abbaskhan2294

I have always been in this confusion about the various approaches followed by various designers for the design of shear walls while using analysis software. One of the most common method followed is to analyze it for moment envelop. I have never been able to understand the philosophy behind this as envelopes are usually done to analyze critical points against moving live loads. Whats your opinion on this?

According to ASTM A615 , the tensile strength for low carbon steel should be 1.25 times the yield strength. The values specified are based on lower bound values and in lab you should be definitely getting higher values than this. However the ratio of 1.25 drops below in case of high strength bars such as grade 90 and 105 where it is limited to 1.1 only. B

Hello. I just came to know about this new foundation method known as post tensioned waffle mat slab foundation. Does anyone know how it is designed? If I can get any reference document it be helpful.

@Simple Structures can you provide any reference for the design procedure of fully and partially moment resistant connections in frames?

This case would usually occur if there is a two span beam with one span having high bending moment and the other one with lesser bending moment demands. In such cases all you need to care about is the development length so that full yield capacity of the bar in high bending moment section is achieved and further you can curtail the bar anywhere near the support. For grade beam the case is opposite i-e top bars will have to be curtailed at same specific location.

Dear all I want to discuss an issue recently encountered at site. Designer had given details of raft with #6 @ 6" o.c reinforcement BW in top and bottom meshes but contractor instead provided#4 @ 7" o.c. the raft and 1st story columns have been casted. On doing its capacity analysis the raft with provided reinforcement was not adequate to support to super structure. Now I'm thinking of doing retrofitting by providing an overlay of 9 to 12 inches. I wanted to know about shear keys/dowels for the connection between old and new slab. Can anyone provide any helping material for it? Thanks

All the design codes are well-mentioned by Umar. As far as timber design is concerned you can refer to NDS (national design supplement) and AWC ( American wood council) documents. The wood engineering properties are based on species found in US and cannot be used for Pakistan timber industry. The potential of timber in Pakistan still needs to be investigated.

@Rana How to calculate reinforcement for pattern live loads in ETABS for a whole structure automatically?

I'm facing an issue in design of strip footing for region with high water table. The water table is situated at a depth of 3 ft from the N.S.L. What measures should I take? Thanks

Sure. Here are the relevant snaps. Just put in all your BFRP properties for each parameter.

Define a typical steel bar then edit its properties by inputting the properties of the BFRP.

If u r using both BFRP and Steel rebar in a section then u have to use section designer. But incase you're intending only to use BFRP instead of Steel rebars then its pretty simple. Define a rebar in materials properties then edit its properties with ur bfrp bars. Thats all you need to do.

Etabs only gives you minimum area of reinforcement incase of slabs and columns provided that the designed reinforcement is less than the minimum area of reinforcement as per design code. However as far as design of beams are concerned, you must always calculate the area of steel manually and check it with the designed reinforcement by etabs. Etabs always give the designed reinforcement for beams whether its less or more than the minimum area of steel.

Following are the limitations imposed by ACI for calculating minimum reinforcement in case of slabs and beams: As (min) = 0.002 bh or 0.0018 bh (for slabs) As(min) = (200/ fy ) b x d ( Governing for beams when fc' < 4500 psi) I tried to calculate isolated footing flexural reinforcement with both these methods and the results were quite different. Eq of slab when used gave me a min reinforcement of #4@8" c/c whereas beams equation gave a minimum reinforcement of #4 @ 4" c/c. My question is when to use each of them in footing and which one is the more reliable approach to use. Thanks

change section modifiers in ur original model and then check. atleast u can do that

Umar I face problems in design of trusses after analysis. As roof trusses are subjected to both wind and snow load also pitching is done on roof, so they are likely to be analyzed for line loads. Analyzing for line loads mean that they would have both bending moments and axial forces. Also for studs, compressive strength is a function of length (tendency to buckle which reduces the strength). How to do design then? Should it be like to use the interaction equation or unity check for combined loads with axial capacity incase of stud being calculated as a function of length, adding them up and the product of summation should be less than or equal to 1?

I heard from a professor to always shift base shear in below ground structures to the ground As the columns below ground has some sort of confining pressure and its mode is different than the structure above ground. This may save ur reinforcement if valid.

Please open the file which i uploaded. I have modified it with Etabs version 16.0. Download Etabstran2013 and convert the file to ur workable version. Again I will suggest u that section modifiers have very important role to play. You need to take care of the case i-e whether it is equilibrium torsion or compatibility torsion in order to decide torsion modifier for section. As far as Ixx and Iyy modifiers are considered, if u go with 1 as section modifier ur section has greater stiffness. Greater stiffness means greater attraction of loads towards itself. As it takes more load in case with section modifier 1 u need more shear reinforcement because more load means more shear demand on the member. Now in such cases the required shear reinforcement exceeds the section capacity and as a result section is dislayed as failed. Also these are short span members and such members are very critical to shear than flexure failure. This failure is referred to as deep beam failure. Please check definition for deep beam in Aci code. If u go with the other option with section modifiers for moment of inertia as 0.35 (as suggested by ACI) which is pretty much valid for under reinforced cracked sections, it is actually the real representation of the beam element. In such cases less loads will be taken by the beams and consequently less shear demand. please use the section modifiers as suggested and then see ur structural elements. Thanks

That is a very nice question asked. Well section modifiers just not only has to do anything with deflection but stiffness of the member too and stiffness in RCC is a major factor controlling the reinforcement. Just as u explained the torsional constant if it is input as 1 it means the beam is so stiff that all the torsional moment will be taken by this beam alone and it will not be distributed in the adjacent members thats why beams always fail in torsion when torsion modifier is used as 1. But if u start playing with the torsion modifier and decrease it u will see that the torsional demand acting on it previously has now been distributed amongst other members and as a result reinforcement in all the other adjacent columns and beams increases. This condition is known as compatibility torsions. Coming back to moment of inertia modifiers case, Building systems are broadly classified into moment resisting frames, Bracing system and dual systems. When designing columns in buildings with braced systems (shear walls), they recommend to use a section modifier for both Ixx and Iyy as 0.1 and when u do such , columns in buildings with greater heights will have 1% reinforcement for the sake that u have assumed that ur column stiffness is too less (from the section modifiers) and all the load is taken by the bracing systems alone. This is what i have understood from the case studies I have performed. sharing other experiences will be great addition.

Please have a look at the file now. As per my understanding it is very much okay now. You were inputting the wrong modifiers. For your reference I have also uploaded an image for appropriate section modifiers. Please find the attached files. Thanks house for thulo kancha uncle 2-check.EDB

As long as the columns are short columns i don't think effective length has anything to do with the area of reinforcement as incase of short columns, area of reinforcement primarily depends upon the cross sectional area provided and not the length of the columns. Yes for slender columns or columns with secondary moments (P delta) effective length has a vital role in deciding the area of reinforcement.

I suspect there may be a concrete wall element at the bottom most story increasing the stiffness and decreasing the demand on the columns or u may have used reduced geometrical properties in the intermediate story as compared to the lowest story. Also there can be another possibility about the base shear level defined for the seismic loads.

If the columns are not slender how come would the braced and unbraced length effect the reinforcement in it?

As far as I believe you have to provide steel in any case when designing for flexure. The conventional conservative approach is that whether the section is cracked or uncracked it is assumed that all the stresses are carried by steel on the tension side. However the concept of cracked and uncracked section is useful in controlling the deflection.

Can u please provide the link of the thread?