for the 21st century”

Therefore, the webinar seeks to explore the contemporary use of stone in a variety of structural applications. It will showcase the versatility of this often-overlooked material.

Date: 29th April, 2020
Time: 12:30 (BST)
Price: £45.5 + VAT (members)
£70 + VAT (Standard)

Key Learning Objectives of Webinar

Gain a basic understanding of the primary considerations when designing in stone
Knowledge of the design parameters required for un-reinforced and reinforced stone structures
Introduction to the basic principles of post-tensioned stone
Improved awareness of structural stone applications through a series of case studies
The live webinar includes an interactive Q&A session with the expert speakers. Booking will close 24 hours prior to the webinar.

To Register for this Webinar, click HERE to be directed to IStructE’s page.

Disclaimer:
Structville.com is not an agent and is in no way affiliated to the organisation of the above named event. We have presented this as news so that the civil engineering family in the world can be aware of reputable online events that can be of benefit to their careers. This is the commitment of Structville.

What is the cause of the failure of this staircase?

By

Ubani Obinna

-

April 24, 2020

Considering structural design and detailing standards, what do you think is the cause of the failure of this staircase?

Write your answer in the comment section. Thank you very much.

BIM can Assist Construction Robots in Object Recognition

By

Ubani Obinna

-

April 30, 2020

The construction industry remains one of the least automated industries which depend on manual human labour for maximum productivity. The role of robots in construction operations is still very insignificant when compared other industries. However, robotic automation stands as a promising field for construction, as it offers a safer and efficient approach for handling building resources on site which extends the limits for more creative and customizable architecture.

Due to the complex and highly dynamic nature of the construction industry, robots cannot blindly execute predefined instructions as obtainable in other more deterministic industries. This makes it difficult for robots to adapt to the ever changing construction sites. It has therefore been found important that robots be equipped with advanced vision capabilities to enable them observe, cope, and adapt to busy construction sites.

construction robots — 3d rendering robotic arms with building

Researchers from Barlett School of Architecture, UCL, London UK, investigated a flexible 3D object recognition approach to easily reference building components to a vision system and be able to deal with objects’ imprecision. This is due to the fact that dealing with unique, irregular structures require robust object estimation approaches to enable on-site autonomous robotic assembly applications. To achieve this, they used a virtual scanning process which takes the “ideal” representation model of all the objects found in the Building Information Modelling (BIM) model as the guide for finding all “best match objects” on site, without the need for physically scanning or labelling the objects to reference them. The study was published in Springer – Construction Robotics.

According to the authors,

Our overarching hypothesis is that using the available BIM data as the reference model will have the potential not only to locate highly matched known objects with a threshold to accommodate material deviations, but also flexibility to find the best match objects from an unknown pile… Our goal was to detect and manipulate the best-matched known objects in an unsorted pile on site according to a given design in the form of the BIM model. Therefore, we developed a holistic approach that includes our detection method, and an automated process to grasp, manipulate, and determine the assembly sequence based on the design scheme of the structure and awareness of the surrounding context.

In their methodology, 3D object recognition was adopted using global feature based approach due to its flexibility to work with noisy data. 3D CAD representation of the objects coming from the BIM model was used for training instead of using physical scan models of the objects. After the recognition stage, object to robot calibration was done.

Recognition framework — Recognition System Framework, which comprises two different phases: an online recognition phase and an offline virtual training stage [1]

The recognition system proposed in the study showed that the method of using the virtual representation coming from the BIM model instead of having to scan the actual model brings advantages regarding flexible setup and affordable recognition. The experiments also showed that the system able to detect and construct several structures with inherited imperfections within acceptable tolerances. However, it also showed several limitations related to the object’s geometrical characteristic and its implication on the successful detection of the whole process success.

Reference for the full research paper:
[1] Mohamed Dawod and Sean Hanna (2019): BIM‑assisted object recognition for the on‑site autonomous robotic assembly of discrete structures. Construction Robotics (2019) 3:69–81 https://doi.org/10.1007/s41693-019-00021-9

Disclaimer:
The findings of this research work has been shown on www.structville.com because it is an open access article which is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Non-linear Analysis of Cable Stayed Structures on Staad Pro

By

Ubani Obinna

-

April 23, 2020

Non-linear analysis can be used when all of the members, elements and support springs are linear except for cables and/or preloaded truss members. This analysis is based on applying the load in steps with equilibrium iterations to converge at each step. In Staad Pro software, the iteration continues at each step until the change in deformations is small before proceeding to the next step. If not converged, then the solution is stopped. The user can then select more steps or modify the structure and rerun.

This method assumes small displacement theory for all members/trusses/elements other than cables and preloaded trusses. The cables and preloaded trusses can have large displacement and moderate/large strain. Pretension is the force necessary to stretch the cable/truss from its unstressed length to enable it to fit between the two end joints. Alternatively, you may enter the unstressed length for cables.

The analysis sequence is as follows:

(1) Compute the unstressed length of the nonlinear members based on joint coordinates, pretension, and temperature.

(2) Member/Element/Cable stiffness is formed. Cable stiffness is from EA/L and the sag formula plus a geometric stiffness based on current tension.

(3) Assemble and solve the global matrix with the percentage of the total applied load used for this load step.

(4) Perform equilibrium iterations to adjust the change in directions of the forces in the nonlinear cables, so that the structure is in static equilibrium in the deformed position. If force changes are too large or convergence criteria not met within 15 iterations then stop the analysis.

(5) Go to step 2 and repeat with a greater percentage of the applied load. The nonlinear members will have an updated orientation with new tension and sag effects.

(6) After 100% of the applied load has converged then proceed to compute member forces, reactions, and static check.

Note that the static check is not exactly in balance due to the displacements of the applied static equivalent joint loads.

Non-linear Cable Analysis Parameters

If you open the cable analysis command on Staad Pro, you will see cable analysis parameters which can be confusing at first sight. Staad Pro advanced training manual offers some insight on the definition of the parameters and the values to use.

Steps: The number of steps if entered should be in the range 5 to 145.

Eq-iterations: This is the maximum number of iterations permitted in each load step. The default value is 15 and should be in the range of 10 to 30.

Eq-tolerance: This is he convergence tolerance for the above iterations. Default value is 0.0005.

Sag minimum: Cables may sag when tension is low. This is accounted for by reducing the E value. Sag minimum may be between 1.0 (no sag E reduction) and 0.0 (full sag E reduction). Default is 1.0. If sag minimum is entered, it should be in the range 0.7 to 1.0 for a relatively simple process. As soon as SAGMIN becomes less than 0.95 the possibility exists that a converged solution will not be achieved without increasing the steps or the pretension loads. The Eq iterations may need to be 30 or more. The Eq tolerance may need to be greater or smaller.

Stability stiffness: A stiffness matrix value that is added to the global matrix at each translational direction for joints connected to cables and nonlinear trusses for the first Load Steps. The amount added linearly decreases with each of the Load Steps (Load Step is 1.0 if omitted). If stability stiffness is entered, use 0.0 to 2.0 but the default value is 0. For Load Steps use a maximum value of 145.

K small stiffness: This is a stiffness matrix value, that is added to the global matrix at each translational direction for joints connected to cables and nonlinear trusses for every load step. If entered, use values between 0.0 to 1.0 but the default value is 0.0. This parameter alters the stiffness of the structure.

Analysis Example
Let us analyse the cable stayed canopy loaded as shown below. The structure comprises of UB 254 x 102 x 22 stanchions supporting cantilevered trusses of UA 50 x 50 x 5 sections. Purlins on the trusses are made of channel sections CH 100 x 150 x 10. Cables are made of 30 mm diameter steel wires and UB 127 x 76 x 13 are used to brace the stanchions in the lateral directions. The loading applied on the structure is the self weight and UDL of 1.5 kN/m on the purlins.

In the analysis, an initial tension of 0.1 kN was applied in the cable members using the default parameters (leaving the cable analysis parameters blank). However, the load case refused to converge after iterations. The parameters that gave acceptable convergence are shown below.

Analysis Results

The displacement of the cable stayed canopy is shown below;

The maximum displacement in the truss was observed to be 17.155 mm.

The bending moment in the intermediate column was observed to be 27.2 kNm. The shear force in the same column was observed to be 9.08 kN with an axial force of 45.5 kN.

The axial forces in the cables and trusses of the intermediate members are shown below;

The axial forces in the cables and trusses of the end members are shown below;

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