Bracing for houses

Summary

This video offers essential knowledge on bracing domestic dwellings, explaining the principles of bracing design and the role of ceiling diaphragms in distributing wind loads. It discusses the importance of bracing walls, the challenges of load path disruptions like garage door installations, and presents various bracing solutions including plywood, K-frames, power trusses, and portal frames. The video also covers the spacing requirements for bracing walls and the necessity of direct fixing for ceiling sheeting to function as a diaphragm, concluding with advice on alternative bracing for suspended ceilings.

Takeaways

• 🏠 The video discusses the importance of bracing in domestic dwellings to withstand horizontal loads from wind.
• 🌪️ Wind forces are transmitted through the building structure to the foundation via the interaction of walls, ceilings, and floors.
• 🔄 The ceiling diaphragm plays a crucial role in distributing wind loads from the walls to the bracing walls.
• 📏 In the absence of a ceiling diaphragm, walls can become unstable, similar to a cardboard box without sides.
• 🔨 For two-story buildings with void regions, such as stairwells, a wind beam can be used to resist horizontal loads and prevent wall bowing.
• 🔍 The script highlights issues in house design, such as the transfer of wind loads to the floor and bracing walls, creating racking forces.
• 🚪 Problems arise when cutting panels for installations like garage doors, as it alters the load path and requires alternative bracing solutions.
• 🛠️ Solutions for bracing include plywood bracing, K-frames, power trusses, and portal frame systems, depending on the wall width left after modifications.
• 📏 The spacing of bracing walls is specified in AS1684, with the maximum distance varying based on wind classification, ceiling depth, and roof pitch.
• 🏗️ The depth of the ceiling diaphragm affects the distribution of forces, with deeper diaphragms having a higher capacity without significant deflection.
• 🔩 For the ceiling diaphragm to function properly, the ceiling sheeting must be directly fixed to the roof or ceiling framing; suspended ceilings are not suitable.
• 💡 If suspended ceilings are desired, alternative bracing solutions must be considered to ensure structural stability.

Q & A

What is the primary purpose of bracing in domestic dwellings?

The primary purpose of bracing in domestic dwellings is to transmit horizontal loads, such as those caused by wind, through the structure to the foundation, ensuring the stability and rigidity of the building.

How do wind forces affect the structural integrity of a building?

Wind forces create horizontal loads that, if not properly managed, can cause the walls to become unstable and potentially bow out, compromising the building's structural integrity.

What is a ceiling diaphragm and why is it important?

A ceiling diaphragm is a structural element that helps distribute wind loads from the walls to the bracings. It is important because it ensures the even distribution of forces and prevents instability in the building structure.

How does the absence of a diaphragm system affect the building's response to wind forces?

Without a diaphragm system, the wind forces are not distributed out to the bracings, leading to instability and a higher risk of the walls bowing out, similar to a cardboard box becoming less rigid when its sides are cut off.

What is a wind beam and how does it help in two-story buildings with void regions?

A wind beam is a structural element used in two-story buildings with void regions, such as stairwells and lobbies, to resist horizontal wind loads and prevent the walls from bowing out, maintaining structural stability.

What happens when a bracing panel is removed, such as for installing a garage door?

When a bracing panel is removed, the load path remains the same until it reaches the lateral panel, which now has only two narrow sections of wall to resist the racking forces, potentially compromising the structural integrity.

What are some solutions to address the problem of a removed bracing panel?

Solutions include leaving 600 to 900 millimeters of wall and using plywood bracing, using a K-frame or power truss for wall portions between 250 and 600 millimeters, or designing a portal frame system for wall widths less than 250 millimeters.

What is the difference between a K-frame and a power truss?

A K-frame is a steel truss frame made of square hollow sections (SHS) welded together, while a power truss is a proprietary lightweight metal truss frame, offering alternative load-bearing solutions.

What is a portal frame system and what materials can it be made of?

A portal frame system is a rigid frame designed to provide high lateral capacity. It can be made of hollow sections like SHS or RHS, or open sections such as UBs, UCS, or PFCs, and can be constructed in either timber or steel.

Why is the spacing of bracing walls important according to AS1684?

The spacing of bracing walls is important because it affects the building's ability to withstand wind loads. AS1684 provides guidelines on the maximum distance between bracing walls based on wind classification, ceiling depth, and roof pitch.

How does the depth of the ceiling diaphragm affect the distribution of forces in a building?

The depth of the ceiling diaphragm influences how effectively forces can be distributed without causing excessive deflection. A deeper diaphragm has a higher capacity to distribute forces to the bracing panels, while a shallow diaphragm may not engage the bracings effectively.

Why is direct fixing of the ceiling sheeting to the roof or ceiling framing necessary for the diaphragm to work properly?

Direct fixing of the ceiling sheeting to the roof or ceiling framing is necessary to ensure that the diaphragm can effectively transfer forces to the bracing system. Suspended ceilings, without direct fixing, will not function as a diaphragm and require alternative bracing solutions.

Outlines

00:00

🏠 Principles of Bracing Domestic Dwellings

This paragraph introduces the fundamentals of bracing design in domestic buildings. It explains how wind creates horizontal loads that must be transferred through the structure to the foundation. The importance of the ceiling diaphragm and floor diaphragm in distributing these loads is emphasized, with the use of diagrams to illustrate the load paths. The potential instability caused by the absence of a diaphragm system is compared to a cardboard box, highlighting the need for proper bracing. Specific bracing options, such as wind beams for void regions like stairwells, are also discussed.

05:02

🛠️ Bracing Solutions and Ceiling Diaphragm Considerations

The second paragraph delves into potential issues encountered during house design, particularly the distribution of wind loads and the creation of racking forces in bracing panels. It discusses the impact of cutting off a bracing panel, such as for a garage door, and how this can affect the load path and the stability of the structure. Solutions to these issues are presented, including plywood bracing, K-frames, power trusses, and portal frame systems, each suitable for different wall widths. The paragraph also addresses the spacing of bracing walls as per wind classification and the importance of direct fixing of ceiling sheeting for the diaphragm to function effectively, noting that suspended ceilings are not suitable for this purpose.

Keywords

💡Bracing

Bracing refers to the structural elements used to resist lateral forces such as wind or seismic activity in a building. In the video, bracing is essential for transferring horizontal loads from the roof and walls to the foundation, ensuring the stability of the dwelling. Examples include the use of plywood bracing, K-frames, and power trusses to maintain structural integrity.

💡Ceiling Diaphragm

A ceiling diaphragm is a structural system that acts like a stiff plate, distributing horizontal forces from the roof to the vertical bracing elements. The video emphasizes its importance in the load path, showing how wind forces are transferred through the diaphragm to the bracing walls. Without a proper ceiling diaphragm, the building's walls could become unstable under wind pressure.

💡Horizontal Loads

Horizontal loads are forces that act perpendicular to the vertical gravitational forces, such as those caused by wind or earthquakes. The script explains how these loads are transmitted through the building's structure, specifically highlighting the role of the ceiling and floor diaphragms in carrying these loads to the foundation.

💡Wind Force

Wind force is the pressure exerted by the wind on the structure of a building. The video script discusses how wind force affects different parts of a building, with the roof and walls being key areas where these forces are transferred to the ceiling diaphragm and then to the bracing walls or floor diaphragm.

💡Racking Force

Racking force is a type of lateral force that can cause structures to move or deform in a diagonal pattern, resembling the motion of a rake. The video describes how wind loads create racking forces in the bracing panels, which must be resisted to prevent structural damage.

💡Bracing Walls

Bracing walls are vertical structural elements designed to resist lateral forces and provide stability to a building. The script describes how these walls are crucial for carrying the wind forces from the ceiling diaphragm down to the foundation, especially in situations where wall sections are interrupted, such as for a garage door opening.

💡Foundation

The foundation is the base upon which a structure rests, transferring the loads from the building to the ground. In the context of the video, the foundation's role is to receive and distribute the horizontal loads transmitted through the bracing system, ensuring the overall stability of the dwelling.

💡K Frame

A K frame is a type of steel truss frame used in construction to provide support and rigidity. The video mentions K frames as a potential solution for bracing in situations where there is limited wall space, such as when installing a garage door.

💡Power Truss

A power truss is a proprietary, lightweight metal truss frame used in construction to carry loads down to the foundation. The script presents power trusses as an alternative to K frames for situations with limited wall space, emphasizing their role in maintaining structural integrity.

💡Portal Frame

A portal frame is a rigid structural frame system that can be made of timber or steel and is designed to resist lateral forces. The video discusses portal frames as a solution when the wall width is less than 250 millimeters, highlighting their ability to provide high lateral capacity.

💡Spacing of Bracing Walls

The spacing of bracing walls refers to the distance between these structural elements, which is crucial for effective load distribution. The video script explains that according to AS1684, the maximum distance between bracing walls varies based on factors like wind classification, ceiling depth, and roof pitch, with shallower ceilings requiring closer bracing.

💡Ceiling Sheeting

Ceiling sheeting is the material used to cover the ceiling structure. The video emphasizes the importance of direct fixing of ceiling sheeting to the roof or ceiling framing for it to function effectively as a diaphragm. It also mentions that suspended ceilings, which do not provide the necessary rigidity, require alternative bracing solutions.

Highlights

Introduction to the principles of bracing design for domestic dwellings.

Explanation of how wind produces horizontal loads on buildings.

The role of the ceiling diaphragm in transmitting wind forces to the floor and bracing walls.

Illustration of the wind force distribution through the diaphragm system.

The importance of the diaphragm system for structural stability, compared to a cardboard box.

Use of a wind beam in two-story buildings with void regions like stairwells.

Discussion on issues encountered in house design related to wind load transfer.

Impact of cutting off a bracing panel on load path and racking forces.

Solutions for dealing with load path issues, such as plywood bracing or k-frames.

Considerations for designing a portal frame system for narrow wall sections.

Spacing of bracing walls as per AS1684 standards and wind classification.

The relationship between ceiling depth, roof pitch, and bracing wall spacing.

Visual analogy of a house as a wind beam to understand force distribution.

Requirement for direct fixing of ceiling sheeting to the framing for diaphragm function.

Challenges with using suspended ceilings as a diaphragm and alternative bracing solutions.

Conclusion summarizing the importance of understanding bracing design for domestic dwellings.

Transcripts

hey guys welcome back to the channel today is  a quick video on the things you need to know  

about bracing domestic dwellings i'm going to  be going over the principles of bracing design  

ceiling diaphragm and show you a couple of bracing  options that you can use in specific situations  

let's get started with this diagram. Wind  produce horizontal loads on buildings  

that must be transmitted through  the structure to the foundation  

in a conventionally constructed house these  loads are transmitted to the ground by a complex  

interaction between the walls ceiling  and roof structure and floor structure

so what we've got here is wind force on the  roof are transferred to the ceiling diaphragm  

and then carry it down to the floor via  bracing walls. The same applies to the wind  

force on the top half of this wall which are  carried to the ceiling and down to the floor  

through the bracing walls the wind force on the  bottom half of this wall represented by the yellow  

arrows are transmitted to the floor diaphragm  and down to the footings through bracing walls  

the same principle applies to the wind force  acting on the ground floor wall so you can  

see here that the wind force on the top half of  this wall are transferred to the floor diaphragm  

then it engages the bracing wall and the load is  carried down to the footing and finally the force  

acting on the bottom half of this wall are carried  direct to the ground via the wall and footing  

to illustrate this explanation better and  reinforce how important the diaphragm system  

is have a look at the image on the right hand side  here we're able to visualize how the ceiling will  

distribute the wind loads carried from the wall to  the bracings now let's have a look at the image on  

the left imagine that the ceiling has been removed  without the diaphragm system the wind force will  

not be distributed out to the bracings and the  walls will become unstable and try to bow out  

think of a cardboard box as soon as you start  cutting off the sides the box becomes less  

rigid and more unstable you will come across  a situation like that in two-story buildings  

where a full height void region exists between the  first floor and ground floor these void regions  

commonly fall within stairwells and entrancy  lobbies in this case a wind beam can be used  

to resist the horizontal wind loads  and prevent the wall from bowing out  

let's have a look at a couple of issues that you  might encounter when you're designing a house as  

i explained before in the first slide of the  presentation half of the wind load acting on  

the wall is transferred to the floor and the other  half goes through the ceiling to the bracing wall  

when it gets to the bracing panel it creates  a racking force and it's carried down to the  

foundation you can also imagine this bracing panel  as a cross bracing for wind forces coming from  

this direction this rod here will be in tension  and this other one will buckle away in compression  

now what happens if we cut off this panel  to install a garage door for example the  

load path will remain the same until it gets to the  lateral panel which now has only these two narrow  

sections of wall to resist the racking forces  so how can we deal with this type of problem  

we've got a couple of solutions the first one  if you can leave between 600 or 900 millimeters  

of wall you might get away with plywood bracing at  both ends these would be the cheapest options that  

you can provide to your client if the portion  of wall left is between 250 and 600 millimeters  

you might get away with k frame or power truss  to carry the loads down to the foundation  

k frame is a steel truss frame that can be made  of shs welded together while power truss is a  

proprietary lightweight metal truss frame on  the other hand if the width of the wall left  

is less than 250 millimeters you might have  to consider designing a portal frame system  

the portal frame can be either in timber or steel  

the idea behind this option is to create a frame  rigid enough to provide high lateral capacity  

the portal frame can be made of hollow sections  like shs or rhs or open sections such as ubs ucs  

or pfcs another important point i would like to  touch upon is the spacing of bracing walls as1684  

presents a table with the maximum distance between  bracing walls according to wind classification  

ceiling depth and roof pitch the shallower the  ceiling diaphragm the closer the bracing the  

bracing walls need to be an easy way to understand  this concept is imagining the house as a wind beam  

the deeper the beam the higher its capacity to  distribute the forces out without deflecting too  

much if you have a look at this deep diaphragm  the force can still reach the bracing panel  

while in the shallow diaphragm the forces cross  the whole ceiling without engaging the bracings  

as a final comment for the ceiling diaphragm  to work properly the ceiling sheeting must have  

direct fixing to the roof or ceiling framing  okay so for instance suspended ceilings will  

not work as a diaphragm and if you wish to use  suspended ceilings you will have to look for  

an alternative solution to brace the structure  maybe speed brace or another type of bracings  

so that was it for today hope you learned  something new if you have any questions  

or suggestions just drop a comment below and  thanks for watching and i'll see you next time