Structural Design 5 - Loading of a building

 There are three major types of loading. Dead, Imposed and lateral loading. We have to clearly and correctly identify the loading nature and the values for a better analysis part. If the loading is not considered well, you will have either a very weak insufficient sizes for elements or an over-design which will lead for an unnecessary money loss to the client. We will look at those different different loading separately. 

(1). Dead loads. 

We call these immovable loads as well. Actually these are the still loads which are definitely act weather the building functioning or not.  We consider the brick weights, Block wall weights, Concrete weights (Self weight) and finishes weight for this load case. We consider the service dead load as it is and when conducting the design works we have to factorize it for an adverse dead load condition. Acoording to BS8110, it is 1.4

gk is used to denote a dead load in units of kN/m or kN/m2 (Dead load intensity)
Gk is usually used to denote a dead load in units of kN (Point dead load)

Here we have small simple examples for calculating the dead loads in two different situations.

(1). Imposed loads.(Live loads) 

We call these movable loads. The things inside utilizable areas including pets and humans in it are taken for live or imposed loads. According to the function of the different different floor areas imposed loads can be varied. As an example, inside a bed room usually 2-5 people can accommodate and relative it has a small imposed load. When we consider a lift machine room there is a relatively large imposed load since the weight of the machines used there are very large. Likewise we have to find the relevant imposed load for the areas. For that we can use a code in British standards. That is BS 6399. All the relevant live loads according to the functionalities are included in this loading code. Weather it is a school, factory or an office all the needed loads are indicated in that book. Since after fixing a partition is immovable, we can change the position and the amount of partitions anytime easily in the floors. So the partition loads are also taken in to imposed loads. In British codes, they have clearly mentioned that a safety factor of 1.6 is needed to taken in to account when converting a service imposed load in to a design or Ultimate imposed load. 

Sop finally we can take the final Design load by multiplying the dead load by 1.4 and multiplying the imposed load by 1,6 and by adding those together.

n=1.4gk+1.6qk

For an example, we can show below the column design philosophy. In that equation, we have the ultimate point or axial load on a column in left hand side. When finding it we have to find the dead loads and live loads on it and factor them and sum up them. That is shown in the below picture.

Now you have a good knowledge about loading. We can proceed these learning from here. Meet us on our next new lesson about Initial sizing of structural elements in Structural Design 6 Lesson. Until then follow our blog to receive the notifications.
Thank You! Cheers!

Structural Design 04 - Bending Moments & Shear Forces

 Today we will discuss about bending moments and shear forces. These parameters are the reinforcement amount governing factors in the design process most of the time. There are few others such as Torsion etc. We can discuss about them in future. 

According to the type of the beam and the nature or support conditions and loading patterns, few formulas are available for finding the values of BM and SFs. If we have several loading patterns we can go for the theorem of super-positioning. Let's discuss few common types  that can be useful in basic structural designs. We focus mainly about beams here. Here we go.

(a). SIMPLY SUPPORTED BEAM - 

This means the support allows the beam for the rotation. that means no any restriction for rotation of the beam. For this support and beam should not be monotonically casted.

In simply support, support can be a Rock, R/R Masonry wall, Another beam, Brick wall or concrete wall.

When we idealize the situation we can find the values of bending moment and shear force using below equations. When we have only a UDL (Uniformly Distributed Load) on the beam and if the value is "n kN/m" and if the length of the two simply supports is "l", the value of Bending Moment at the middle bottom of the beams will be, (nl^2/)8 as shown. Shear force at a support would be (nl)/2. Shear force is coming from equating the loads in Up-Down direction and the BM values come from by taking the area of that shear force diagram.

We can show you several types of BM and SF diagrams according to few different common situation below. 

(b). Simply support beam with a point load at the middle

(c). Simply support beam with a UDL and a point load at the middle

(d). Two end fixed beam with a UDL

(e). Two end fixed beam with a pint load at the middle

(f). Cantilever beam (One end free beam) with a UDL

(g). Propped cantilever beam with a UDL

Beam Reactions, Bending Moments and Deflections values and equations can be directly taken from the Appendix 3 of the book of "Design of Structural Elements" by W.M.C.McKenzie. He is a teaching Fellow of Napier University of Edinburgh. I will mention those in below for your kind references. (All rights should belongs to the original author of the book)

structural design 03 - COVER TO THE REINFORCEMENT

 Concrete cover f=to the reinforcement should be maintained very well during the construction stage in order to avoid the corrosion of the reinforcement bars. So we usually use several cover values such as 25mm, 40mm, 50mm, etc. In order to decide the value for the cover we have to first look at the environment condition. Because the reinforcement c corrosion capacity depends on that. If we have a concrete structure under sea water, it is more vulnerable for corrosion. Inside a covered building which is protected from a roof slab it is less vulnerable for corrosion. So we have to decide the cover according to the conditions.

  

Reinforcement corrosion is an Electo-Chemical reaction. The below reactions are expected to happen there.

In BS 8110 table 3.3 these conditions are defined with related proposed covers. There are 5 main conditions mentioned in it. There may be cases where extra precautions needed beyond those given in the table 3.3

(1).MILD (2).MODERATE (3).SEVERE (4).VERY SEVERE (5).EXTREME are those five.

After the deciding the exposure condition, we can find the related cover from table 3.4 of BS 8110.

Structural Design 02 - About Steel & Concrete for Designs

 When we talk about steel, we use two types of steel for design and constructions. Tor Steel and Mild steel. Hot rolled tor steel strength is 460 Newton per square millimetres. Hot rolled mild steel strength is 250 Newton per square millimetres.

fy=460N/mm^2 & fy=250N/mm^2

In reinforcement calculation formulas we use these values.
Ex: As(req.)=M/(0.87fyz)  Here we can use fy=460N/mm^2 for reinforcement calculation.(Tor)

Say we have a stirrup or link design of a beam. We may have below equation for that.

Asv>=[(Vd-Vc)bvSv/(0.87fyv)]    Here we can use fy=250N/mm^2 for reinforcement calculation.(mild)

According to the diameter sizes, Reinforcement area of cross sections are as follows.

Nominal diameter of a bar is taken without the RIB around a rebar. It is shown in below image.

As an example we can use the above values inorder to calculate the the provided reinforcement areas in below shown beam sections as mentioned below. 

We have shown similar examples below as well.

Mild steel rebar or reinforcement bar section is circular as it doesn't have any rib. We call them as round bars. Usually we can find them in 10 mm, 12 mm, 8 mm and 6 mm diameter sizes. In house constructions, 6 mm or R6 mild steel bars are commonly used for fabricating the beam stirrups. 

When we talk about concretes, we have two major types. Mass concrete and reinforced concrete are those two types. In mass concretes we don't use reinforcements. Just Concrete. Usually we use these for screed concrete which is placed before casting a footing on the ground. The purpose of that is to level the uneven ground surface and avoiding bleeding the liquid part of foundation concrete to the ground. It passively act as an additional covering to the reinforcement as well.

Reinforced concrete is also known as structural concrete. We use several grades of concretes for constructions according to their different different mix proportions. Few of them are shown below along with their water cement ratios and cement contents.You can find this table in the British standards BS8110 table 3.4

There is a guideline for the aggregate sizes to be used in BS8110. 

We can discuss about the Reinforcement cover and it's requirement from our next article. Until then please subscribe us. Thank You!