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Design Guide: Shear Wall to Australian Standards

Design Guide: Trusses

Design Guide: Shear Wall to ACI Standards

Design Guide: Pad Footings

Design Guide: Purlins and Girts for Structural Support in Buildings

Design Guide: Structural Load Calculations

Design Guide: RC Staircase

Design Guide: Limit State Design of steel members to AS4100

Design Guide: Steel Bracing Systems

Design Guide: Concrete Footing to AS3600

Design Guide: Timber Members

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Structures Design Guidelines

Civil Engineering Design Guide Articles

Trusses comprise of individual members connected at joints or nodes, creating a system of interconnected triangular elements that work together to form a rigid structure. 

All loads and reactions are assumed to act only at 

 nodes, so no shear or moment is transferred between members. As such, truss members are subject to axial loading only which is beneficial because it means section sizes can be greatly reduced. Pin connections found in trusses are also much cheaper compared to moment connections, making trusses a relatively cheap option for a structure.

This guide will discuss truss forms, their features, and how to design them. But first, check out the activity below to understand how triangular elements work in a truss.

Why are triangles and resulting trusses so effective at resisting load? 

These shapes perform well since no shear or bending develops in the system’s individual members when the load is applied at nodes. As the nodes are pinned, equilibrium at the connection is achieved through opposing vectorised loads in each connecting member.

Each member connected to the node will take a percentage of the imposed load in axial force proportional to its angular position. In the example above, the horizontal members take no vertical load, given they're at 90 degrees to the force F, but they take the horizontal reaction load induced by the angled chords. 

So in short, trusses - the triangular marvel - are great because they:

Take a deep dive into the world of trusses and check out:

Common Truss Types

Truss Analysis

Steps for Designing Truss Members to Australian Standards

, the app you're reading this one is a calculation management platform. You can sign-up and build hosted, shareable web apps (complete with an API and a web publishing module) with tools like Python and Spreadsheets. Learn more 

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Structural truss design guide and details information

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Design Of Shear Wall As Per Australian Standards

Shear Wall Design Guide to Australian Standards

Shear walls play a critical role in the structural stability of buildings by providing stronger resistance to lateral loads. As trends in construction shifted towards high-rises and tall buildings in general, they have become even more important.

Lateral loads, such as wind and seismic forces, cause structures to sway and deflect (as shown in Figure 2). Without proper bracing and resistance against these loads, the structure's occupants may experience discomfort, or in the worst case, the structure may fail completely. One of the ways to limit the lateral sway and deflection is to increase the size of beams and columns - however, this method is costly and increases dimensions, which may not be permitted.

Shear walls are an effective solution to this problem. They are thick blocks of reinforced concrete that extend from a structure’s foundation to the top, adding stiffness and rigidity. 

 can be constructed in many shapes, as shown in Figure 3.

Arrangement (A) is common in structures as the shear walls are easier to construct and have no corners. Arrangement (B) has an aesthetic advantage as spaces between the shear wall allow the installation of decorative features such as large windows. In most tall structures, shear walls are placed in the center in a ‘c-shape’ (generally used as elevator shafts or stairways) to form a ‘core’. These cores prevent lateral torsion of the structure. Of course, there are other arrangements of shearwalls, and the design engineer must select one that suits the project at hand.

Often, openings are required in shear walls for access (e.g. elevator shafts), windows, or doorways, as seen in Figure 4. In these situations, the areas straight above the opening are significantly weaker than others. To counteract these internal stresses, ‘lintels’ are placed to act as beams to support additional height. For more, you can read this guide by StructuralGuide 

 follows ACI 318-14 and ACI 318R-14 Chapter 11[1]. This chapter only applies to walls resisting vertical and 

, and the ‘special walls’ design must follow Section 18 instead. A ‘special wall’ is a shear wall including coupling beams and wall piers forming part of a seismic-force-resisting system.

Step 1. Determine the minimum shear wall thickness

ACI 318-14 Table 11.3.1.1 provides the minimum shear wall thickness h for different types of walls. If justified with appropriate structural analysis, then thinner walls may be used.

Step 2. Determine factored axial force and moment

Shear walls are to be designed for the factored axial force, Pu, which produces the maximum factored moment Mu (if Pu is eccentric) for each applicable load combination. Pu must be smaller than ϕPn, max what is given in Table 22.4.2.1. Pn, max is a function of the nominal axial strength Po, at zero 

Step 3. Determine axial and flexural strength

‍ACI 318-14 Clause 11.5.2 - Axial load and in-plane or out-of-plane flexure

For bearing walls, Pn and in- or out-of-plane nominal flexural strength Mn is calculated using Pn, max obtained from Step 2.

For non-bearing walls, Mn can be calculated ignoring the effect of axial forces, as they have an insignificant effect on its flexural behavior.

ACI 318-14 Clause 11.5.3 - Axial load and out-of-plane flexure

If the location of the resultant forces acting on the wall lies within the middle third of the wall (as shown in Figure 4) and it is subject to an axial load that causes out-of-plane flexure, then Pn is calculated as:

Step 4. Determine the in-plane shear strength

ACI 318-14 Clause11.5.4.1 ~ 7 - Contribution from concrete

The total in plane shear strength of a wall Vn is the sum of the strengths of the concrete Vc and steel reinforcement Vs. The portion of total in-plane shear strength provided by the concrete section is calculated using Table 11.5.4.6:

It should also be noted that if the length of the wall is greater or equal to its height (i.e. hw <= 2lw), then it requires a more analytical design approach and must follow the strut-tie-method outlined in Chapter 23.

ACI 318-14 Clause11.5.4.8 - Contribution from reinforcement

The portion of total in-plane shear strength provided by reinforcement  is calculated as:

Once V_c and V_s have been calculated, check the total shear strength of the wall against in-plane design action effects:

Step 5. Determine the out-of-plane shear strength

Identical to the calculation of in-plane shear strength, equations and design procedures for out-of-plane shear strength change depending on the presence of prestressing and axial forces.

ACI 318-14 Clause 22.5.5 - Contribution from concrete (non-prestressed members without axial force)

If the wall is non-prestressed and there is no axial force applied to it, then the portion of the total out-of-plane shear strength provided by concrete V_c can be calculated as:

Or, using Table 22.5.5.1, which provides more detailed formulae:

ACI 318-14 Clause 22.5.6 - Contribution from concrete (non-prestressed members with axial compression)

If the wall is non-prestressed and axial compression is present, then the portion of the total out-of-plane shear strength provided by concrete Vc can be calculated as:

Or, using Table 22.5.6.1, which provides more detailed formulae:

ACI 318-14 Clause 22.5.7- Contribution from concrete (non-prestressed members with significant axial tension)

If the wall is non-prestressed and there is significant axial tension present, then the portion of the total out-of-plane shear strength provided by concrete Vc can be calculated as:

The equation above is similar to Clause 22.5.6; however, as concrete is significantly weaker in tension than in compression, the provided shear strength is deemed to be lower. This is reflected in the equation as when Nu becomes negative, Vc is reduced.

ACI 318-14 Clause 22.5.8- Contribution from concrete (prestressed members)

This clause is applicable for pre- and post-tensioned flexural members in regions of the structure where the effective stress in the prestressed reinforcement is fully transferred to the concrete. Effective stress If the member satisfies the following condition, then Vc can be calculated using Table 22.5.8.2:

It should also be noted that equations in Table 7 may be applied to walls with prestressed reinforcements only as well as walls with a combination of prestressed and non-prestressed reinforcement.

Additionally, Vc for prestressed members can be the lesser of the out-of-plane flexural-shear strength Vci and web-shear strength Vw:

ACI 318-14 Clause22.5.10 - Contribution from reinforcement

In sections of the wall where concrete alone isn’t strong enough, the portion of total out-of-plane shear strength required from transverse reinforcement V_s is calculated such that:

Once Vc and Vs have been calculated, check the total shear strength of the wall against out-of-plane design action effects:

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Shear Wall design Guide - ACI - CalcTree

Foundations are unsung heros of structures, rarely seen, but critical for structural integrity. They are often aesthetically underwhelming compared to more flashy structural components above ground, but foundations are responsible for the structure’s stability and the safety of its occupants. This guide specifically outlines procedures for the design of a very 

The primary role of a foundation is to transfer loads from the superstructures above to the ground below. Different types of foundations have been developed to accommodate different ground and loading conditions, and are generally categorized as either deep or shallow. Shallow foundations transfer loads to the upper soil layers which are relatively shallow compared to the effective size of the bearing area. If the upper soil layer is not strong enough, deep foundations can be used to reach stronger soil layers deeper in the ground, much deeper in comparison to the pad’s width. Their behavior, and therefore the method of design for shallow foundations, is different from deep foundations.

The design of a foundation varies from project to project and depends on various factors such as the design of the structure above, applied loads, site accessibility, proximity to nearby structures, soil conditions, budget, etc. Pad foundations are commonly used in structures with smaller heights which have relatively small deck areas supported by each column, like residential dwellings or single-story industrial buildings.

ad foundation design follows AS 3600 [1] and AS 2159 [2]. The table below shows import parameters and their definitions that are used constantly throughout this guide:

Step 1. Calculate the ultimate soil-bearing capacity

The first step in designing a pad footing is calculating the ultimate bearing capacity of the surrounding soil (qu), a nominal pressure at the foundation base at which the soil fails. The actual pressure beneath a foundation is often not uniform, but the nominal pressure takes the overall distribution into account, provided that a compliant set of equations are used end to end of the design procedure. The most relevant ultimate bearing capacity formula are those proposed by Terzaghi and Hansen. For cohesionless soils, Terzaghi and Hansen’s equations are equal; however, for cohesive soils, Terzaghi’s equations are more conservative than Hensen’s. Generally, Terzaghi’s equations only apply to shallow footings of certain shapes (i.e. circular, strip, and square), but Hansen’s equation applies to all. 

s = shape factor accounts for the shape of the footing

d = depth factor accounts for the depth of the footing base

i = inclination factor accounts for cases where loads act other than normal

g = ground factor accounts for the slope of the ground

b = base factor accounts for the inclined surface of the footing

Step 2. Calculate the required area of the footing

Generally, the ultimate bearing capacity is reduced by a ‘factor of safety’ to obtain the allowable bearing capacity qa. The allowable bearing capacity is used as the limit for the design of the footing instead of the ultimate bearing capacity for safety. This factor of safety generally varies between 2 ~ and 4, depending on the type of structure.

The required footing area can be calculated by rearranging the ‘pressure = force/area’ equation. P represents the column axial force, which includes the loads transferred from the superstructure and the column’s self-weight. For footings, the shorter side is referred to as width (denoted B), and the longer side as length (denoted L). 

You should note that some of the bearing capacity coefficients depend on the pad geometry. Therefore, you often need to do a bearing capacity ‘check’ after the first design iteration.

Step 3. Calculate the minimum reinforcement and spacing

A footing is responsible for the transfer of loads to the surrounding stratum and soil. Much like a beam or a slab, footings are subject to flexure and shear forces. Therefore, reinforcements are necessary.

Find the minimum requirement steel reinforcement:

Then, select the appropriate bar size and the number of bars required, considering the cover on each side.

Step 4. Find the ultimate flexural strength 

The critical flexural failure (or two-way shear failure) occurs at the face of the column. The flexural strength of the footing can be calculated below.

Critical shear failure (or one-way shear failure) occurs at some distance away from the face of the column. The shear strength of the footing (specifically the concrete) can be calculated below.

This section of the standard presents two methods for estimating the ultimate shear strength of concrete. In cases where the following are satisfied, a simplified method outlined in Clause 8.2.4.3 may be used:

The aggregate particle size of the concrete > 10mm

In such cases, the following can be assumed as per Clause 8.2.4.3:

If the above conditions are not met, then Clauses 8.2.4.2.1 ~ 3 must be followed.

The calculated flexural and shear strength must satisfy the following:

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Pad Foundation Design Guide - CalcTree

Purlins and girts are horizontal beams used for structural support in buildings. Purlins are a roof element and provide support for the weight of the roof sheeting while girts are a wall element and provide support for wall cladding. 

Purlins and girts are a very common building element, so it is worth 

 them. Look no further, we’ve got the guide for you! Below we'll answer a few key questions; 

📟 We also have an accompanying calculator 

Purlins and girts are structural members in roofs and walls that support loads from sheathing and cladding and are themselves supported by rafters,  trusses, or 

. Purlins and girts can be made from timber or steel. As a rule of thumb,  purlins are frequently proportioned for a depth-to-length ratio of  1/32. Dead, live, snow, and wind loads should be calculated by the designer and checked against the bending, shear, torsion, buckling, and deflection capacity of the girt or purlin by relevant design codes.

Steel purlin sections are namely C or Z sections. Steels are typically hot or cold-rolled. Hot rolling involves rolling steel at high temperatures to allow easy shaping and forming. Compared with hot-rolled sections of the same thickness, cold-rolled purlins tend to be lighter, stronger, and easier to assemble. 

The following factors should be considered when determining roof purlin spacing:  

2. Assume the depth and width of the cross-section:

: the orientation of purlins should provide a shorter distance from the shear center to the loading vector to minimize torsional impacts.

The x-axis of the local coordinate is parallel to the rafter. The y-axis of the local coordinate is perpendicular to the rafter. The process of transferring loads to local coordinates is illustrated in Figure 5:

 Do not add dead load and live load together when projecting them on the x and y axis because the load factor in the load combination will change it.

If the design check is not satisfied, the sections' area must be increased or decreased.

Wind load is directly normal to the roof, which is on the y-axis of the local coordinate, so there is no need to convert it. Different design codes specify different approaches to calculating wind load on roofs. Wind load depends heavily on the region the structure is located in, the type of structure, and topographic factors.

The Design Codes that guide the wind load calculation are:

The result will propose different wind load magnitudes (in kPa) on different locations on the roof, engineers usually choose the most significant wind load to design all purlins.

Calculate the uniformly distributed live load in kN/m along the purlin length by taking 

wind load on the effective area x spacing of purlins

. The same steps could be repeated if there are any other environmental loads. Calculating wind loads on pitched roofs is a detailed affair, so we’ve written a separate article here that details the process.

Each Design Code will propose a different methodology to 

 with different design factors. Due to the design factor, more loads on the structure don’t mean it will be the critical condition, so all load combinations should be checked and the most critical condition chosen. Here are

Choose the most critical scenario among those combinations to model and analyze the purlins. If the purlin system is simple enough, hand calculations can be performed.

Usually, bending is the deciding design factor for purlins; that is why it is commonly checked first. 

‍❗If not verified, increase the section size and recalculate

Verify the beam for a combination of bending and axial load. 

The bending and axial loading combination is the second deciding factor for purlins. This occurs when a beam is subjected to loading that causes bending in both the major and minor axes. To verify combined loading refer to appropriate sections within steel or timber design Codes - 

❗If not verified, increase the width or height of the beam and recalculate.

The shear stress in the most critical section of the cross-section can be calculated using the maximum shear force. 

Shear stress for beams can be calculated using: 

V = total shear force at the point of location (N.m)

I = moment of inertia of entire cross-section (m4) 

t  = material thickness perpendicular to shear (m)

Verify that shear stresses are less than the resistance/allowable stress of material cross-section by referring to the appropriate design codes for timber or steel member design‍

9. Verify the beam for instantaneous and final deflection criteria.  

❗If not verified, increase the width or height of the beam and recalculate. 

consider sag rods (or struts)  for purlins and girts.

‍While cold-formed sections (particularly Z purlins or girts) can provide substantial material and cost savings due to their higher strength-to-weight ratios, it gives rise to conditions that may see these members deform under many loading conditions and fail by yielding or local buckling as a result of induced stresses. Sag rods (struts) may be added as lateral supports to provide alignment and reduce their tendency to twist and bend and allow the transfer of the lateral loads. Typical sag rods are 10-12mm in diameter. Generally, manufacturer’s tables will provide details on the application and installation of a bridging system such as sag rods. 

10. If all those checks are verified, you have safe and suitable beam dimensions. 

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Purlins and Girts Design Guide - CalcTree

Defining and determining load combinations and calculations is essential in the design of structures to ensure their safety and stability. This process involves identifying and analyzing all the loads a structure will be subjected to, and assigning the correct safety factors and loading scenarios to simulation models. 

Design loads encompass everything from the self-weight of the structure to the weight of occupants, wind, snow, and seismic forces as well as the weight of any materials, semi-permanent members, and equipment used in construction or operation. Engineers must also determine the appropriate design load combinations to develop strength and serviceability design to withstand impact and prevent fatigue failure over time.

Design codes in varying geographical jurisdictions also have different recommendations when it comes to loading combinations, which must be considered. This is due to specific features of the environment structures in a given jurisdiction will be subject to, as well as the principles and nature of regulators.

In this article, we’ll summarise design 

 and combinations as recommended by AS/NZS 1170: 2002, ASCE7-10, and EN1991-1-1 to gain an understanding of commonalities and differences in loading requirements in these three major code sets. 

Unsurprisingly, design load, load combinations, materials, and structural systems vary significantly between national and local authorities. Nevertheless, engineers need to be familiar with specific design codes and standards that apply to their projects, regardless of location.

What are structural loads, and how do you calculate them?

A structure can be affected by various loads, the nature of which will vary depending on its design, location, usage, and more. In most cases, the maximum loads a structure must withstand are used to define design requirements. Typically loading can be considered as 

. This is an important distinction, as it determines safety factors applied to the underlying design action.

Dead loads on a structure are always present and cannot be removed. As such, a structure must be designed to safely support the weight of its own dead loads.

Dead loads, also referred to as permanent or static loads, are those expected to remain relatively constant over time.  Examples include the self-weight of a building's structural elements, flooring components and finishes, building services and operational equipment, like the HVAC system, non-structural permanent partitions, immovable fixtures, and even built-in cabinets. 

Most dead loads can be calculated by examining the specified materials' weights and volumes as depicted on drawings and/or in-situ and factoring them by the area they are imposed over. As a result, it is possible to quite accurately calculate dead loads. 

Structural engineers also tend to be conservative in their estimates, minimizing acceptable deflections, allowing for a margin of error, and accounting for potential changes in conditions over time. As a result, design dead loads frequently far exceed actual loads.

By calculating the volume of each member and multiplying it by the unit weight of the materials from which it is composed, an accurate dead load can be determined for each component. 

The different components can be added together to determine the total dead load. 

In most instances, engineers will translate loads into a UDL (uniformly distributed load) measured and record it in kPa or psf and an equivalent allowable concentrated loads (point loads). Dead load UDLs will be combined with superimposed loads and live loads to calculate ultimate and service loading on the structure.

Superimposed Dead Loads (SDLs) are permanent loads added to a structure, but not part of the structure itself. Examples include movable partitions, planter boxes, fixed office equipment, and base building equipment such as mechanical and electrical systems. They may seem permanent but can be relocated during renovations.

Similarly to pure dead loads, SDLs will typically be presented as a UDL used generally in a design. An important departure between pure dead load and SDLs is that the SDLs are sometimes treated as allowable loading for architects and building occupants. 

That is to say, the engineers have allowed for 5kPa of SDL in a certain area. Other stakeholders then use this information to design layout etc.

DL and SDL are typically combined and referred to as a total dead load, also sometimes referred to as 

, in structural engineering codes and calculations.

Live loads, also known as applied or imposed loads, can change over short timeframes. Live loads on a structure are not always present and can vary in location and magnitude. These loads include the weight of people, furniture, and other objects on the structure. A structure must be designed to safely support the weight of the maximum possible live loads it may be subject to. The weight of the audience in an auditorium, the books in a library, traffic loads, and so on are all examples of typical live loads.

Given the dynamic nature of live loads, it’s rare that an engineer will calculate them from scratch, as they do with dead loads. Rather, rates and allowable loading requirements are determined using design codes.

Typical characteristic live loads for different building purposes are determined according to tables given across various standards:

Environmental loads, such as seismic movement, wind, waves, rain, and snow, can impact structures in a short time frame similar to live loads. These loads, however, vary in their application and impact, so they have specific calculation protocols and loading rules. They are considered separate from live or dead loads as they may act horizontally and dynamically.

Regional differences greatly affect environmental loads. Climate, topography, and seismic activity vary from region to region, causing loading requirements to differ. For instance, Christchurch, New Zealand has stricter seismic loading rules than London, England.

A building's snow load is the downward force caused by the weight of accumulated snow and ice on its roof. If the snow load is greater than the building was designed to support, the roof or the entire structure may fail.

Table of clauses referring to Snow Loads and separated by code/jurisdiction 

The wind is a mass of air that moves mostly horizontally from a high-pressure region to a low-pressure region. Wind load is the intensity of pressure, i.e. the load, in pounds per square foot, placed on the exterior of a structure by the wind. High winds can cause a lot of damage because they pressure a building's surface. 

Table of clauses referring to Wind Loads and separated by code/jurisdiction 

Additionally, for certain structures, a dedicated wind analysis will be undertaken to provide more accurate wind-loading data for structural designers. This may be required due to a structure's importance or risk exposure levels (i.e. a hospital or stadium). And typically involves the use of dedicated specialist CFD (computational fluid dynamics) software or a physical model of the structure being created and run through a wind tunnel experiment.

The application of earthquake-induced agitation to a structure is the focus of seismic engineering. Damages most often occur where the structure touches the ground or another building. 

The calculation of a seismic load is included in the international building code. Seismic load tells how much seismic energy (energy waves that travel through the earth) a building would need to withstand in a given area.

The following factors are considered when calculating seismic loads:

General steps to determine seismic loading: 

The seismic load can be calculated by multiplying the Total Seismic Weight by the seismic response coefficient Cs in a horizontal direction.

Table of clauses referring to Seismic Loads and separated by code/jurisdiction 

To guarantee the structure's safety under a variety of maximum expected loading scenarios, building codes typically specify a variety of load combinations along with load factors (weightings) for each type of load.